Re: [Xen-devel] [PATCH] shared_info size and padding fixes

[John Levon <levon@xxxxxxxxxxxxxxxxx>]

On Thu, Aug 10, 2006 at 02:29:25PM +0100, Keir Fraser wrote:

> Your patch looked fine, by the way, except that adding padding to every
> arch-specific portion of shared info seems unnecessary.

On the presumption that silence means no, here's a patch without the
padding. I sincerely hope whoever next changes these structures
remembers the padding!

regards
john

# HG changeset patch
# User levon@xxxxxxxxxxxxxxxxx
# Date 1155298435 -3600
# Node ID 43cc94ddfa1c31b1c0d288344ec3147fd61180e5
# Parent  b60ea69932b1a4d10c3aae945a1ce1aa160c689b
Add a clear warning that shared_info_t can change in size even in
compatible ABI revisions. Fix a few places relying on its size.

Signed-off-by: John Levon <john.levon@xxxxxxx>

diff -r b60ea69932b1 -r 43cc94ddfa1c tools/libxc/xc_hvm_build.c
--- a/tools/libxc/xc_hvm_build.c        Wed Aug 09 18:04:20 2006 +0100
+++ b/tools/libxc/xc_hvm_build.c        Fri Aug 11 13:13:55 2006 +0100
@@ -304,7 +304,7 @@ static int setup_guest(int xc_handle,
               xc_handle, dom, PAGE_SIZE, PROT_READ | PROT_WRITE,
               shared_info_frame)) == 0 )
         goto error_out;
-    memset(shared_info, 0, sizeof(shared_info_t));
+    memset(shared_info, 0, PAGE_SIZE);
     /* Mask all upcalls... */
     for ( i = 0; i < MAX_VIRT_CPUS; i++ )
         shared_info->vcpu_info[i].evtchn_upcall_mask = 1;
diff -r b60ea69932b1 -r 43cc94ddfa1c tools/libxc/xc_linux_build.c
--- a/tools/libxc/xc_linux_build.c      Wed Aug 09 18:04:20 2006 +0100
+++ b/tools/libxc/xc_linux_build.c      Fri Aug 11 13:13:55 2006 +0100
@@ -593,7 +593,7 @@ static int setup_guest(int xc_handle,
         xc_handle, dom, PAGE_SIZE, PROT_READ|PROT_WRITE, shared_info_frame);
     printf("shared_info = %p, err=%s frame=%lx\n",
            shared_info, strerror (errno), shared_info_frame);
-    //memset(shared_info, 0, sizeof(shared_info_t));
+    //memset(shared_info, 0, PAGE_SIZE);
     /* Mask all upcalls... */
     for ( i = 0; i < MAX_VIRT_CPUS; i++ )
         shared_info->vcpu_info[i].evtchn_upcall_mask = 1;
@@ -1064,7 +1064,7 @@ static int setup_guest(int xc_handle,
     /* shared_info page starts its life empty. */
     shared_info = xc_map_foreign_range(
         xc_handle, dom, PAGE_SIZE, PROT_READ|PROT_WRITE, shared_info_frame);
-    memset(shared_info, 0, sizeof(shared_info_t));
+    memset(shared_info, 0, PAGE_SIZE);
     /* Mask all upcalls... */
     for ( i = 0; i < MAX_VIRT_CPUS; i++ )
         shared_info->vcpu_info[i].evtchn_upcall_mask = 1;
diff -r b60ea69932b1 -r 43cc94ddfa1c tools/libxc/xc_linux_restore.c
--- a/tools/libxc/xc_linux_restore.c    Wed Aug 09 18:04:20 2006 +0100
+++ b/tools/libxc/xc_linux_restore.c    Fri Aug 11 13:13:55 2006 +0100
@@ -737,7 +737,7 @@ int xc_linux_restore(int xc_handle, int 
     /* Copy saved contents of shared-info page. No checking needed. */
     page = xc_map_foreign_range(
         xc_handle, dom, PAGE_SIZE, PROT_WRITE, shared_info_frame);
-    memcpy(page, shared_info, sizeof(shared_info_t));
+    memcpy(page, shared_info, PAGE_SIZE);
     munmap(page, PAGE_SIZE);
 
     /* Uncanonicalise the pfn-to-mfn table frame-number list. */
diff -r b60ea69932b1 -r 43cc94ddfa1c xen/include/public/xen.h
--- a/xen/include/public/xen.h  Wed Aug 09 18:04:20 2006 +0100
+++ b/xen/include/public/xen.h  Fri Aug 11 13:13:55 2006 +0100
@@ -376,7 +376,11 @@ typedef struct vcpu_info vcpu_info_t;
 
 /*
  * Xen/kernel shared data -- pointer provided in start_info.
- * NB. We expect that this struct is smaller than a page.
+ *
+ * This structure is defined to be both smaller than a page, and the
+ * only data on the page, but may vary in actual size even within
+ * compatible Xen versions; domains should never rely on the actual
+ * size of this structure.
  */
 struct shared_info {
     struct vcpu_info vcpu_info[MAX_VIRT_CPUS];

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