Re: [PATCH] clflush instruction emulation

[Wei Huang <wei.huang2@amd.com>]

WBINVD is a bit heavy since it flushes all caches. I saw 1170 times of 
CLFLUSH during FreeBSD booting. After guest boots, 0 was observed. 1170 
is not huge though.

-Wei

Keir Fraser wrote:
> On 15/04/2010 17:38, "Wei Huang" <wei.huang2@amd.com> wrote:
>
>   
>> I created a patch which enables clflush emulation. I have verified that
>> this patch solves FreeBSD issue. This patch returns immediately without
>> doing anything. Note that we don't have to intercept clflush in SVM. So
>> the only time we have page_fault for clflush instruction is when guest
>> VM flushes MMIO space. In this case, do we need to send this command
>> over to QEMU? I didn't do it in this patch anyway.
>>     
>
> I don't think we need to anything since all guest reads/writes to the mmio
> space will get intercepted to qemu, and hence qemu's view of the space is
> coherent with the guest with no need for special handling of CLFLUSH.
>
> Still the emulator should call out and let the caller decide what to do. We
> could safely turn CLFLUSH into WBINVD (i.e., call the existing wbinvd hook).
> Do you know if these CLFLUSH emulations happen often or only very rarely?
> That might help us decide how smart we need to be about emulating CLFLUSH
> (i.e., add a hook specifically for clflush, separately from wbinvd).
>
>  -- Keir
>
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