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Re: [Xen-devel] [PATCH v6 2/4] x86/hvm: Treat non-instruction fetch nested page faults also as read violations
- To: "Boris Ostrovsky" <boris.ostrovsky@xxxxxxxxxx>
- From: "Jan Beulich" <JBeulich@xxxxxxxx>
- Date: Fri, 15 Aug 2014 16:01:22 +0100
- Cc: Kevin Tian <kevin.tian@xxxxxxxxx>, "ian.campbell@xxxxxxxxxx" <ian.campbell@xxxxxxxxxx>, "stefano.stabellini@xxxxxxxxxxxxx" <stefano.stabellini@xxxxxxxxxxxxx>, Jun Nakajima <jun.nakajima@xxxxxxxxx>, Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, "ian.jackson@xxxxxxxxxxxxx" <ian.jackson@xxxxxxxxxxxxx>, "xen-devel@xxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxx>, Eddie Dong <eddie.dong@xxxxxxxxx>, "Aravind.Gopalakrishnan@xxxxxxx" <Aravind.Gopalakrishnan@xxxxxxx>, "suravee.suthikulpanit@xxxxxxx" <suravee.suthikulpanit@xxxxxxx>, Tamas Lengyel <tamas.lengyel@xxxxxxxxxxxx>
- Delivery-date: Fri, 15 Aug 2014 15:01:29 +0000
- List-id: Xen developer discussion <xen-devel.lists.xen.org>
>>> 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.
Jan
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