Re: migration: broken snapshot saves appear on s390 when small fields in migration stream removed

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Claudio Fontana (cfontana@suse.de) wrote:
> Hello all,
> 
> during unrelated work for splitting QTest from the TCG instruction counting module,
> 
> I encountered what seems to be a migration stream issue, which is apparent only on s390, and only shows in block test 267.
> 
> ./check -qcow2 267
> 
> when it comes to snapshot save and load using backing file.
> 
> Here is a minimal reproducer patch that causes the issue on s390 only.
> 
> --------------------------------------------cut-------------------------------------------
> diff --git a/cpus.c b/cpus.c
> index 41d1c5099f..443b88697a 100644
> --- a/cpus.c
> +++ b/cpus.c
> @@ -643,7 +643,7 @@ static void qemu_account_warp_timer(void)
>  
>  static bool icount_state_needed(void *opaque)
>  {
> -    return use_icount;
> +    return 0;
>  }

That's weird; I mean that's just turning a subsection on and off;
so you'd hope if this is a test that generates it's own snapshot and
then uses it then it should be consistent.

>  static bool warp_timer_state_needed(void *opaque)
> --------------------------------------------cut-------------------------------------------
> 
> config.status configure line:
> exec '/home/cfontana/qemu-build/../qemu/configure' '--enable-tcg' '--disable-kvm' '--disable-hax' '--target-list=s390x-softmmu' '--enable-debug'
> 
> $ make check-block
> 
> TEST    iotest-qcow2: 267 [fail]
> QEMU          -- "/home/cfontana/qemu-build/tests/qemu-iotests/../../s390x-softmmu/qemu-system-s390x" -nodefaults -display none -accel qtest
> QEMU_IMG      -- "/home/cfontana/qemu-build/tests/qemu-iotests/../../qemu-img" 
> QEMU_IO       -- "/home/cfontana/qemu-build/tests/qemu-iotests/../../qemu-io"  --cache writeback --aio threads -f qcow2
> QEMU_NBD      -- "/home/cfontana/qemu-build/tests/qemu-iotests/../../qemu-nbd" 
> IMGFMT        -- qcow2 (compat=1.1)
> IMGPROTO      -- file
> PLATFORM      -- Linux/s390x s390zp13 5.3.18-21-default
> TEST_DIR      -- /home/cfontana/qemu-build/tests/qemu-iotests/scratch
> SOCK_DIR      -- /tmp/tmp.bLJcJVtzk5
> SOCKET_SCM_HELPER -- /home/cfontana/qemu-build/tests/qemu-iotests/socket_scm_helper
> 
> --- /home/cfontana/qemu/tests/qemu-iotests/267.out	2020-07-12 05:10:07.948262675 -0400
> +++ /home/cfontana/qemu-build/tests/qemu-iotests/267.out.bad	2020-07-12 05:27:03.358362781 -0400
> @@ -137,6 +137,9 @@
>  ID        TAG                 VM SIZE                DATE       VM CLOCK
>  --        snap0                  SIZE yyyy-mm-dd hh:mm:ss   00:00:00.000
>  (qemu) loadvm snap0
> +Unexpected storage key flag data: 0
> +error while loading state for instance 0x0 of device 's390-skeys'
> +Error: Error -22 while loading VM state
>  (qemu) quit
>  
>  
> -----------
> 
> 
> Not run: 172 186 192 259 287
> Failures: 267
> Failed 1 of 115 iotests
> make: *** [/home/cfontana/qemu/tests/Makefile.include:880: check-tests/check-block.sh] Error 1
> 
> -----------
> 
> Note: only the === -blockdev with a backing file === part of test 267 fails. -blockdev with NBD is ok, like all the rest.
> 
> 
> Interesting facts about s390 in particular: its save/load code includes the transfer of "storage keys",
> which include a buffer of 32768 bytes of keydata in the stream.
> 
> The code (hw/s390x/s390-skeys.c),
> is modeled similarly to RAM transfer (like in migration/ram.c), with an EOS (end of stream) marker.
> 
> Countrary to RAM transfer code though, after qemu_put_be64(f, EOS), the s390 code does not qemu_fflush(f).
> 
> ----------
> Observation: the migration/qemu-file.c shows an IO_BUF_SIZE of 32768.
> 
> --
> 
> The following workarounds hide the problem (make the test pass):
> 
> 1) always including the icount field in the (unrelated) timers field that are sent before in the migration stream (ie not applying the reproducer patch).
> 
> 2) increasing the IO_BUF_SIZE also hides the problem:
> 
> ----------------------cut--------------------------
> diff --git a/migration/qemu-file.c b/migration/qemu-file.c
> index be21518c57..f81d1272eb 100644
> --- a/migration/qemu-file.c
> +++ b/migration/qemu-file.c
> @@ -30,7 +30,7 @@
>  #include "trace.h"
>  #include "qapi/error.h"
>  
> -#define IO_BUF_SIZE 32768
> +#define IO_BUF_SIZE 65536
>  #define MAX_IOV_SIZE MIN_CONST(IOV_MAX, 64)
>  
>  struct QEMUFile {
> ----------------------cut--------------------------
> 
> 3) adding a qemu_fflush in hw/s390x/s390-skeys.c after EOS also "fixes" the problem:
> 
> ----------------------cut--------------------------
> diff --git a/hw/s390x/s390-skeys.c b/hw/s390x/s390-skeys.c
> index 1e036cc602..47c9a015af 100644
> --- a/hw/s390x/s390-skeys.c
> +++ b/hw/s390x/s390-skeys.c
> @@ -252,6 +252,8 @@ static const TypeInfo qemu_s390_skeys_info = {
>      .class_size    = sizeof(S390SKeysClass),
>  };
>  
> +extern void qemu_fflush(QEMUFile *f);
> +
>  static void s390_storage_keys_save(QEMUFile *f, void *opaque)
>  {
>      S390SKeysState *ss = S390_SKEYS(opaque);
> @@ -302,6 +304,7 @@ static void s390_storage_keys_save(QEMUFile *f, void *opaque)
>      g_free(buf);
>  end_stream:
>      qemu_put_be64(f, eos);
> +    qemu_fflush(f);
>  }
>  
>  static int s390_storage_keys_load(QEMUFile *f, void *opaque, int version_id)
> ----------------------cut--------------------------
> 
> Do any of you with better understanding of migration/, block and s390 have a suggestion on what could be the issue here,
> and what could be the next step in the investigation?
> 
> Is the fact that migration/ram.c always does fflush after writing the EOS have any relevance here? why does it do it,
> and should s390 code also follow the same pattern?

I didn't think it was required.
And qemu_put_buffer loops if needed and calls qemu_fflush internally.
It's possible here that the storage key code is just the canary - the
first thing that detects that the stream is invalid after it all goes
wrong.

Dave


> Thanks,
> 
> Claudio
> 
> 
> -- 
> Claudio Fontana
> Engineering Manager Virtualization, SUSE Labs Core
> 
> SUSE Software Solutions Italy Srl
> 
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK