linux-kernel - Re: [5.2 regression] x86/fpu changes cause crashes in KVM guest

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Date:   Mon, 22 Jul 2019 12:28:42 +0800
From:   Wanpeng Li <kernellwp@...il.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Thomas Lambertz <mail@...maslambertz.de>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Rik van Riel <riel@...riel.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Radim Krcmar <rkrcmar@...hat.com>, kvm <kvm@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Marc Orr <marcorr@...gle.com>
Subject: Re: [5.2 regression] x86/fpu changes cause crashes in KVM guest

On Fri, 19 Jul 2019 at 19:09, Paolo Bonzini <pbonzini@...hat.com> wrote:
>
> On 19/07/19 10:59, Wanpeng Li wrote:
> > https://lkml.org/lkml/2017/11/14/891, "The scheduler will save the
> > guest fpu context when a vCPU thread is preempted, and restore it when
> > it is scheduled back in." But I can't find any scheduler codes do
> > this.
>
> That's because applying commit 240c35a37 was completely wrong.  The idea
> before commit 240c35a37 was that you have the following FPU states:
>
>                userspace (QEMU)             guest
> ---------------------------------------------------------------------------
>                processor                    vcpu->arch.guest_fpu
> >>> KVM_RUN: kvm_load_guest_fpu
>                vcpu->arch.user_fpu          processor
> >>> preempt out
>                vcpu->arch.user_fpu          current->thread.fpu
> >>> preempt in
>                vcpu->arch.user_fpu          processor
> >>> back to userspace
> >>> kvm_put_guest_fpu
>                processor                    vcpu->arch.guest_fpu
> ---------------------------------------------------------------------------
>
> After removing user_fpu, QEMU's FPU state is destroyed when KVM_RUN is
> preempted.  So that's already messed up (I'll send a revert), and given
> the diagram above your patch makes total sense.
>
> With the new lazy model we want to hook into kvm_vcpu_arch_load and get
> the state back to the processor from current->thread.fpu, and indeed
> switch_fpu_return is essentially copy_kernel_to_fpregs(&current->thread.
> fpu->state).
>
> However I would keep the fpregs_assert_state_consistent in
> kvm_arch_vcpu_load, and also
> WARN_ON_ONCE(test_thread_flag(TIF_NEED_FPU_LOAD)) in vcpu_enter_guest.

Looks good to me, just send out two patches rebase on the revert.

Regards,
Wanpeng Li