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SubjectRe: [PATCH RFC 2/7] kvm: x86: Introduce XFD MSRs as passthrough to guest

On 6/30/2021 1:58 AM, Dave Hansen wrote:
> On 6/27/21 7:00 PM, Liu, Jing2 wrote:
>> On 6/24/2021 1:50 AM, Dave Hansen wrote:
>>> On 5/24/21 2:43 PM, Sean Christopherson wrote:
>>>> On Sun, Feb 07, 2021, Jing Liu wrote:
>>>>> Passthrough both MSRs to let guest access and write without vmexit.
>>>> Why? Except for read-only MSRs, e.g. MSR_CORE_C1_RES,
>>>> passthrough MSRs are costly to support because KVM must context
>>>> switch the MSR (which, by the by, is completely missing from the
>>>> patch).
>>>> In other words, if these MSRs are full RW passthrough, guests
>>>> with XFD enabled will need to load the guest value on entry, save
>>>> the guest value on exit, and load the host value on exit. That's
>>>> in the neighborhood of a 40% increase in latency for a single
>>>> VM-Enter/VM-Exit roundtrip (~1500 cycles =>
>>>>> 2000 cycles).
>>> I'm not taking a position as to whether these _should_ be passthrough or
>>> not.  But, if they are, I don't think you strictly need to do the
>>> RDMSR/WRMSR at VM-Exit time.
>> Hi Dave,
>> Thanks for reviewing the patches.
>> When vmexit, clearing XFD (because KVM thinks guest has requested AMX) can
>> be deferred to the time when host does XSAVES, but this means need a new
>> flag in common "fpu" structure or a common macro per thread which works
>> only dedicated for KVM case, and check the flag in 1) switch_fpu_prepare()
>> 2) kernel_fpu_begin() . This is the concern to me.
> Why is this a concern? You're worried about finding a single bit worth
> of space somewhere?
A bit of flag can be found so far though the space is somehow nervous.
What I
am worrying about is, we introduce a flag per thread and add the check
in core
place like softirq path and context switch path, to handle a case only
for KVM
thread + XFD=1 + AMX usage in guest. This is not a quite frequent case
but we
need check every time for every thread.

I am considering using XGETBV(1) (~24 cycles) to detect if KVM really need
wrmsr(0) to clear XFD for guest AMX state when vmexit. And this is not a
frequent case I think. Only one concern is, does/will kernel check
somewhere that
thread's memory fpu buffer is already large but thread's XFD=1? (I
believe not)



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