Re: [PATCH] KVM: use slowpath for cross page cached accesses

Paolo Bonzini
2015-04-08T12:23:06Z

On 08/04/2015 14:16, Radim Krčmář wrote:
> 2015-04-08 12:43+0200, Paolo Bonzini:
>> On 08/04/2015 11:26, Radim Krčmář wrote:
>>> Only cross-memslot writes have NULL memslot.
>>
>> The power of wrong comments...
>>
>> Considering how kvm_gfn_to_hva_cache_init is used (one 1-byte field, two
>> 4-byte fields, one 28-bytes struct that is 32-bytes aligned, one
>> 32-bytes field that is in practice cacheline-aligned), I wonder if we
>> should just use ghc->memslot = NULL for cross page writes. This would
>> bypass the bug you are fixing here, and avoid worries about partial writes.
>
> Good idea, and it could make those comments right :)
> (Though in general, I prefer less constraints on APIs ...)

It doesn't put constraints, it still handles cross page writes right
(just slower). copy_to_user in some sense is the API that constrains us
to do this.

> Partial writes would be a pain; copy_to_user API does not define which
> bytes were not written. I think the write can't fail mid-page, which
> makes our implementation ok

No, writes can't fail mid-page (I guess in atomic context it's
theoretically possible, but we're equipped to handle the failure in that
case).

Patch applied, thanks!

Paolo