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

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

On 08/14/2014 06:59 PM, Jan Beulich wrote:
> > > > 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.
Is this specific to VMX or does SVM have the same problem (I am not 
aware of this but I might be wrong). Because if it doesn't then I think 
Tamas' [PATCH v6 2/4] should have SVM report actual bits.
If, OTOH, you need both return same results for consistency then I 
wonder whether we could move this up the stack into HVM common code.
-boris


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