Re: [PATCH v4 21/23] multifd: Zero pages transmission

[Juan Quintela <quintela@redhat.com>]

"Dr. David Alan Gilbert" <dgilbert@redhat.com> wrote:
> * Juan Quintela (quintela@redhat.com) wrote:
>> "Dr. David Alan Gilbert" <dgilbert@redhat.com> wrote:
>> > * Juan Quintela (quintela@redhat.com) wrote:
>> >> This implements the zero page dection and handling.
>> >> 
>> >> Signed-off-by: Juan Quintela <quintela@redhat.com>
>> >> 
>> >> ---
>> >> 
>> >> Add comment for offset (dave)
>> >>              }
>> >>          }
>> >>  
>> >> +        for (int i = 0; i < p->zero_num; i++) {
>> >> +            memset(p->host + p->zero[i], 0, qemu_target_page_size());
>> >> +        }
>> >> +
>> >
>> > On the existing code, it tries to avoid doing the memset if the target
>> > page size matches; that avoids allocating the zero pages on the
>> > destination host; should we try and do the same here?
>> >
>> > Dave
>> 
>> Hi Dave
>> 
>> That only happens on postcopy.
>> With precopy we have to do the memset, because we can have:
>> 
>> write non zero to page 50
>> migrate page 50
>> write zeros to page 50
>> Another migration pass
>> If we don't write here, we have garbage on the page.
>> 
>> Or I am missing something?
>
> You're missing the call to buffer_is_zero:
>
> void ram_handle_compressed(void *host, uint8_t ch, uint64_t size)
> {
>     if (ch != 0 || !buffer_is_zero(host, size)) {
>         memset(host, ch, size);
>     }
> }

Aha, I didn't understood you the 1st time.

Thanks, will add that.

Later, Juan.