Re: [Xen-devel] [PATCH v2] x86/vpmu_intel: Fix hypervisor crash by masking PC bit in MSR_P6_EVNTSEL

["Tian, Kevin" <kevin.tian@xxxxxxxxx>]

> From: Boris Ostrovsky [mailto:boris.ostrovsky@xxxxxxxxxx]
> Sent: Thursday, April 27, 2017 11:18 PM
> 
> On 04/27/2017 11:05 AM, Jan Beulich wrote:
> >>>> On 27.04.17 at 16:57, <boris.ostrovsky@xxxxxxxxxx> wrote:
> >> On 04/27/2017 03:32 AM, Jan Beulich wrote:
> >>>>>> On 26.04.17 at 20:50, <mohit.gambhir@xxxxxxxxxx> wrote:
> >>>> On 04/26/2017 02:19 PM, Andrew Cooper wrote:
> >>>>> On 26/04/17 19:11, Mohit Gambhir wrote:
> >>>>>> Setting Pin Control (PC) bit (19) in MSR_P6_EVNTSEL results in a
> General
> >>>>>> Protection Fault and thus results in a hypervisor crash. This patch
> fixes
> >> the
> >>>>>> crash by masking PC bit and returning an error in case any guest tries
> to
> >> write
> >>>>>> to it.
> >>>>>>
> >>>>>> Signed-off-by: Mohit Gambhir <mohit.gambhir@xxxxxxxxxx>
> >>>>> Out of interest, which hardware has this been observed on?
> >>>> I have tested this on two Intel Broadwell machines.
> >>> Since by now all we have are indications that this is an erratum,
> >>> this information belongs into the commit message. As it is written
> >>> now, it means the bit can't be set on any hardware. If there are
> >>> reasons beyond this erratum to uniformly disallow the bit to be
> >>> set by guests, these should be named here too. After all the
> >>> way you do the change, you now refuse values with the bit set
> >>> everywhere.
> >> I don't think this is specific to Broadwell. I tried this on a Haswell
> >> (model 60) and got a #GPF as well.
> >>
> >> If I understand what this bit does, it is pretty pointless in a guest.
> >> It is only useful in some sort of embedded setup, where something is
> >> hooked up to a particular pin on the board. So perhaps this is not an
> >> erratum but rather a not fully documented feature, where if nothing is
> >> connected to that pin this bit should not be set.
> >>
> >> Or maybe it is documented but I can't find anything on that.
> > Kevin, Jun?

I asked internally but didn't get a clarification yet.

> >
> >> Either way, we should mask this bit.
> > Sure, but: Refuse attempts to set it, or silently ignore such?
> 
> I think the former, especially if what I surmised above is correct ---
> the user shouldn't try to set it.
> 

I'm with the former too.

Thanks
Kevin

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