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SubjectRe: Thoughts of AMX KVM support based on latest kernel
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On 11/16/21 21:36, Thomas Gleixner wrote:
> local_irq_enable(); <- Problem starts here
>
> preempt_enable(); <- Becomes wider here
>
>> It doesn't become that much wider because there's always preempt
>> notifiers. So if it's okay to save XFD in the XSAVES wrapper and in
>> kvm_arch_vcpu_put(), that might be already remove the need to do it
>> schedule().
>
> Did not think about preemption notifiers. Probably because I hate
> notifiers with a passion since I had to deal with the CPU hotplug
> notifier trainwreck.
>
> But yes that would work. So the places to do that would be:
>
> 1) kvm_sched_out() -> kvm_arch_vcpu_put() > 2) kernel_fpu_begin_mask()

... which calls save_fpregs_to_fpstate

> 3) kvm_put_guest_fpu()

... which calls save_fpregs_to_fpstate via fpu_swap_kvm_fpstate

So perhaps it could be done in save_fpregs_to_fpstate (for the sched out
path, it would be called by switch_fpu_prepare()). But for now I would
also start with the trivial version.

> I'd be really surprised if that RDMSR is truly noticeable within all the
> other crud this path is doing.

I agree.

Paolo

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