Re: [PATCH v3 for-4.15] x86/msr: introduce an option for compatible MSR behavior selection

[Jan Beulich <jbeulich@xxxxxxxx>]

On 10.03.2021 12:13, Roger Pau Monné wrote:
> On Tue, Mar 09, 2021 at 07:57:42PM +0000, Andrew Cooper wrote:
>> On 09/03/2021 11:36, Jan Beulich wrote:
>>> So what are your thoughts wrt my change to this file? Drop it
>>> altogether and require people to use this new option? Or do you
>>> see both coexist? In the latter case, since you had suggested
>>> that I drop the write side of my change - does your changing of
>>> the write path indicate you've changed your mind?
>>
>> I don't think we should legitimise buggy PV behaviour, either by
>> codifying in the ABI, or providing a knob beyond this one.
> 
> In that case - can we try to figure out which MSRs is PV Linux trying
> to access without having the #GP handler setup?
> 
> Maybe we can try to handle the ones we know the buggy Linux versions
> will try to access without having #GP setup?
> 
> I know it's not possible to test all possible Linux versions, but we
> could at least try to get the ones we know about fixed. Is the only
> offending one we know about MSR_K8_HWCR?

No, that was the secondary observation. The crash was when reading
MSR_K8_TSEG_ADDR. See my patch'es description.

Handling this on an MSR-by-MSR basis is what Andrew suggested when
I first posted that patch. I continue to not agree, because even if
we limited our auditing to just a single Linux version, how could
we be sure to catch all cases (including rarely taken paths)? IOW I
think auditing isn't a workable approach, and waiting for bug
reports isn't one either.

Jan