[Xen-changelog] [qemu-xen master] migration/savevm.c: set MAX_VM_CMD_PACKAGED_SIZE to 1ul << 32

[patchbot@xxxxxxx]

commit 2095c5a2e38a51c6777640396bb471dde3948c27
Author:     Daniel Henrique Barboza <danielhb@xxxxxxxxxxxxxxxxxx>
AuthorDate: Fri Jan 26 13:59:40 2018 -0200
Commit:     Michael Roth <mdroth@xxxxxxxxxxxxxxxxxx>
CommitDate: Sun Feb 11 21:06:01 2018 -0600

    migration/savevm.c: set MAX_VM_CMD_PACKAGED_SIZE to 1ul << 32
    
    MAX_VM_CMD_PACKAGED_SIZE is a constant used in qemu_savevm_send_packaged
    and loadvm_handle_cmd_packaged to determine whether a package is too
    big to be sent or received. qemu_savevm_send_packaged is called inside
    postcopy_start (migration/migration.c) to send the MigrationState
    in a single blob to the destination, using the MIG_CMD_PACKAGED subcommand,
    which will read it up using loadvm_handle_cmd_packaged. If the blob is
    larger than MAX_VM_CMD_PACKAGED_SIZE, an error is thrown and the postcopy
    migration is aborted. Both MAX_VM_CMD_PACKAGED_SIZE and MIG_CMD_PACKAGED
    were introduced by commit 11cf1d984b ("MIG_CMD_PACKAGED: Send a packaged
    chunk ..."). The constant has its original value of 1ul << 24 (16MB).
    
    The current MAX_VM_CMD_PACKAGED_SIZE value is not enough to support postcopy
    migration of bigger pseries guests. The blob size for a postcopy migration 
of
    a pseries guest with the following setup:
    
    qemu-system-ppc64 --nographic -vga none -machine pseries,accel=kvm -m 64G \
    -smp 1,maxcpus=32 -device virtio-blk-pci,drive=rootdisk \
    -drive file=f27.qcow2,if=none,cache=none,format=qcow2,id=rootdisk \
    -netdev user,id=u1 -net nic,netdev=u1
    
    Goes around 12MB. Bumping the RAM to 128G makes the blob sizes goes to 20MB.
    With 256G the blob goes to 37MB - more than twice the current maximum size.
    At this moment the pseries machine can handle guests with up to 1TB of RAM,
    making this postcopy blob goes to 128MB of size approximately.
    
    Following the discussions made in [1], there is a need to understand what
    devices are aggressively consuming the blob in that manner and see if that
    can be mitigated. Until then, we can set MAX_VM_CMD_PACKAGED_SIZE to the
    maximum value allowed. Since the size is a 32 bit int variable, we can set
    it as 1ul << 32, giving a maximum blob size of 4G that is enough to support
    postcopy migration of 32TB RAM guests given the above constraints.
    
    [1] https://lists.nongnu.org/archive/html/qemu-devel/2018-01/msg06313.html
    
    Signed-off-by: Daniel Henrique Barboza <danielhb@xxxxxxxxxxxxxxxxxx>
    Reported-by: Balamuruhan S <bala24@xxxxxxxxxxxxxxxxxx>
    Reviewed-by: Juan Quintela <quintela@xxxxxxxxxx>
    Signed-off-by: Juan Quintela <quintela@xxxxxxxxxx>
    Signed-off-by: Dr. David Alan Gilbert <dgilbert@xxxxxxxxxx>
    (cherry picked from commit ee555cdf4d495ddd83633406e3099c5d1ef22e0a)
    Signed-off-by: Michael Roth <mdroth@xxxxxxxxxxxxxxxxxx>
---
 migration/savevm.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/migration/savevm.c b/migration/savevm.c
index 4d6c794..b024ee3 100644
--- a/migration/savevm.c
+++ b/migration/savevm.c
@@ -81,7 +81,7 @@ enum qemu_vm_cmd {
     MIG_CMD_MAX
 };
 
-#define MAX_VM_CMD_PACKAGED_SIZE (1ul << 24)
+#define MAX_VM_CMD_PACKAGED_SIZE UINT32_MAX
 static struct mig_cmd_args {
     ssize_t     len; /* -1 = variable */
     const char *name;
--
generated by git-patchbot for /home/xen/git/qemu-xen.git#master

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