[Xen-devel] [PATCH LINUX RFC] xen: privcmd: implement IOCTL_PRIVCMD_MMAPBATCH_V2_UNCACHED

[Ian Campbell <ian.campbell@xxxxxxxxxx>]

On ARM we want to use uncached foreign mappings while building the domain
because the guests start with MMU and caches disabled.

Flushing the caches before launching the guest is problematic because there is
a window between flush and unmap where the processor might speculatively fill
a cache line. Using a non-cacheable mapping completely avoids this.

Signed-off-by: Ian Campbell <ian.campbell@xxxxxxxxxx>
Cc: Stefano.Stabellini@xxxxxxxxxx>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
Cc: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
Cc: David Vrabel <david.vrabel@xxxxxxxxxx>
Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx
---
 drivers/xen/privcmd.c      |   15 ++++++++++-----
 include/uapi/xen/privcmd.h |    2 ++
 2 files changed, 12 insertions(+), 5 deletions(-)

diff --git a/drivers/xen/privcmd.c b/drivers/xen/privcmd.c
index 569a13b..b5561d1 100644
--- a/drivers/xen/privcmd.c
+++ b/drivers/xen/privcmd.c
@@ -253,6 +253,7 @@ struct mmap_batch_state {
        domid_t domain;
        unsigned long va;
        struct vm_area_struct *vma;
+       pgprot_t prot;
        int index;
        /* A tristate:
         *      0 for no errors
@@ -285,8 +286,7 @@ static int mmap_batch_fn(void *data, void *state)
                cur_page = pages[st->index++];
 
        ret = xen_remap_domain_mfn_range(st->vma, st->va & PAGE_MASK, *mfnp, 1,
-                                        st->vma->vm_page_prot, st->domain,
-                                        &cur_page);
+                                        st->prot, st->domain, &cur_page);
 
        /* Store error code for second pass. */
        if (st->version == 1) {
@@ -367,7 +367,7 @@ static int alloc_empty_pages(struct vm_area_struct *vma, 
int numpgs)
 
 static struct vm_operations_struct privcmd_vm_ops;
 
-static long privcmd_ioctl_mmap_batch(void __user *udata, int version)
+static long privcmd_ioctl_mmap_batch(void __user *udata, int version, int 
cached)
 {
        int ret;
        struct privcmd_mmapbatch_v2 m;
@@ -464,6 +464,8 @@ static long privcmd_ioctl_mmap_batch(void __user *udata, 
int version)
 
        state.domain        = m.dom;
        state.vma           = vma;
+       state.prot          = cached ? vma->vm_page_prot
+                                    : pgprot_noncached(vma->vm_page_prot);
        state.va            = m.addr;
        state.index         = 0;
        state.global_error  = 0;
@@ -514,13 +516,16 @@ static long privcmd_ioctl(struct file *file,
                break;
 
        case IOCTL_PRIVCMD_MMAPBATCH:
-               ret = privcmd_ioctl_mmap_batch(udata, 1);
+               ret = privcmd_ioctl_mmap_batch(udata, 1, 1);
                break;
 
        case IOCTL_PRIVCMD_MMAPBATCH_V2:
-               ret = privcmd_ioctl_mmap_batch(udata, 2);
+               ret = privcmd_ioctl_mmap_batch(udata, 2, 1);
                break;
 
+       case IOCTL_PRIVCMD_MMAPBATCH_V2_UNCACHED:
+               ret = privcmd_ioctl_mmap_batch(udata, 2, 0);
+               break;
        default:
                ret = -EINVAL;
                break;
diff --git a/include/uapi/xen/privcmd.h b/include/uapi/xen/privcmd.h
index a853168..be7e72b 100644
--- a/include/uapi/xen/privcmd.h
+++ b/include/uapi/xen/privcmd.h
@@ -94,5 +94,7 @@ struct privcmd_mmapbatch_v2 {
        _IOC(_IOC_NONE, 'P', 3, sizeof(struct privcmd_mmapbatch))
 #define IOCTL_PRIVCMD_MMAPBATCH_V2                             \
        _IOC(_IOC_NONE, 'P', 4, sizeof(struct privcmd_mmapbatch_v2))
+#define IOCTL_PRIVCMD_MMAPBATCH_V2_UNCACHED                    \
+       _IOC(_IOC_NONE, 'P', 5, sizeof(struct privcmd_mmapbatch_v2))
 
 #endif /* __LINUX_PUBLIC_PRIVCMD_H__ */
-- 
1.7.10.4


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