Messages in this thread | | | Date | Wed, 27 Jul 2011 09:47:52 +0800 | From | Xiao Guangrong <> | Subject | Re: [PATCH 03/11] KVM: x86: fast emulate repeat string write instructions |
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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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