Re: [Xen-devel] [PATCH 5/6] libxl: support mapping static shared memory areas during domain creation

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Wed, 23 Aug 2017, Zhongze Liu wrote:
> Add libxl__sshm_add to map shared pages from one DomU to another, The mapping
> process involves the follwing steps:
> 
>   * Set defaults and check for further errors in the static_shm configs:
>     overlapping areas, invalid ranges, duplicated master domain,
>     no master domain etc.
>   * Write infomation of static shared memory areas into the appropriate
>     xenstore paths.
>   * use xc_domain_add_to_physmap_batch to do the page sharing.
> 
> Temporarily mark this as unsupported on x86 because calling p2m_add_foregin on
> two domU's is currently not allowd on x86 (see the comments in
> x86/mm/p2m.c:p2m_add_foregin for more details).
> 
> This is for the proposal "Allow setting up shared memory areas between VMs
> from xl config file" (see [1]).
> 
> [1] https://lists.xenproject.org/archives/html/xen-devel/2017-07/msg03047.html
> 
> Signed-off-by: Zhongze Liu <blackskygg@xxxxxxxxx>
> 
> Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> Cc: George Dunlap <George.Dunlap@xxxxxxxxxxxxx>
> Cc: Ian Jackson <ian.jackson@xxxxxxxxxxxxx>
> Cc: Jan Beulich <jbeulich@xxxxxxxx>
> Cc: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
> Cc: Stefano Stabellini <sstabellini@xxxxxxxxxx>
> Cc: Tim Deegan <tim@xxxxxxx>
> Cc: Wei Liu <wei.liu2@xxxxxxxxxx>
> Cc: Julien Grall <julien.grall@xxxxxxx>
> Cc: xen-devel@xxxxxxxxxxxxx
> ---
>  tools/libxl/Makefile         |   2 +-
>  tools/libxl/libxl_arch.h     |   6 +
>  tools/libxl/libxl_arm.c      |  15 ++
>  tools/libxl/libxl_create.c   |  27 ++++
>  tools/libxl/libxl_internal.h |  14 ++
>  tools/libxl/libxl_sshm.c     | 336 
> +++++++++++++++++++++++++++++++++++++++++++
>  tools/libxl/libxl_x86.c      |  18 +++
>  tools/libxl/libxl_xshelp.c   |   8 ++
>  8 files changed, 425 insertions(+), 1 deletion(-)
>  create mode 100644 tools/libxl/libxl_sshm.c
> 
> diff --git a/tools/libxl/Makefile b/tools/libxl/Makefile
> index 3b63fb2cad..fd624b28f3 100644
> --- a/tools/libxl/Makefile
> +++ b/tools/libxl/Makefile
> @@ -138,7 +138,7 @@ LIBXL_OBJS = flexarray.o libxl.o libxl_create.o 
> libxl_dm.o libxl_pci.o \
>                       libxl_dom_suspend.o libxl_dom_save.o libxl_usb.o \
>                       libxl_vtpm.o libxl_nic.o libxl_disk.o libxl_console.o \
>                       libxl_cpupool.o libxl_mem.o libxl_sched.o libxl_tmem.o \
> -                     libxl_9pfs.o libxl_domain.o \
> +                     libxl_9pfs.o libxl_domain.o libxl_sshm.o \
>                          $(LIBXL_OBJS-y)
>  LIBXL_OBJS += libxl_genid.o
>  LIBXL_OBJS += _libxl_types.o libxl_flask.o _libxl_types_internal.o
> diff --git a/tools/libxl/libxl_arch.h b/tools/libxl/libxl_arch.h
> index 5e1fc6060e..1d681d8863 100644
> --- a/tools/libxl/libxl_arch.h
> +++ b/tools/libxl/libxl_arch.h
> @@ -71,6 +71,12 @@ int libxl__arch_extra_memory(libxl__gc *gc,
>                               const libxl_domain_build_info *info,
>                               uint64_t *out);
>  
> +_hidden
> +bool libxl__arch_domain_support_sshm(const libxl_domain_build_info *b_info);
> +
> +_hidden
> +int libxl__arch_domain_sshm_cachepolicy_setdefault(libxl_static_shm *sshm);
> +
>  #if defined(__i386__) || defined(__x86_64__)
>  
>  #define LAPIC_BASE_ADDRESS  0xfee00000
> diff --git a/tools/libxl/libxl_arm.c b/tools/libxl/libxl_arm.c
> index d842d888eb..0975109c0c 100644
> --- a/tools/libxl/libxl_arm.c
> +++ b/tools/libxl/libxl_arm.c
> @@ -1065,6 +1065,21 @@ void libxl__arch_domain_build_info_acpi_setdefault(
>      libxl_defbool_setdefault(&b_info->acpi, false);
>  }
>  
> +bool libxl__arch_domain_support_sshm(const libxl_domain_build_info *b_info)
> +{
> +    return true;
> +}
> +
> +int libxl__arch_domain_sshm_cachepolicy_setdefault(libxl_static_shm *sshm)
> +{
> +    if (sshm->cache_policy == LIBXL_SSHM_CACHEPOLICY_UNKNOWN)
> +        sshm->cache_policy = LIBXL_SSHM_CACHEPOLICY_ARM_NORMAL;
> +    if (sshm->cache_policy >= LIBXL_SSHM_CACHEPOLICY_X86_NORMAL)
> +        return ERROR_INVAL;
> +
> +    return 0;
> +}
> +
>  /*
>   * Local variables:
>   * mode: C
> diff --git a/tools/libxl/libxl_create.c b/tools/libxl/libxl_create.c
> index 1158303e1a..8e5ec486d2 100644
> --- a/tools/libxl/libxl_create.c
> +++ b/tools/libxl/libxl_create.c
> @@ -501,6 +501,14 @@ int libxl__domain_build(libxl__gc *gc,
>          ret = ERROR_INVAL;
>          goto out;
>      }
> +
> +    /* the p2m has been setup, we could map the static shared memory now. */
> +    ret = libxl__sshm_add(gc, domid, d_config->sshms, d_config->num_sshms);
> +    if (ret != 0) {
> +        LOG(ERROR, "failed to map static shared memory");
> +        goto out;
> +    }
> +
>      ret = libxl__build_post(gc, domid, info, state, vments, localents);
>  out:
>      return ret;
> @@ -918,6 +926,25 @@ static void initiate_domain_create(libxl__egc *egc,
>          goto error_out;
>      }
>  
> +    if (d_config->num_sshms != 0 &&
> +        !libxl__arch_domain_support_sshm(&d_config->b_info)) {
> +        LOGD(ERROR, domid, "static_shm is not supported by this domain 
> type.");
> +        ret = ERROR_INVAL;
> +        goto error_out;
> +    }
> +
> +    ret = libxl__sshm_check_overlap(gc, domid,
> +                                    d_config->sshms, d_config->num_sshms);
> +    if (ret) goto error_out;

I think it makes sense to call libxl__sshm_check_overlap only if
num_sshms != 0.


> +    for (i = 0; i < d_config->num_sshms; ++i) {
> +        ret = libxl__sshm_setdefault(gc, domid, &d_config->sshms[i]);
> +        if (ret) {
> +            LOGD(ERROR, domid, "Unable to set defaults for static sshm");
> +            goto error_out;
> +        }
> +    }
> +
>      ret = libxl__domain_make(gc, d_config, &domid, &state->config);
>      if (ret) {
>          LOGD(ERROR, domid, "cannot make domain: %d", ret);
> diff --git a/tools/libxl/libxl_internal.h b/tools/libxl/libxl_internal.h
> index 724750967c..74bc0acb21 100644
> --- a/tools/libxl/libxl_internal.h
> +++ b/tools/libxl/libxl_internal.h
> @@ -721,6 +721,7 @@ _hidden char **libxl__xs_directory(libxl__gc *gc, 
> xs_transaction_t t,
>                                     const char *path, unsigned int *nb);
>     /* On error: returns NULL, sets errno (no logging) */
>  _hidden char *libxl__xs_libxl_path(libxl__gc *gc, uint32_t domid);
> +_hidden char *libxl__xs_get_sshmpath(libxl__gc *gc, const char *id);
>  
>  _hidden int libxl__backendpath_parse_domid(libxl__gc *gc, const char 
> *be_path,
>                                             libxl_domid *domid_out);
> @@ -4352,6 +4353,19 @@ static inline bool libxl__acpi_defbool_val(const 
> libxl_domain_build_info *b_info
>  }
>  #endif
>  
> +/*
> + * Set up static shared ram pages for HVM domains to communicate
> + *
> + * This function should only be called after the memory map is constructed
> + * and before any further memory access. */
> +_hidden int libxl__sshm_add(libxl__gc *gc, uint32_t domid,
> +                            libxl_static_shm *sshm, int len);
> +
> +_hidden int libxl__sshm_check_overlap(libxl__gc *gc, uint32_t domid,
> +                                      libxl_static_shm *sshms, int len);
> +_hidden int libxl__sshm_setdefault(libxl__gc *gc, uint32_t domid,
> +                                   libxl_static_shm *sshm);
> +
>  /*
>   * Local variables:
>   * mode: C
> diff --git a/tools/libxl/libxl_sshm.c b/tools/libxl/libxl_sshm.c
> new file mode 100644
> index 0000000000..e16c24ccb9
> --- /dev/null
> +++ b/tools/libxl/libxl_sshm.c
> @@ -0,0 +1,336 @@
> +#include "libxl_osdeps.h"
> +#include "libxl_internal.h"
> +#include "libxl_arch.h"
> +
> +#define SSHM_ERROR(domid, sshmid, f, ...)                               \
> +    LOGD(ERROR, domid, "static_shm id = %s: " f, sshmid, ##__VA_ARGS__)
> +
> +
> +/* Set default values for libxl_static_shm */
> +int libxl__sshm_setdefault(libxl__gc *gc, uint32_t domid,
> +                                  libxl_static_shm *sshm)
> +{
> +    int rc;
> +
> +    if (sshm->role == LIBXL_SSHM_ROLE_UNKNOWN) {
> +        sshm->role = LIBXL_SSHM_ROLE_SLAVE;
> +    }
> +    if (sshm->prot == LIBXL_SSHM_PROT_UNKNOWN) {
> +        sshm->prot = LIBXL_SSHM_PROT_RW;
> +    }
> +    /* role-specific checks */
> +    if (LIBXL_SSHM_ROLE_SLAVE == sshm->role) {
> +        if (sshm->offset == LIBXL_SSHM_RANGE_UNKNOWN) {
> +            sshm->offset = 0;
> +        }
> +        if (sshm->cache_policy != LIBXL_SSHM_CACHEPOLICY_UNKNOWN) {
> +            SSHM_ERROR(domid, sshm->id,
> +                       "cache_policy is only applicable to master domains");
> +            return ERROR_INVAL;
> +        }
> +    } else {
> +        if (sshm->offset != LIBXL_SSHM_RANGE_UNKNOWN) {
> +            SSHM_ERROR(domid, sshm->id,
> +                       "offset is only applicable to slave domains");
> +            return ERROR_INVAL;
> +        }
> +        rc = libxl__arch_domain_sshm_cachepolicy_setdefault(sshm);
> +        if (rc) {
> +            SSHM_ERROR(domid, sshm->id,
> +                       "cache policy not supported on this platform");
> +            return ERROR_INVAL;
> +        }
> +    }
> +
> +    return 0;
> +}
> +
> +/* Compare function for sorting sshm ranges by sshm->begin */
> +static int sshm_range_cmp(const void *a, const void *b)
> +{
> +    libxl_static_shm *const *sshma = a, *const *sshmb = b;
> +    return (*sshma)->begin > (*sshmb)->begin ? 1 : -1;
> +}
> +
> +/* check if the sshm slave configs in @sshm overlap */
> +int libxl__sshm_check_overlap(libxl__gc *gc, uint32_t domid,
> +                                     libxl_static_shm *sshms, int len)
> +{
> +
> +    const libxl_static_shm **slave_sshms = NULL;
> +    int num_slaves;
> +    int i;
> +
> +    slave_sshms = libxl__calloc(gc, len, sizeof(slave_sshms[0]));
> +    num_slaves = 0;
> +    for (i = 0; i < len; ++i) {
> +        if (LIBXL_SSHM_ROLE_SLAVE == sshms[i].role)
> +            slave_sshms[num_slaves++] = sshms + i;
> +    }
> +    qsort(slave_sshms, num_slaves, sizeof(slave_sshms[0]), sshm_range_cmp);
> +
> +    for (i = 0; i < num_slaves - 1; ++i) {
> +        if (slave_sshms[i+1]->begin < slave_sshms[i]->end) {
> +            SSHM_ERROR(domid, slave_sshms[i+1]->id, "slave ranges overlap.");
> +            return ERROR_INVAL;
> +        }
> +    }
> +
> +    return 0;
> +}
> +
> +/* The caller have to guarentee that sshm->begin < sshm->end */
> +static int libxl__sshm_do_map(libxl__gc *gc, uint32_t mid, uint32_t sid,
> +                              libxl_static_shm *sshm,
> +                              uint64_t mbegin, uint64_t mend)
> +{
> +    int rc;
> +    int i;
> +    unsigned int num_mpages, num_spages, offset;
> +    int *errs;
> +    xen_ulong_t *idxs;
> +    xen_pfn_t *gpfns;
> +
> +    num_mpages = (mend - mbegin) >> 12;

Please don't hardcode 12, please use XC_PAGE_SHIFT (also in other libxl
patches).


> +    num_spages = (sshm->end - sshm->begin) >> 12;
> +    offset = sshm->offset >> 12;
> +
> +    /* Check range. Test offset < mpages first to avoid overflow */
> +    if ((offset >= num_mpages) || (num_mpages - offset < num_spages)) {
> +        SSHM_ERROR(sid, sshm->id, "exceeds master's address space.");
> +        rc = ERROR_INVAL;
> +        goto out;
> +    }
> +
> +    /* fill out the pfn's and do the mapping */
> +    errs = libxl__calloc(gc, num_spages, sizeof(int));
> +    idxs = libxl__calloc(gc, num_spages, sizeof(xen_ulong_t));
> +    gpfns = libxl__calloc(gc, num_spages, sizeof(xen_pfn_t));
> +    for (i = 0; i < num_spages; i++) {
> +        idxs[i] = (mbegin >> 12) + offset + i;
> +        gpfns[i]= (sshm->begin >> 12) + i;
> +    }
> +    rc = xc_domain_add_to_physmap_batch(CTX->xch,
> +                                        sid, mid,
> +                                        XENMAPSPACE_gmfn_foreign,
> +                                        num_spages,
> +                                        idxs, gpfns, errs);
> +
> +    for (i = 0; i< num_spages; i++) {
> +        if (errs[i]) {
> +            SSHM_ERROR(sid, sshm->id,
> +                       "can't map at address 0x%"PRIx64".",
> +                       sshm->begin + (offset << 12) );
> +            rc = ERROR_FAIL;
> +        }
> +    }
> +    if (rc) goto out;
> +
> +    rc = 0;
> +
> +out:
> +    return rc;
> +}
> +
> +static int libxl__sshm_add_slave(libxl__gc *gc, uint32_t domid,
> +                                 libxl_static_shm *sshm)
> +{
> +    int rc;
> +    char *sshm_path, *slave_path, *dom_path, *dom_sshm_path, *dom_role_path;
> +    char *ents[9];
> +    const char *xs_value;
> +    libxl_static_shm master_sshm;
> +    uint32_t master_domid;
> +    xs_transaction_t xt = XBT_NULL;
> +
> +    sshm_path = libxl__xs_get_sshmpath(gc, sshm->id);
> +    slave_path = GCSPRINTF("%s/slaves/%"PRIu32, sshm_path, domid);
> +    dom_path = libxl__xs_get_dompath(gc, domid);
> +    /* the domain should be in xenstore by now */
> +    assert(dom_path);
> +    dom_sshm_path = GCSPRINTF("%s/static_shm/%s", dom_path, sshm->id);
> +    dom_role_path = GCSPRINTF("%s/role", dom_sshm_path);
> +
> +    /* prepare the slave xenstore entries */
> +    ents[0] = "begin";
> +    ents[1] = GCSPRINTF("0x%"PRIx64, sshm->begin);
> +    ents[2] = "end";
> +    ents[3] = GCSPRINTF("0x%"PRIx64, sshm->end);
> +    ents[4] = "offset";
> +    ents[5] = GCSPRINTF("0x%"PRIx64, sshm->offset);
> +    ents[6] = "prot";
> +    ents[7] = libxl__strdup(gc, libxl_sshm_prot_to_string(sshm->prot));
> +    ents[8] = NULL;
> +
> +    for (;;) {
> +        rc = libxl__xs_transaction_start(gc, &xt);
> +        if (rc) goto out;
> +
> +        if (!libxl__xs_read(gc, xt, sshm_path)) {
> +            SSHM_ERROR(domid, sshm->id, "no master found.");
> +            rc = ERROR_FAIL;
> +            goto out;
> +        }
> +
> +        /* every ID can appear in each domain at most once */
> +        if (libxl__xs_read(gc, xt, dom_sshm_path)) {
> +            SSHM_ERROR(domid, sshm->id,
> +                       "domain tried to map the same ID twice.");
> +            rc = ERROR_FAIL;
> +            goto out;
> +        }
> +
> +        /* look at the master info and see if we could do the mapping */
> +        rc = libxl__xs_read_checked(gc, xt,
> +                                    GCSPRINTF("%s/prot", sshm_path),
> +                                    &xs_value);
> +        if (rc) goto out;
> +        libxl_sshm_prot_from_string(xs_value, &master_sshm.prot);
> +
> +        rc = libxl__xs_read_checked(gc, xt,
> +                                    GCSPRINTF("%s/begin", sshm_path),
> +                                    &xs_value);
> +        if (rc) goto out;
> +        master_sshm.begin = strtoull(xs_value, NULL, 16);
> +
> +        rc = libxl__xs_read_checked(gc, xt,
> +                                    GCSPRINTF("%s/end", sshm_path),
> +                                    &xs_value);
> +        if (rc) goto out;
> +        master_sshm.end = strtoull(xs_value, NULL, 16);
> +
> +        rc = libxl__xs_read_checked(gc, xt,
> +                                    GCSPRINTF("%s/master", sshm_path),
> +                                    &xs_value);
> +        if (rc) goto out;
> +        master_domid = strtoull(xs_value, NULL, 16);
> +
> +        /* check if the slave is asking too much permission */

this comment doesn't seem to match the check below


> +        if (LIBXL_SSHM_PROT_UNKNOWN == sshm->prot) {
> +            sshm->prot = master_sshm.prot;
> +        }
> +        if (master_sshm.prot < sshm->prot) {
> +            SSHM_ERROR(domid, sshm->id, "slave is asking too much 
> permission.");
> +            rc = ERROR_INVAL;
> +            goto out;
> +        }
> +
> +        /* all checks passed, do the job */
> +        rc = libxl__sshm_do_map(gc, master_domid, domid, sshm,
> +                                master_sshm.begin, master_sshm.end);

Doesn't libxl__sshm_do_map risk being called twice on xenstore
transaction retries?



> +        if (rc) {
> +            rc = ERROR_INVAL;
> +            goto out;
> +        }
> +
> +        /* write the result to xenstore and commit */
> +        rc = libxl__xs_write_checked(gc, xt, dom_role_path, "slave");
> +        if (rc) goto out;
> +        libxl__xs_writev(gc, xt, slave_path, ents);
> +
> +        rc = libxl__xs_transaction_commit(gc, &xt);
> +        if (!rc) break;
> +        if (rc < 0) goto out;
> +    }
> +
> +    rc = 0;
> +out:
> +    libxl__xs_transaction_abort(gc, &xt);
> +    return rc;
> +}
> +
> +static int libxl__sshm_add_master(libxl__gc *gc, uint32_t domid,
> +                                  libxl_static_shm *sshm)
> +{
> +    int rc;
> +    char *sshm_path, *dom_path, *dom_role_path;
> +    char *ents[13];
> +    struct xs_permissions noperm;
> +    xs_transaction_t xt = XBT_NULL;
> +
> +    sshm_path = libxl__xs_get_sshmpath(gc, sshm->id);
> +    dom_path = libxl__xs_get_dompath(gc, domid);
> +    /* the domain should be in xenstore by now */
> +    assert(dom_path);
> +    dom_role_path = GCSPRINTF("%s/static_shm/%s/role", dom_path, sshm->id);
> +
> +    /* prepare the xenstore entries */
> +    ents[0] = "master";
> +    ents[1] = GCSPRINTF("%"PRIu32, domid);
> +    ents[2] = "begin";
> +    ents[3] = GCSPRINTF("0x%"PRIx64, sshm->begin);
> +    ents[4] = "end";
> +    ents[5] = GCSPRINTF("0x%"PRIx64, sshm->end);
> +    ents[6] = "prot";
> +    ents[7] = libxl__strdup(gc, libxl_sshm_prot_to_string(sshm->prot));
> +    ents[8] = "cache_policy";
> +    ents[9] = libxl__strdup(gc,
> +                            
> libxl_sshm_cachepolicy_to_string(sshm->cache_policy));
> +    ents[10] = "status";
> +    ents[11] = "alive";
> +    ents[12] = NULL;
> +
> +    /* could only be accessed by Dom0 */
> +    noperm.id = 0;
> +    noperm.perms = XS_PERM_NONE;
> +
> +    for (;;) {
> +        rc = libxl__xs_transaction_start(gc, &xt);
> +        if (rc) goto out;
> +
> +        if (!libxl__xs_read(gc, xt, sshm_path)) {
> +            /* every ID can appear in each domain at most once */
> +            if (libxl__xs_read(gc, xt, dom_role_path)) {
> +                SSHM_ERROR(domid, sshm->id,
> +                           "domain tried to map the same ID twice.");
> +                rc = ERROR_FAIL;
> +                goto out;
> +            }
> +            rc = libxl__xs_write_checked(gc, xt, dom_role_path, "master");
> +            if (rc) goto out;;
> +
> +            libxl__xs_mknod(gc, xt, sshm_path, &noperm, 1);
> +            libxl__xs_writev(gc, xt, sshm_path, ents);
> +        } else {
> +            SSHM_ERROR(domid, sshm->id, "can only have one master.");
> +            rc = ERROR_FAIL;
> +            goto out;
> +        }
> +
> +        rc = libxl__xs_transaction_commit(gc, &xt);
> +        if (!rc) break;
> +        if (rc < 0) goto out;
> +    }
> +
> +    rc = 0;
> +out:
> +    libxl__xs_transaction_abort(gc, &xt);
> +    return rc;
> +}
> +
> +int libxl__sshm_add(libxl__gc *gc,  uint32_t domid,
> +                    libxl_static_shm *sshms, int len)
> +{
> +    int rc, i;
> +
> +    if (!len) return 0;
> +
> +    for (i = 0; i < len; ++i) {
> +        if (LIBXL_SSHM_ROLE_SLAVE == sshms[i].role) {
> +            rc = libxl__sshm_add_slave(gc, domid, sshms+i);
> +        } else {
> +            rc = libxl__sshm_add_master(gc, domid, sshms+i);
> +        }
> +        if (rc)  return rc;
> +    }
> +
> +    return 0;
> +}
> +
> +/*
> + * Local variables:
> + * mode: C
> + * c-basic-offset: 4
> + * indent-tabs-mode: nil
> + * End:
> + */
> diff --git a/tools/libxl/libxl_x86.c b/tools/libxl/libxl_x86.c
> index 455f6f0bed..8dd34fd7ba 100644
> --- a/tools/libxl/libxl_x86.c
> +++ b/tools/libxl/libxl_x86.c
> @@ -587,6 +587,24 @@ void libxl__arch_domain_build_info_acpi_setdefault(
>      libxl_defbool_setdefault(&b_info->acpi, true);
>  }
>  
> +bool libxl__arch_domain_support_sshm(const libxl_domain_build_info *b_info)
> +{
> +    /* FIXME: Mark this as unsupported since calling p2m_add_foreign on two
> +     * DomU's is currently not allowd on x86, see the comments in 
> +     * x86/mm/p2m.c: p2m_add_foreign */
> +     return false;
> +}
> +
> +int libxl__arch_domain_sshm_cachepolicy_setdefault(libxl_static_shm *sshm)
> +{
> +    if (sshm->cache_policy == LIBXL_SSHM_CACHEPOLICY_UNKNOWN)
> +        sshm->cache_policy = LIBXL_SSHM_CACHEPOLICY_X86_NORMAL;
> +    if (sshm->cache_policy < LIBXL_SSHM_CACHEPOLICY_X86_NORMAL)
> +        return ERROR_INVAL;
> +
> +    return 0;
> +}
> +
>  /*
>   * Local variables:
>   * mode: C
> diff --git a/tools/libxl/libxl_xshelp.c b/tools/libxl/libxl_xshelp.c
> index c4a18df353..d91fbf5fda 100644
> --- a/tools/libxl/libxl_xshelp.c
> +++ b/tools/libxl/libxl_xshelp.c
> @@ -193,6 +193,14 @@ char *libxl__xs_libxl_path(libxl__gc *gc, uint32_t domid)
>      return s;
>  }
>  
> +char *libxl__xs_get_sshmpath(libxl__gc *gc, const char *id)
> +{
> +    char *s = GCSPRINTF("/local/static_shm/%s", id);
> +    if (!s)
> +        LOGE(ERROR, "cannot allocate static shm path");
> +    return s;
> +}
> +
>  int libxl__xs_read_mandatory(libxl__gc *gc, xs_transaction_t t,
>                               const char *path, const char **result_out)
>  {

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