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 4:00 PM
> 
> >>> On 15.08.14 at 00:34, <kevin.tian@xxxxxxxxx> wrote:
> >>  From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
> >> Sent: Thursday, August 14, 2014 1:40 PM
> >>
> >> >>> On 14.08.14 at 18:49, <andrew.cooper3@xxxxxxxxxx> wrote:
> >> > On 14/08/14 17:43, Tian, Kevin wrote:
> >> >>
> >> >> but doing so just moves from one incomplete solution (where
> >> >> read-modify-write is not treated as read-violation) to another
> >> >> incomplete solution (where all writes are treated read-violation). If
> >> >> there's actual usage relying on accurate read-violation information,
> >> >> either solution doesn't work. So I don't see the value of this change.
> >> >>
> >> >
> >> > I would agree.  Anything using this information will have to have
> >> > detailed knowledge of what the hardware is capable of reporting, to
> >> > understand the information it has to hand.
> >> >
> >> > I think Xen should faithfully pass on what hardware reports.  It will be
> >> > more useful to the consumer than blurring the details like this.
> >>
> >> Not if it's unreliable. Plus on x86 elsewhere write access implies
> >> read access anyway. If you look at the draft patch I had sent
> >> Tamas (which I intend to rebase on his series), you'll see that
> >> there the change here is actually strictly needed.
> >>
> >
> > 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...

Thanks
Kevin

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