Re: [Qemu-devel] [PATCH 2/3] migration: Test for disabled features on reception

[Juan Quintela <quintela@redhat.com>]

"Dr. David Alan Gilbert" <dgilbert@redhat.com> wrote:
> * Juan Quintela (quintela@redhat.com) wrote:
>> Right now, if we receive a compressed page or a xbzrle page while this
>> features are disabled, Bad Things (TM) can happen.  Just add a test for
>> them.
>
> This confuses me; I didn't think until recently we could
> guarantee anything about having the capabilities set on the destination
> side.  Until -incoming defer the destination didn't have a way of setting
> capabilities in a known state before starting the reception.

Ouch.

So, here we are after further investigation.

- xbzrle: we don't need anything special on the reception side, we just
          decode inplace.  So we are good.  If we don't use xbzrle, we
          don't waste any resources either.
- compression: We create the decompression threads always, with all its
          associated configuration.


Why do I wanted to do this?  For multifd, I don't really want to create
multiple fd's except if configured, because I wait for the fd's to be
"accepted" before continue, so I need a way to know if it is configured
or no.  So I need something like this.

I started doing this for compression because when I was debugging I had
too many threads waiting on qemu_cond_wait(), and I only wanted to look
at the multifd ones.

I had not througth that we didn't need to set capabilities on
destination.

Suggestions?

Later, Juan.

>
> Dave
>
>> Signed-off-by: Juan Quintela <quintela@redhat.com>
>> ---
>>  migration/ram.c | 23 ++++++++++++++++++++++-
>>  1 file changed, 22 insertions(+), 1 deletion(-)
>> 
>> diff --git a/migration/ram.c b/migration/ram.c
>> index fb9252d..4bb707c 100644
>> --- a/migration/ram.c
>> +++ b/migration/ram.c
>> @@ -2464,7 +2464,7 @@ static int ram_load_postcopy(QEMUFile *f)
>> 
>>  static int ram_load(QEMUFile *f, void *opaque, int version_id)
>>  {
>> -    int flags = 0, ret = 0;
>> +    int flags = 0, ret = 0, invalid_flags;
>>      static uint64_t seq_iter;
>>      int len = 0;
>>      /*
>> @@ -2479,6 +2479,15 @@ static int ram_load(QEMUFile *f, void *opaque, int version_id)
>>          ret = -EINVAL;
>>      }
>> 
>> +    invalid_flags = 0;
>> +
>> +    if (!migrate_use_xbzrle()) {
>> +        invalid_flags |= RAM_SAVE_FLAG_XBZRLE;
>> +    }
>> +
>> +    if (!migrate_use_compression()) {
>> +        invalid_flags |= RAM_SAVE_FLAG_COMPRESS_PAGE;
>> +    }
>>      /* This RCU critical section can be very long running.
>>       * When RCU reclaims in the code start to become numerous,
>>       * it will be necessary to reduce the granularity of this
>> @@ -2499,6 +2508,18 @@ static int ram_load(QEMUFile *f, void *opaque, int version_id)
>>          flags = addr & ~TARGET_PAGE_MASK;
>>          addr &= TARGET_PAGE_MASK;
>> 
>> +        if (flags & invalid_flags) {
>> +            if (flags & invalid_flags  & RAM_SAVE_FLAG_XBZRLE) {
>> +                error_report("Received an unexpected XBRLE page");
>> +            }
>> +            if (flags & invalid_flags  & RAM_SAVE_FLAG_COMPRESS_PAGE) {
>> +                error_report("Received an unexpected compressed page");
>> +            }
>> +
>> +            ret = -EINVAL;
>> +            break;
>> +        }
>> +
>>          if (flags & (RAM_SAVE_FLAG_COMPRESS | RAM_SAVE_FLAG_PAGE |
>>                       RAM_SAVE_FLAG_COMPRESS_PAGE | RAM_SAVE_FLAG_XBZRLE)) {
>>              RAMBlock *block = ram_block_from_stream(f, flags);
>> -- 
>> 2.7.4
>> 
> --
> Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK