Re: [Xen-devel] [PATCH] x86: ignore guest microcode loading attempts

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 13.03.18 at 11:36, <andrew.cooper3@xxxxxxxxxx> wrote:
> On 13/03/2018 10:13, Jan Beulich wrote:
>> The respective MSRs are write-only, and hence attempts by guests to
>> write to these are - as of 1f1d183d49 ("x86/HVM: don't give the wrong
>> impression of WRMSR succeeding") no longer ignored. Restore original
>> behavior for the two affected MSRs.
>>
>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
>> ---
>> While what is being logged for the current osstest failure on the 4.7
>> branch (I have to admit that I don't understand why it's only that
>> branch which shows a regression)
> 
> Differences in advertised viridian?
> 
>>  doesn't fully prove this to be the
>> problem, RCX holding 0x79 and there being a recorded hypervisor level
>> #GP recovery immediately before the guest triple fault is sufficient
>> indication imo.
>> What I'm unsure about is whether we want to ignore such writes also for
>> PV guests. If not, at least the WRMSR change would need to move into
>> hvm/hvm.c.
> 
> Hmm - I'd very much like not to make this change, because it sets a bad
> precedent for making the MSR handling sane.  We shouldn't be silently
> dropping MSR writes, as that will cause more problems for the guests,
> rather than less.
> 
> Given that it is appears to be just 4.7 which is affected, I think it is
> worth trying to work out what is causing 4.8 and later to be fine, and
> whether that is a better solution to backport.

The latest flight on 4.9 shows the same issue.

Jan


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