Messages in this thread | | | Date | Wed, 17 Nov 2021 08:39:27 +0100 | Subject | Re: Thoughts of AMX KVM support based on latest kernel | From | Paolo Bonzini <> |
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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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