linux-kernel - Re: [PATCH 0/7] KVM: Alleviate mmu_lock hold time when we start dirty logging

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Message-ID: <20121220125946.GB7750@amt.cnet>
Date:	Thu, 20 Dec 2012 10:59:46 -0200
From:	Marcelo Tosatti <mtosatti@...hat.com>
To:	Takuya Yoshikawa <yoshikawa_takuya_b1@....ntt.co.jp>
Cc:	Alex Williamson <alex.williamson@...hat.com>,
	Takuya Yoshikawa <takuya.yoshikawa@...il.com>, gleb@...hat.com,
	kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/7] KVM: Alleviate mmu_lock hold time when we start
 dirty logging

On Thu, Dec 20, 2012 at 02:02:32PM +0900, Takuya Yoshikawa wrote:
> On Wed, 19 Dec 2012 08:42:57 -0700
> Alex Williamson <alex.williamson@...hat.com> wrote:
> 
> > Please let me know if you can identify one of these as the culprit.
> > They're all very simple, but there's always a chance I've missed a hard
> > coding of slot numbers somewhere.  Thanks,
> 
> I identified the one:
>   commit b7f69c555ca430129b6cde81e9f0927531420c5c
>   KVM: Minor memory slot optimization
> 
> IIUC, the problem was that you did not care about the generation of
> slots which was updated by update_memslots():
> 
>   Your patch reused the old memory slots which was there before
>   doing the update for invalidating the slot, and badly, we did flush
>   shadow pages after that before doing the second update for finally
>   installing the new slot.  As a result, the generation did not change
>   from that of the invalidated one, although the ghc(gfn to hva cache)
>   might be stale.
> 
>   After that, kvm_write_guest_cached() checked if ghc should be
>   initialized by comparing ghc's generation with that old one,
>   resulting mark_page_dirty_in_slot() was called with the invalid
>   cache contents.
> 
> Although we can do something to correct the generation alone, I do not
> think such a trick is worth it because this is not a hot path.  Let's
> just revert the patch.

Agreed. No dependencies by the following patches on it?

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