Re: [Qemu-devel] [PATCH for-4.1] q35: acpi: do not create dummy MCFG table

[Wei Yang]

On Thu, Apr 11, 2019 at 01:46:27PM +0200, Igor Mammedov wrote:
>On Thu, 11 Apr 2019 09:32:11 +0800
>Wei Yang <address@hidden> wrote:
>

[...]

>> To be honest, this is what I feel confused in your previous reply.
>> 
>> First I want to confirm both fields in RAMBlock affects the migration:
>> 
>> * used_length
>> * max_length
>> 
>> Both of them should be the same on both source/destination, otherwise the
>> migration would fail.
>well, it works fine for me.
>Where do you see max_length being used during migration?
>

I asked my colleague, but seems you are right.

>
>> Then I thought the migration would be broken if source/destination has
>> different knowledge about acpi table size. Because this will introduce
>> different value of used_length, even we have resizable MemoryRegion.
>> 
>> The 1st time ACPI generation flow:
>> 
>>     acpi_add_rom_blob
>>         rom_add_blob
>>             rom_set_mr
>>                 memory_region_init_resizable_ram
>>                     qemu_ram_alloc_resizable
>>                      new_block->used_length = size
>>                      new_block->max_length = max_size
>> 
>> The 2nd time ACPI generation flow:
>> 
>>     acpi_ram_update
>>         memory_regioin_ram_resize
>>             qemu_ram_resize
>>                 block->used_length = new_size
>> 
>> The max_length is always the same, while used_length would be changed to the
>> actual table_blob size.
>> 
>> In case source/destination has different knowledge about acpi table size, the
>> table_blob size(even after aligned) could be different.
>> 
>> This is why I thought there is still some chance to break migration after
>> resizable MemoryRegion.
>> 
>> Do I miss something?
>yes, you did, max_length does not influence migration stream.
>see what above mentioned commits and ram_load() -> "if (length != 
>block->used_length)" do.
>

I see this in the code.

So the destination will check length and adjust self's length if it
doesn't equal to the source.

Thanks :-)

-- 
Wei Yang
Help you, Help me