On Tue, Jan 23, 2007 at 11:38:29PM -0000, Ian Pratt wrote:
> We should make xc_linux_save cope more gracefully with
> arch.pfn_to_mfn_frame_list_list being NULL, have xc_linux_save zero the
> field in shared_info before writing it out (its an MFN and not valid
> anyhow), and then move the field's re-initialisation a little later in
> the post_suspend function to close the race.
I've done some testing with the below, and it seems to do the trick.
regards
john
# HG changeset patch
# User john.levon@xxxxxxx
# Date 1169600022 28800
# Node ID 3121e0cb7be68fa46c59f61422f05adc03ec5a5e
# Parent a75413c0072fa5b892bd8b6a05c1f1d3435bb093
Close save-after-restore race. Make xc_linux_save() wait for the
frame_list_list MFN to be updated by the domain before trying to use it.
Signed-off-by: John Levon <john.levon@xxxxxxx>
diff --git a/tools/libxc/xc_linux_save.c b/tools/libxc/xc_linux_save.c
--- a/tools/libxc/xc_linux_save.c
+++ b/tools/libxc/xc_linux_save.c
@@ -403,6 +403,33 @@ static int suspend_and_state(int (*suspe
return -1;
}
+/*
+** Map the top-level page of MFNs from the guest. The guest might not have
+** finished resuming from a previous restore operation, so we wait a while for
+** it to update the MFN to a reasonable value.
+*/
+static void *map_frame_list_list(int xc_handle, uint32_t dom,
+ shared_info_t *shinfo)
+{
+ int count = 3000;
+ void *p;
+
+ while (count-- && shinfo->arch.pfn_to_mfn_frame_list_list == 0)
+ usleep(10000);
+
+ if (shinfo->arch.pfn_to_mfn_frame_list_list == 0) {
+ ERROR("Timed out waiting for frame list updated.");
+ return NULL;
+ }
+
+ p = xc_map_foreign_range(xc_handle, dom, PAGE_SIZE, PROT_READ,
+ shinfo->arch.pfn_to_mfn_frame_list_list);
+
+ if (p == NULL)
+ ERROR("Couldn't map p2m_frame_list_list (errno %d)", errno);
+
+ return p;
+}
/*
** During transfer (or in the state file), all page-table pages must be
@@ -663,14 +690,11 @@ int xc_linux_save(int xc_handle, int io_
max_pfn = live_shinfo->arch.max_pfn;
- live_p2m_frame_list_list =
- xc_map_foreign_range(xc_handle, dom, PAGE_SIZE, PROT_READ,
- live_shinfo->arch.pfn_to_mfn_frame_list_list);
-
- if (!live_p2m_frame_list_list) {
- ERROR("Couldn't map p2m_frame_list_list (errno %d)", errno);
- goto out;
- }
+ live_p2m_frame_list_list = map_frame_list_list(xc_handle, dom,
+ live_shinfo);
+
+ if (!live_p2m_frame_list_list)
+ goto out;
live_p2m_frame_list =
xc_map_foreign_batch(xc_handle, dom, PROT_READ,
@@ -1169,8 +1193,14 @@ int xc_linux_save(int xc_handle, int io_
ctxt.ctrlreg[3] =
xen_pfn_to_cr3(mfn_to_pfn(xen_cr3_to_pfn(ctxt.ctrlreg[3])));
+ /*
+ * Reset the MFN to be a known-invalid value. See map_frame_list_list().
+ */
+ memcpy(page, live_shinfo, PAGE_SIZE);
+ ((shared_info_t *)page)->arch.pfn_to_mfn_frame_list_list = 0;
+
if (!write_exact(io_fd, &ctxt, sizeof(ctxt)) ||
- !write_exact(io_fd, live_shinfo, PAGE_SIZE)) {
+ !write_exact(io_fd, page, PAGE_SIZE)) {
ERROR("Error when writing to state file (1) (errno %d)", errno);
goto out;
}
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