Re: [Xen-devel] [PATCH v2 5/6] libxl: support mapping static shared memory areas during domain creation

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Sun, Aug 27, 2017 at 04:36:14PM +0800, 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.
>   * Set the refcount of the shared region accordingly
> 
> 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.xen.org/archives/html/xen-devel/2017-08/msg03242.html
> 
> Signed-off-by: Zhongze Liu <blackskygg@xxxxxxxxx>
[...]
> +
> +        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);
> +
> +        if (sshm->prot == LIBXL_SSHM_PROT_UNKNOWN) {
> +            sshm->prot = master_sshm.prot;
> +        }
> +        /* check if the slave is asking too much permission */
> +        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 */
> +        if (!isretry) {
> +            rc = libxl__sshm_do_map(gc, master_domid, domid, sshm,
> +                                    master_sshm.begin, master_sshm.end);
> +            if (rc) goto out;

You probably need to roll back the mapping should the transaction gets
aborted in this loop.

> +        }
> +
> +        /* write the result to xenstore and commit */
> +        rc = libxl__xs_write_checked(gc, xt, dom_role_path, "slave");
> +        if (rc) goto out;
> +        rc = libxl__xs_writev(gc, xt, slave_path, ents);
> +        if (rc) goto out;
> +        rc = libxl__sshm_incref(gc, xt, sshm_path);
> +        if (rc) goto out;
> +
> +        rc = libxl__xs_transaction_commit(gc, &xt);
> +        if (!rc) break;
> +        if (rc < 0) goto out;
> +        isretry = true;
> +    }
> +
> +    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)
> +{
> +

No refcount increment here?

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