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 11:29 AM, Jan Beulich wrote:
> > > > On 15.08.14 at 17:09, <boris.ostrovsky@xxxxxxxxxx> wrote:
> > 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.
> Ah, no, clearly not: Again - read-modify-write instructions _have_
> to be reported as being reads and writes. Reporting simply writes
> as reads too is the smaller of the two "evils" here. If anything we
> could introduce a "maybe-read" flag that gets set when don't know
> for sure.
I think an explicit comment in VMX and SVM code explaining why the bits 
are set the way they are may be sufficient (I know this is mentioned in 
the commit message but having it in the code is better IMO).
-boris

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