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

[Paolo Bonzini]

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).

> 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)
        new     new     works (choose your favorite workaround)

Paolo