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

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Date:   Fri, 19 Jul 2019 13:09:03 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Wanpeng Li <kernellwp@...il.com>,
        Thomas Lambertz <mail@...maslambertz.de>
Cc:     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>,
        Dave Hansen <dave.hansen@...el.com>
Subject: Re: [5.2 regression] x86/fpu changes cause crashes in KVM guest

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.

Paolo