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

[Liu, Jing2]

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 
quite
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)

Thanks,
Jing

>