Re: [PATCH RFC] memory: Don't allow to resize RAM while migrating

From: David Hildenbrand <david@redhat.com>
To: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
Cc: "Eduardo Habkost" <ehabkost@redhat.com>,
	"Michael S. Tsirkin" <mst@redhat.com>,
	"Juan Quintela" <quintela@redhat.com>,
	"Richard Henderson" <richard.henderson@linaro.org>,
	qemu-devel@nongnu.org,
	"Shameerali Kolothum Thodi"
	<shameerali.kolothum.thodi@huawei.com>,
	"Shannon Zhao" <shannon.zhao@linaro.org>,
	"Igor Mammedov" <imammedo@redhat.com>,
	"Paolo Bonzini" <pbonzini@redhat.com>,
	"Alex Bennée" <alex.bennee@linaro.org>
Subject: Re: [PATCH RFC] memory: Don't allow to resize RAM while migrating
Date: Fri, 14 Feb 2020 11:32:31 +0100	[thread overview]
Message-ID: <30a66b1d-184e-a684-d0d2-c3921366b478@redhat.com> (raw)
In-Reply-To: <20200214102514.GB3283@work-vm>

On 14.02.20 11:25, Dr. David Alan Gilbert wrote:
> * David Hildenbrand (david@redhat.com) wrote:
>> Resizing while migrating is dangerous and does not work as expected.
>> The whole migration code works on the usable_length of ram blocks and does
>> not expect this to change at random points in time.
>>
>> Precopy: The ram block size must not change on the source, after
>> ram_save_setup(), so as long as the guest is still running on the source.
>>
>> Postcopy: The ram block size must not change on the target, after
>> synchronizing the RAM block list (ram_load_precopy()).
>>
>> AFAIKS, resizing can be trigger *after* (but not during) a reset in
>> ACPI code by the guest
>> - hw/arm/virt-acpi-build.c:acpi_ram_update()
>> - hw/i386/acpi-build.c:acpi_ram_update()
>>
>> I see no easy way to work around this. Fail hard instead of failing
>> somewhere in migration code due to strange other reasons. AFAIKs, the
>> rebuilts will be triggered during reboot, so this should not affect
>> running guests, but only guests that reboot at a very bad time and
>> actually require size changes.
>>
>> Let's further limit the impact by checking if an actual resize of the
>> RAM (in number of pages) is required.
>>
>> Don't perform the checks in qemu_ram_resize(), as that's called during
>> migration when syncing the used_length. Update documentation.
> 
> Interesting; we need to do something about this - but banning resets
> during migration is a bit harsh; and aborting the source VM is really
> nasty - for a precopy especially we shouldn't kill the source VM,
> we should just abort the migration.

Any alternative, easy solutions to handle this? I do wonder how often
this will actually trigger in real life.

> 
> The other thing that worries me is that acpi_build_update calls
>    acpi_ram_update->memory_region_ram_resize
> multiple times.

It's different memory regions, no? table_mr, rsdp_mr, linker_mr.

> So, it might be that the size you end up with at the end of
> acpi_build_update is actually the same size as the original - so
> the net effect is the RAMBlock didn't really get resized.

Are you sure?

-- 
Thanks,

David / dhildenb