linux-kernel - Re: [PATCH 2/4] KVM: Avoid checking huge page mappings in get_dirty

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Message-ID: <4F5F1D91.6030706@redhat.com>
Date:	Tue, 13 Mar 2012 12:12:33 +0200
From:	Avi Kivity <avi@...hat.com>
To:	Takuya Yoshikawa <yoshikawa.takuya@....ntt.co.jp>
CC:	mtosatti@...hat.com, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/4] KVM: Avoid checking huge page mappings in get_dirty_log()

On 03/13/2012 11:20 AM, Takuya Yoshikawa wrote:
> Avi Kivity <avi@...hat.com> wrote:
>
> > It occurs to me that we should write-protect huge page tables, since it
> > makes write protection much faster (we make up for this later at write
> > fault time, but that might not occur, and even if it does we reduce
> > guest jitter).  In fact I once proposed a more involved variant, called
>
> Do you mean protecting huge page tables when we start logging and dropping
> them, one by one, at the time of write fault?

Yes.  Even more generally, protecting PML4Es, then at fault time
un-protecting the faulting PML4E and write-protecting all underlying
PDPEs except the one for the faulting address, and similarly for PDEs
and PTEs.

> If so, we do not need to change get_dirty_log() implementation.
> Changing kvm_mmu_remove_write_access() and fault handler seems to be enough.
>
> > O(1) write protection, in which we write-protect the topmost page table
> > only and only un-write-protect the paths that fault.
>
> > That can be done later however and shouldn't affect this patchset.
>
> I have some additional patches to optimize rmap handling which seems to
> improve get_dirty_log() about 10% in the worst case.

Great, looking forward.

> After that, I want to take some time to prepare for further developments
> because my current test tools are not enough now.
>

-- 
error compiling committee.c: too many arguments to function

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