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 08/15/2014 11:01 AM, Jan Beulich wrote:
> > > > On 15.08.14 at 16:31, <boris.ostrovsky@xxxxxxxxxx> wrote:
> > On 08/14/2014 06:59 PM, Jan Beulich wrote:
> > > 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.
> You as the SVM maintainer should know better than me... With
> NPT using "normal" page fault error codes, there is not even an
> indication for read access. Tamas's patches adjust the current
> misbehavior too in that at least instruction fetches no longer get
> reported as reads.
What I am asking is whether

   .read_access = !(pfec & (PFEC_insn_fetch | PFEC_write_access))

would be more appropriate.

-boris




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