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

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

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?
>
>> 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.

-boris

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxx
https://lists.xen.org/xen-devel