Re: [Qemu-devel] [RFC PATCH qemu] qapi: Add query-memory-checksum

[Alexey Kardashevskiy <aik@ozlabs.ru>]

On 22/08/2019 11:33, Eric Blake wrote:
> On 8/21/19 8:16 PM, Alexey Kardashevskiy wrote:
>> This returns MD5 checksum of all RAM blocks for migration debugging
>> as this is way faster than saving the entire RAM to a file and checking
>> that.
>>
>> Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
>> ---
>>
>>
>> I am actually wondering if there is an easier way of getting these
>> checksums and I just do not see it, it cannot be that we fixed all
>> memory migration bugs :)
> 
> I'm not sure whether the command itself makes sense, but for the interface:
> 
> 
>> +++ b/qapi/misc.json
>> @@ -1194,6 +1194,33 @@
>>   ##
>>   { 'command': 'query-memory-size-summary', 'returns': 'MemoryInfo' }
>>   
>> +##
>> +# @MemoryChecksum:
>> +#
>> +# A string with MD5 checksum of all RAMBlocks.
>> +#
>> +# @checksum: the checksum.
>> +#
>> +# Since: 3.2.0
> 
> This should be 4.2, not 3.2.
> 
>> +##
>> +{ 'struct': 'MemoryChecksum',
>> +  'data'  : { 'checksum': 'str' } }
>> +
>> +##
>> +# @query-memory-checksum:
>> +#
>> +# Return the MD5 checksum of all RAMBlocks.
>> +#
>> +# Example:
>> +#
>> +# -> { "execute": "query-memory-checksum" }
>> +# <- { "return": { "checksum": "a0880304994f64cb2edad77b9a1cd58f" } }
>> +#
>> +# Since: 3.2.0
> 
> and again
> 
>> +##
>> +{ 'command': 'query-memory-checksum',
>> +  'returns': 'MemoryChecksum' }
>> +
>>   
> 
>> +++ b/exec.c
>> @@ -2050,6 +2050,22 @@ void *qemu_ram_get_host_addr(RAMBlock *rb)
>>       return rb->host;
>>   }
>>   
>> +gchar *qemu_ram_chksum(void)
> 
> gchar is a pointless glib type.  Use 'char' instead.
> 
>> +{
>> +    struct RAMBlock *rb;
>> +    GChecksum *chksum = g_checksum_new(G_CHECKSUM_MD5);
>> +    gchar *ret;
>> +
>> +    RAMBLOCK_FOREACH(rb) {
>> +        g_checksum_update(chksum, qemu_ram_get_host_addr(rb),
>> +                          qemu_ram_get_used_length(rb));
>> +    }
>> +    ret = g_strdup(g_checksum_get_string(chksum));
>> +    g_checksum_free(chksum);
>> +
>> +    return ret;
>> +}
> 
> How long does this take to run?  Is it something where you really want
> to block the guest while chewing over the guest's entire memory?


10-20 times faster than "pmemsave" and blocking the guest is not a 
problem here as both - source and destination - guests are stopped 
(otherwise the checksum does not make sense).



-- 
Alexey