linux-kernel - Re: [PATCH 03/11] KVM: x86: fast emulate repeat string write instructions

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Date:	Wed, 27 Jul 2011 07:26:02 +0300
From:	Gleb Natapov <gleb@...hat.com>
To:	Xiao Guangrong <xiaoguangrong@...fujitsu.com>
Cc:	Avi Kivity <avi@...hat.com>, Marcelo Tosatti <mtosatti@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>, KVM <kvm@...r.kernel.org>
Subject: Re: [PATCH 03/11] KVM: x86: fast emulate repeat string write
 instructions

On Wed, Jul 27, 2011 at 09:47:52AM +0800, Xiao Guangrong wrote:
> On 07/26/2011 08:27 PM, Gleb Natapov wrote:
> > On Tue, Jul 26, 2011 at 07:26:46PM +0800, Xiao Guangrong wrote:
> >> We usually use repeat string instructions to clear the page, for example,
> > By "we" do you mean Linux guest?
> > 
> 
> I do not know other guests except linux, but, generally rep instruction is
> not used to update a page table which is been using.
> 
> >> we call memset to clear a page table, stosb is used in this function, and 
> >> repeated for 1024 times, that means we should occupy mmu lock for 1024 times
> >> and walking shadow page cache for 1024 times, it is terrible
> >>
> >> In fact, if it is the repeat string instructions emulated and it is not a
> >> IO/MMIO access, we can zap all the corresponding shadow pages and return to the
> >> guest, then the mapping can became writable and directly write the page
> >>
> 
> 
> > So this patch does two independent things as far as I can see. First it
> > stops reentering guest if rep instruction is done on memory and second
> 
> No.
> Oppositely, it enters guest as soon as possible if rep instruction is done
> on memory ;-)
Oops. Indeed. I read it other way around. So why not just return
X86EMUL_UNHANDLEABLE from emulator_write_emulated_onepage() which should
have the same effect?

> After this patch, we only need to emulate one time for a rep instruction.
> 
> > it drops shadow pages if access to shadowed page table is rep. Why not
> > separate those in different patches?  
> 
> Umm, i will zap shadow page firstly and stop emulation rep instruction in
> the second patch.
> 
> > BTW not entering guest periodically
> > increases interrupt latencies.
> 
> Oppositely, It reduces the latencies. :-)
> 
> > Why not zap shadow, make page writable
> > and reenter the guest instead of emulation, it should be much faster (do we
> > care to optimize for old cpus by complicating the code anyway?).
> > 
> 
> We do better lazily update the mapping to writable, it can be done by the later access.

--
			Gleb.
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