[Qemu-devel] [0/2] Migration bugfixes that were forgotten for 1.3

[Qemu-devel] [0/2] Migration bugfixes that were forgotten for 1.3

[David Gibson]

I've sent these before, and reminded several times about the first one
at least, but they still slipped through the cracks and didn't make it
into 1.3.  The first at least should go into the stable tree - it is a
very simple bugfix for a serious bug (qemu and possibly also guest
memory corruption).

[Qemu-devel] [PATCH 1/2] Fix off-by-1 error in RAM migration code

[David Gibson]

The code for migrating (or savevm-ing) memory pages starts off by creating
a dirty bitmap and filling it with 1s.  Except, actually, because bit
addresses are 0-based it fills every bit except bit 0 with 1s and puts an
extra 1 beyond the end of the bitmap, potentially corrupting unrelated
memory.  Oops.  This patch fixes it.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
---
 arch_init.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch_init.c b/arch_init.c
index e6effe8..b75a4c5 100644
--- a/arch_init.c
+++ b/arch_init.c
@@ -568,7 +568,7 @@ static int ram_save_setup(QEMUFile *f, void *opaque)
     int64_t ram_pages = last_ram_offset() >> TARGET_PAGE_BITS;
 
     migration_bitmap = bitmap_new(ram_pages);
-    bitmap_set(migration_bitmap, 1, ram_pages);
+    bitmap_set(migration_bitmap, 0, ram_pages);
     migration_dirty_pages = ram_pages;
 
     bytes_transferred = 0;
-- 
1.7.10.4

[Qemu-devel] [PATCH 2/2] migration: Fix madvise breakage if host and guest have different page sizes

[David Gibson]

madvise(DONTNEED) will throw away the contents of the whole page at the
given address, even if the given length is less than the page size.  One
can argue about whether that's the correct behaviour, but that's what it's
done for a long time in Linux at least.

That means that the madvise() in ram_load(), on a setup where
TARGET_PAGE_SIZE is smaller than the host page size, can throw away data
in guest pages adjacent to the one it's actually processing right now,
leading to guest memory corruption on an incoming migration.

This patch therefore, disables the madvise() if the host page size is
larger than TARGET_PAGE_SIZE.  This means we don't get the benefits of that
madvise() in this case, but a more complete fix is more difficult to
accomplish.  This at least fixes the guest memory corruption.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Reported-by: Alexey Kardashevskiy <aik@ozlabs.ru>
---
 arch_init.c |    3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch_init.c b/arch_init.c
index b75a4c5..83dcc53 100644
--- a/arch_init.c
+++ b/arch_init.c
@@ -840,7 +840,8 @@ static int ram_load(QEMUFile *f, void *opaque, int version_id)
             memset(host, ch, TARGET_PAGE_SIZE);
 #ifndef _WIN32
             if (ch == 0 &&
-                (!kvm_enabled() || kvm_has_sync_mmu())) {
+                (!kvm_enabled() || kvm_has_sync_mmu()) &&
+                getpagesize() <= TARGET_PAGE_SIZE) {
                 qemu_madvise(host, TARGET_PAGE_SIZE, QEMU_MADV_DONTNEED);
             }
 #endif
-- 
1.7.10.4

Re: [Qemu-devel] [0/2] Migration bugfixes that were forgotten for 1.3

[Anthony Liguori]

David Gibson <david@gibson.dropbear.id.au> writes:

> I've sent these before, and reminded several times about the first one
> at least, but they still slipped through the cracks and didn't make it
> into 1.3.  The first at least should go into the stable tree - it is a
> very simple bugfix for a serious bug (qemu and possibly also guest
> memory corruption).

Juan,

Are you going to pick these up or do you want me to?

Regards,

Anthony Liguori

Re: [Qemu-devel] [0/2] Migration bugfixes that were forgotten for 1.3

[Juan Quintela]

Anthony Liguori <aliguori@us.ibm.com> wrote:
> David Gibson <david@gibson.dropbear.id.au> writes:
>
>> I've sent these before, and reminded several times about the first one
>> at least, but they still slipped through the cracks and didn't make it
>> into 1.3.  The first at least should go into the stable tree - it is a
>> very simple bugfix for a serious bug (qemu and possibly also guest
>> memory corruption).
>
> Juan,
>
> Are you going to pick these up or do you want me to?

Picking them.

Thanks, Juan.