linux-kernel - Re: [PATCH 1/2] KVM: fix cache stale memslot info with correct mmio generation number

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Date:	Tue, 19 Aug 2014 11:03:10 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>,
	David Matlack <dmatlack@...gle.com>
CC:	Gleb Natapov <gleb@...nel.org>, Avi Kivity <avi.kivity@...il.com>,
	mtosatti@...hat.com, linux-kernel@...r.kernel.org,
	kvm@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH 1/2] KVM: fix cache stale memslot info with correct mmio
 generation number

Il 19/08/2014 10:50, Xiao Guangrong ha scritto:
> Okay, what confused me it that it seems that the single line patch
> is ok to you. :)

No, it was late and I was confused. :)

> Now, do we really need to care the case 2? like David said:
> "Sorry I didn't explain myself very well: Since we can get a single wrong
> mmio exit no matter what, it has to be handled in userspace. So my point
> was, it doesn't really help to fix that one very specific way that it can
> happen, because it can just happen in other ways. (E.g. update memslots
> occurs after is_noslot_pfn() and before mmio exit)."
> 
> What's your idea?
> 
> > I think if you always treat the low bit as zero in mmio sptes, you can 
> > do that without losing a bit of the generation.
> 
> What's you did is avoiding cache a invalid generation number into spte, but
> actually if we can figure it out when we check mmio access, it's ok. Like the
> updated patch i posted should fix it, that way avoids doubly increase the number.

Yes.

> Okay, if you're interested increasing the number doubly, there is the simpler
> one:

This wastes a bit in the mmio spte though.  My idea is to increase the
memslots generation twice, but drop the low bit in the mmio spte.

Paolo
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