Re: [Qemu-devel] [PATCH] pc: memhotplug: rise minimum DIMM addr/size alignment to 128Mb

[Paolo Bonzini]

On 21/09/2015 15:32, Igor Mammedov wrote:
> On Mon, 21 Sep 2015 15:13:17 +0200
> Paolo Bonzini <address@hidden> wrote:
> 
>>
>>
>> On 21/09/2015 15:05, Igor Mammedov wrote:
>>>>> To some extend, enforcing natural alignment would be okay as a
>>>>> workaround for the virtio bug as well.  It would also make it easier to
>>>>> ensure that hotplugged hugetlbfs-backed memory can use hugepages in the
>>>>> guest.  Does it make sense to you?
>>> in current machine types we already enforce backend-s address/size 
>>> alignment,
>>> which is file's page size for hugetlbfs-backed memory and 2Mb for RAM 
>>> backend.
>>
>> Right, but it's not enough if the guest's physical address is not
>> aligned to 2Mb/1Gb too.  This is why we changed i440FX and q35 to have
>> only 3 and 2 gigabytes of low memory (down from 3.5 and 2 IIRC).
> 
> DIMM's GPA is aligned to backend's alignment since 2.2,
> it should be aligned 2Mb/1Gb depending on what hugetlbfs file is used
> so that already works as expected

Oh, ok.  This is what pcms->enforce_aligned_dimm does when true, and
this is also what I was missing.  Great!

>>> So I guess we could try to apply workaround to virtio on guest side,
>>> aligning and limiting max buffer size to 2Mb, it should work for 'old'
>>> machine types as well.
>>
>> That would make sense and it would be complementary to natural alignment
>> of DIMMs in the host.  This would give:
>>
>>      host    guest
>>      old     old     fails
>>      old     new     works (virtio workaround)
>>      new     old     works (natural alignment)
>             + not sure if it would work,
>               I've though that virtio refactoring, that drops requirement
>               for buffer to be inside of only one MemoryRegion, would 
>               touch virtio in QEMU and on guest side too.

Right, it would theoretically not be enough.  However, I think it would
work in practice because hot-plugged DIMMs should be bigger than 128 MiB.

Paolo