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

[Dave Hansen <dave.hansen@intel.com>]

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.

Just like the "FPU", XFD isn't be used in normal kernel code.  This is
why we can be lazy about FPU state with TIF_NEED_FPU_LOAD.  I _suspect_
that some XFD manipulation can be at least deferred to the same place
where the FPU state is manipulated: places like switch_fpu_return() or
kernel_fpu_begin().

Doing that would at least help the fast VM-Exit/VM-Enter paths that
really like TIF_NEED_FPU_LOAD today.

I guess the nasty part is that you actually need to stash the old XFD
MSR value in the vcpu structure and that's not available at
context-switch time.  So, maybe this would only allow deferring the
WRMSR.  That's better than nothing I guess.