Re: [PATCH 5/6] migration/postcopy: enable random order target page arrival

[Wei Yang <richardw.yang@linux.intel.com>]

On Wed, Nov 06, 2019 at 08:08:28PM +0000, Dr. David Alan Gilbert wrote:
>* Wei Yang (richardw.yang@linux.intel.com) wrote:
>> After using number of target page received to track one host page, we
>> could have the capability to handle random order target page arrival in
>> one host page.
>> 
>> This is a preparation for enabling compress during postcopy.
>> 
>> Signed-off-by: Wei Yang <richardw.yang@linux.intel.com>
>> ---
>>  migration/ram.c | 16 +++-------------
>>  1 file changed, 3 insertions(+), 13 deletions(-)
>> 
>> diff --git a/migration/ram.c b/migration/ram.c
>> index b5759793a9..da0596411c 100644
>> --- a/migration/ram.c
>> +++ b/migration/ram.c
>> @@ -4015,7 +4015,6 @@ static int ram_load_postcopy(QEMUFile *f)
>>      MigrationIncomingState *mis = migration_incoming_get_current();
>>      /* Temporary page that is later 'placed' */
>>      void *postcopy_host_page = mis->postcopy_tmp_page;
>> -    void *last_host = NULL;
>>      bool all_zero = false;
>>      int target_pages = 0;
>>  
>> @@ -4062,24 +4061,15 @@ static int ram_load_postcopy(QEMUFile *f)
>>               * that's moved into place later.
>>               * The migration protocol uses,  possibly smaller, target-pages
>>               * however the source ensures it always sends all the components
>> -             * of a host page in order.
>> +             * of a host page in one chunk.
>>               */
>>              page_buffer = postcopy_host_page +
>>                            ((uintptr_t)host & (block->page_size - 1));
>>              /* If all TP are zero then we can optimise the place */
>>              if (target_pages == 1) {
>>                  all_zero = true;
>> -            } else {
>> -                /* not the 1st TP within the HP */
>> -                if (host != (last_host + TARGET_PAGE_SIZE)) {
>> -                    error_report("Non-sequential target page %p/%p",
>> -                                  host, last_host);
>> -                    ret = -EINVAL;
>> -                    break;
>> -                }
>
>I think this is losing more protection than needed.
>I think you can still protect against a page from a different host-page
>arriving until we've placed the current host-page.
>So something like:
>
>    if (((uintptr_t)host & ~(block->page_size - 1)) !=
>        last_host)
>

OK, looks reasonable.

>and then set last_host to the start of the host page.
>

I think it is not necessary to update the last_host on each target page. We
can just set it at the first target page.

>Then you'll check if that flush is really working.
>
>Dave
>
>>              }
>>  
>> -
>>              /*
>>               * If it's the last part of a host page then we place the host
>>               * page
>> @@ -4090,7 +4080,6 @@ static int ram_load_postcopy(QEMUFile *f)
>>              }
>>              place_source = postcopy_host_page;
>>          }
>> -        last_host = host;
>>  
>>          switch (flags & ~RAM_SAVE_FLAG_CONTINUE) {
>>          case RAM_SAVE_FLAG_ZERO:
>> @@ -4143,7 +4132,8 @@ static int ram_load_postcopy(QEMUFile *f)
>>  
>>          if (!ret && place_needed) {
>>              /* This gets called at the last target page in the host page */
>> -            void *place_dest = host + TARGET_PAGE_SIZE - block->page_size;
>> +            void *place_dest = (void *)QEMU_ALIGN_DOWN((unsigned long)host,
>> +                                                       block->page_size);
>>  
>>              if (all_zero) {
>>                  ret = postcopy_place_page_zero(mis, place_dest,
>> -- 
>> 2.17.1
>> 
>--
>Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK

-- 
Wei Yang
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