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

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Date:	Wed, 27 Jul 2011 09:47:52 +0800
From:	Xiao Guangrong <xiaoguangrong@...fujitsu.com>
To:	Gleb Natapov <gleb@...hat.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 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 ;-)
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.
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