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

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Date:	Wed, 08 Apr 2015 14:23:06 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Radim Krčmář <rkrcmar@...hat.com>
CC:	linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH] KVM: use slowpath for cross page cached accesses



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
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