linux-kernel - Re: [PATCH] KVM: dirty all pages in kvm_write_guest

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Date:	Wed, 08 Apr 2015 12:43:15 +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: dirty all pages in kvm_write_guest_cached()



On 08/04/2015 11:26, Radim Krčmář wrote:
> 2015-04-08 10:49+0200, Paolo Bonzini:
>> On 07/04/2015 22:34, Radim Krčmář wrote:
>>> We dirtied only one page because writes originally couldn't span more.
>>> Use improved syntax for '>> PAGE_SHIFT' while at it.
>>>
>>> Fixes: 8f964525a121 ("KVM: Allow cross page reads and writes from cached translations.")
>>> Signed-off-by: Radim Krčmář <rkrcmar@...hat.com>
>>
>> Cross-page reads and writes should never get here; they have
>> ghc->memslot set to NULL and go through the slow path in kvm_write_guest.
> 
> 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.

Paolo
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