Re: [PATCH] x86/entry/64: Disallow RDPID in paranoid entry if KVM is enabled

[hpa]

On August 21, 2020 2:28:32 AM PDT, Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
>On Fri, Aug 21 2020 at 10:09, Paolo Bonzini wrote:
>> On 21/08/20 09:47, Borislav Petkov wrote:
>>> On Thu, Aug 20, 2020 at 07:50:50PM -0700, Sean Christopherson wrote:
>>>> +	 * Disallow RDPID if KVM is enabled as it may consume a guest's
>TSC_AUX
>>>> +	 * if an NMI arrives in KVM's run loop.  KVM loads guest's
>TSC_AUX on
>>>> +	 * VM-Enter and may not restore the host's value until the CPU
>returns
>>>> +	 * to userspace, i.e. KVM depends on the kernel not using
>TSC_AUX.
>>>>  	 */
>>> And frankly, this is really unfair. The kernel should be able to use
>any
>>> MSR. IOW, KVM needs to be fixed here. I'm sure it context-switches
>other
>>> MSRs so one more MSR is not a big deal.
>>
>> The only MSR that KVM needs to context-switch manually are XSS and
>> SPEC_CTRL.  They tend to be the same on host and guest in which case
>> they can be optimized away.
>>
>> All the other MSRs (EFER and PAT are those that come to mind) are
>> handled by the microcode and thus they don't have the slowness of
>> RDMSR/WRMSR
>>
>> One more MSR *is* a big deal: KVM's vmentry+vmexit cost is around
>1000
>> cycles, adding 100 clock cycles for 2 WRMSRs is a 10% increase.
>
>We all know that MSRs are slow, but as a general rule I have to make it
>entirely clear that the kernel has precedence over KVM.
>
>If the kernel wants to use an MSR for it's own purposes then KVM has to
>deal with that and not the other way round. Preventing the kernel from
>using a facility freely is not an option ever.
>
>The insanities of KVM performance optimizations have bitten us more
>than
>once.
>
>For this particular case at hand I don't care much and we should just
>rip the whole RDPID thing out unconditionally. We still have zero
>numbers about the performance difference vs. LSL.
>
>Thanks,
>
>        tglx

It is hardly going to be a performance difference for paranoid entry, which is hopefully rare enough that it falls into the noise.
-- 
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