Re: [Xen-devel] [PATCH v8 2/4] x86/hvm: Treat non-instruction fetch nested page faults also as read violations

[Tim Deegan <tim@xxxxxxx>]

At 21:40 +0000 on 28 Aug (1409258457), Tian, Kevin wrote:
> > From: Tim Deegan [mailto:tim@xxxxxxx]
> > Sent: Thursday, August 28, 2014 3:14 AM
> > 
> > At 15:40 +0200 on 19 Aug (1408459219), Tamas K Lengyel wrote:
> > > As pointed out by Jan Beulich in
> > http://lists.xen.org/archives/html/xen-devel/2014-08/msg01269.html:
> > "Read-modify-write instructions absolutely need to be treated as read
> > accesses, yet hardware doesn't guarantee to tell us so (they may surface as
> > just write accesses)." This patch addresses the issue in both the VMX and 
> > the
> > SVM side.
> > >
> > > VMX: Treat all write data access violations also as read violations (in
> > addition to those that were already reported as read violations).
> > > SVM: Refine the meaning of read data access violations to distinguish
> > between read/write and instruction fetch access violations.
> > >
> > > With this patch both VMX and SVM specific nested page fault handling code
> > reports violations the same way, thus abstracting the hardware specific
> > behaviour from the layers above.
> > >
> > > Suggested-by: Jan Beulich <JBeulich@xxxxxxxx>
> > > Signed-off-by: Tamas K Lengyel <tamas.lengyel@xxxxxxxxxxxx>
> > > Reviewed-by: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
> > 
> > Reviewed-by: Tim Deegan <tim@xxxxxxx>
> 
> As I commented earlier in another mail, I still think the right way is to
> do the treatment in general place... 

Understood; I guess it's clear that I disagree, but I guess I shuld
elaborate.  AIUI you're suggesting that we should adjust the consumers
of this field to handle the fact that r/m/w operations might be
reported as writes.  But that:
 - duplicates logic at a number of sites;
 - requires new consumers of the field to be aware of this wrinkle in
   the spec; and
 - makes changes to common code that are only needed on Intel hardware.

Whereas this patch just sorts the problem out once and for all, in one
place.  And as far as I can see it has no big disadvantages -- after
all now that we're aware of this behaviour, we have to treat all
writes as r/m/w anyway for safety.

Cheers,

Tim.

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