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

[Wei Yang]

On Wed, Apr 10, 2019 at 05:01:50PM +0200, Igor Mammedov wrote:
>On Wed, 10 Apr 2019 22:27:56 +0800
>Wei Yang <address@hidden> wrote:
>
>[...]
>> >@@ -2411,19 +2410,7 @@ build_mcfg_q35(GArray *table_data, BIOSLinker 
>> >*linker, AcpiMcfgInfo *info)
>> >     mcfg->allocation[0].start_bus_number = 0;
>> >     mcfg->allocation[0].end_bus_number = PCIE_MMCFG_BUS(info->mcfg_size - 
>> > 1);
>> > 
>> >-    /* MCFG is used for ECAM which can be enabled or disabled by guest.  
>> 
>> I want to cnfirm what is "enabled or disabled by guest" here.
>
>Firmware theoretically during PCI initialization may disable ECAM support
>and that's when we do no need MCFG. In practice that's not happening
>(SeaBIOS or UEFI) but we in case there is out there a firmware that does
>disable ECAM we do not generate MCFG.
>
>Note:
>ACPI tables generated twice, 1st when QEMU starts and the second time
>when firmware accesses fwcfg to read blobs for the 1st time.
>The later happens after PCI subsystem was initialized by firmware.
>At that time we know if ECAM was enabled or not.
>

That's much clear, thanks :-)

So this is the guest BIOS instead of guest kernel who may disable/enable it.

>> If we don't reserve mcfg and "guest" enable mcfg during running, the ACPI
>> table size changed. But the destination still has the original table size,
>> since destination "guest" keep sleep during this period.
>> 
>> Now the migration would face table size difference
>
>with commit a1666142db we do not care as all the tables created on
>source will be migrated to destination as is overwriting whatever blobs
>destination created on startup.
>
>> and break migration?
>nope,
>
>to help you figure out why it works
>look at what following git commits did:
>  git log c8d6f66ae7..a1666142db
>and pay attention to 'used_length'
>

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.

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?

-- 
Wei Yang
Help you, Help me