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

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Date:	Thu, 20 Dec 2012 15:22:55 +0200
From:	Gleb Natapov <gleb@...hat.com>
To:	Marcelo Tosatti <mtosatti@...hat.com>
Cc:	Takuya Yoshikawa <yoshikawa_takuya_b1@....ntt.co.jp>,
	Alex Williamson <alex.williamson@...hat.com>,
	Takuya Yoshikawa <takuya.yoshikawa@...il.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 10:59:46AM -0200, Marcelo Tosatti wrote:
> 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?
Heh, this generation management looks subtle. Would be easy to break by
other changes to the code. I wounder can we make it less subtle somehow.

--
			Gleb.
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