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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> 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:
>> >>  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...

Right, but as said - that's the more safe of the two alternatives.

Jan


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