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

["Tian, Kevin" <kevin.tian@xxxxxxxxx>]

> From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
> Sent: Thursday, August 14, 2014 4:37 PM
> 
> >>> On 15.08.14 at 01:20, <kevin.tian@xxxxxxxxx> wrote:
> >>  From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
> >> Sent: Thursday, August 14, 2014 4:08 PM
> >>
> >> >>> On 15.08.14 at 01:04, <kevin.tian@xxxxxxxxx> wrote:
> >> >>  From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
> >> >> Sent: Thursday, August 14, 2014 4:00 PM
> >> >>
> >> >> >>> On 15.08.14 at 00:34, <kevin.tian@xxxxxxxxx> wrote:
> >> >> > I think you're mixing the behavior and policy here. from behavior 
> >> >> > p.o.v,
> >> >> > we should keep whatever hardware reports, which describes the
> behavior
> >> >> > of the instruction causing violation whether it's a write operation or
> read
> >> >> > operation. From policy p.o.v, you may treat a write operation as read
> >> >> > operation in specific code paths (if access==read || access ==write).
> >> >>
> >> >> No - the hardware specifically does _not_ guarantee to report the
> >> >> actual characteristics of a read-modify-write instruction. Or at least
> >> >> that's what your documentation warns about. And to be on the safe
> >> >> side, treating all writes as also being reads is the better option than
> >> >> to mistakenly treat r-m-w as just w.
> >> >>
> >> >
> >> > but then you are mistakenly treating all other writes as reads too...
> >>
> >> Right, but as said - that's the more safe of the two alternatives.
> >>
> >
> > to decide which one is more 'safe', could you give some examples of
> > how read violation is used today? If there's an usage very relying on
> > the exact read behavior, treating 'w' as 'r' could lead to significant
> > difference.
> 
> 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?

Thanks,
Kevin

Attachment: EPT-fault-GLA.patch
Description: EPT-fault-GLA.patch

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