Re: [Qemu-devel] HEAD is failing virt-test on migration tests

[Alexander Graf <agraf@suse.de>]

On 16.02.15 19:57, Dr. David Alan Gilbert wrote:
> * Alexander Graf (agraf@suse.de) wrote:
> 
> <snip>
> 
>> Can you please test whether the patch below makes things work for you again?
> 
> The patch below fixes RDMA migration (same host); however, see comments.
> 
>> Alex
>>
>> From ef6fde21007e62529799264f57a65c6bb3d0d414 Mon Sep 17 00:00:00 2001
>> From: Alexander Graf <agraf@suse.de>
>> Date: Sat, 14 Feb 2015 00:21:01 +0100
>> Subject: [PATCH] migration: Read JSON VM description on incoming migration
>>
>> One of the really nice things about the VM description format is that it
>> goes
>> over the wire when live migration is happening. Unfortunately QEMU today
>> closes
>> any socket once it sees VM_EOF coming, so we never give the VMDESC the
>> chance to
>> actually land on the wire.
>>
>> This patch makes QEMU read the description as well. This way we ensure that
>> anything wire tapping us in between will get the chance to also
>> interpret the
>> stream.
>>
>> Along the way we also fix virt tests that assume that number_bytes_sent
>> on the
>> sender side is equal to number_bytes_read which was true before the VMDESC
>> patches and is true again with this patch.
>>
>> Signed-off-by: Alexander Graf <agraf@suse.de>
>>
>> diff --git a/savevm.c b/savevm.c
>> index 8040766..ff4bead 100644
>> --- a/savevm.c
>> +++ b/savevm.c
>> @@ -929,6 +929,7 @@ int qemu_loadvm_state(QEMUFile *f)
>>      uint8_t section_type;
>>      unsigned int v;
>>      int ret;
>> +    int file_error_after_eof = -1;
>>
>>      if (qemu_savevm_state_blocked(&local_err)) {
>>          error_report("%s", error_get_pretty(local_err));
>> @@ -1034,6 +1035,22 @@ int qemu_loadvm_state(QEMUFile *f)
>>          }
>>      }
>>
>> +    file_error_after_eof = qemu_file_get_error(f);
>> +
>> +    /*
>> +     * Try to read in the VMDESC section as well, so that dumping tools
>> that
>> +     * intercept our migration stream have the chance to see it.
>> +     */
>> +    if (qemu_get_byte(f) == QEMU_VM_VMDESCRIPTION) {
> 
> You could use qemu_peek_byte for that?

It's what I had originally, but qemu_peek_byte() at the end of the day
is the exact same as qemu_get_byte, but doesn't increment the internal
buffer counter. So any error conditions that incur because the read
failed still happen with peek_byte and are a lot less intuitive.

> 
>> +        uint32_t size = qemu_get_be32(f);
>> +        uint8_t *buf = g_malloc(size);
>> +
>> +        if (buf) {
>> +            qemu_get_buffer(f, buf, size);
>> +            g_free(buf);
>> +        }
> 
> This is slightly dangerous; a malformed file could send you a huge
> value and get you to allocate lots of memory for no good reason.
> 
> You could do some clever; but personally I'd just loop around a
> nice small buffer until it's gone.

Good idea. Will change.

> As mentioned on IRC; I'm still worried though that this is only
> a fix for loading on newer versions; migration to an older QEMU
> with the same machine type would fail.
> (Yes I know mythically that no one cares about this; but I do).

Yeah, I guess I'll follow up with a fix to disable VMDESC submission on
older versions, just to be on the safe side.


Alex