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
