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Re: [Xen-devel] [PATCH v6 2/4] x86/hvm: Treat non-instruction fetch nested page faults also as read violations

> From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
> Sent: Friday, August 15, 2014 7:58 AM
> >>> On 15.08.14 at 01:40, <kevin.tian@xxxxxxxxx> wrote:
> >>  From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
> >> The intended use is that pending patch I referred to, aiming at
> >> reducing the number of GLA->GPA translations we're doing during
> >> instruction emulation, which makes a severe difference to many-
> >> vCPU guests e.g. heavily using the HPET from all their vCPU-s.
> >
> > Is attached patch what you're talking here?
> Yes, that one.

Though I understand the merit of your overall patch, the discussion
around r/w violations are not directly related to the optimization
your patch brings (the real reason is from available GLA info at
VM-exit). It' just because you code the patch that way, so 
__hvmemul_read purely assumes NPFEC_read_access if not is
NPFEC_insn_fetch, and then read-modify-write can't enter the
fast path w/o faking read.

If my understanding is correct, why not just do whatever tricks
here in nestedhvm_hap_nested_page_fault, while keeping arch
specific code unchanged?


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