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SubjectRe: [PATCH RFC 2/7] kvm: x86: Introduce XFD MSRs as passthrough to guest
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On 5/25/2021 5:43 AM, 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 saying these can't be passhthrough, but there needs to be strong
> justification for letting the guest read/write them directly.
For IA32_XFD, it's per task and switched during task switch(if
different). Meanwhile,
hardware uses IA32_XFD to prevent any instruction per task touching AMX
state at the
first time and generate #NM. This means if vcpu running with AMX
enabled, hardware
IA32_XFD should keep a guest value, and once guest #NM handler finished,
the bit
should not be set anymore for this task.
No matter passthrough or not, IA32_XFD need be restored to guest value
when vm-enter,
and load host value on exit (or load zero to prevent losing guest AMX in
use state).
And passthrough makes guest IA32_XFD switch during task switch without
trapped.

For IA32_XFD_ERR, hardware automatically set the bit once #NM fault. #NM
handler
detect and clear the bit.
No matter passthrough or not, KVM doesn't know if guest has #NM, it need
read and save
IA32_XFD_ERR when vmexit to prevent preemption task from clearing bit.
Emulating need not restore guest non-zero IA32_XFD_ERR when vmenter and
effort is guest
#NM handler takes extra trap(s). But restoring a non-zero IA32_XFD_ERR
only occurs for
the vmexit case: between task's first AMX instruction(causing #NM) and
end of clearing
in guest #NM handler.

Thanks,
Jing

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