linux-kernel - Re: Thoughts of AMX KVM support based on latest kernel

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Date:   Wed, 17 Nov 2021 08:39:27 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Sean Christopherson <seanjc@...gle.com>
Cc:     "Liu, Jing2" <jing2.liu@...el.com>,
        LKML <linux-kernel@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "Nakajima, Jun" <jun.nakajima@...el.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Arjan van de Ven <arjan@...ux.intel.com>,
        Jing Liu <jing2.liu@...ux.intel.com>,
        "Cooper, Andrew" <andrew.cooper3@...rix.com>,
        "Bae, Chang Seok" <chang.seok.bae@...el.com>
Subject: Re: Thoughts of AMX KVM support based on latest kernel

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