linux-kernel - Re: [PATCH RESEND] KVM: X86: Fix load bad host fpu state

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Date:   Tue, 12 Dec 2017 17:05:29 +0800
From:   Wanpeng Li <kernellwp@...il.com>
To:     Peter Xu <peterx@...hat.com>
Cc:     David Hildenbrand <david@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        kvm <kvm@...r.kernel.org>, Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Wanpeng Li <wanpeng.li@...mail.com>,
        Rik van Riel <riel@...hat.com>,
        "# v3 . 10+" <stable@...r.kernel.org>
Subject: Re: [PATCH RESEND] KVM: X86: Fix load bad host fpu state

2017-12-12 13:40 GMT+08:00 Wanpeng Li <kernellwp@...il.com>:
> 2017-12-12 11:36 GMT+08:00 Peter Xu <peterx@...hat.com>:
>> On Tue, Dec 12, 2017 at 05:51:26AM +0800, Wanpeng Li wrote:
>>> 2017-12-12 4:48 GMT+08:00 David Hildenbrand <david@...hat.com>:
>>> > On 10.12.2017 22:44, Wanpeng Li wrote:
>>> >> From: Wanpeng Li <wanpeng.li@...mail.com>
>>> >>
>>> >> ------------[ cut here ]------------
>>> >>  Bad FPU state detected at kvm_put_guest_fpu+0xd8/0x2d0 [kvm], reinitializing FPU registers.
>>> >>  WARNING: CPU: 1 PID: 4594 at arch/x86/mm/extable.c:103 ex_handler_fprestore+0x88/0x90
>>> >>  CPU: 1 PID: 4594 Comm: qemu-system-x86 Tainted: G    B      OE    4.15.0-rc2+ #10
>>> >>  RIP: 0010:ex_handler_fprestore+0x88/0x90
>>> >>  Call Trace:
>>> >>   fixup_exception+0x4e/0x60
>>> >>   do_general_protection+0xff/0x270
>>> >>   general_protection+0x22/0x30
>>> >>  RIP: 0010:kvm_put_guest_fpu+0xd8/0x2d0 [kvm]
>>> >>  RSP: 0018:ffff8803d5627810 EFLAGS: 00010246
>>> >>   kvm_vcpu_reset+0x3b4/0x3c0 [kvm]
>>> >>   kvm_apic_accept_events+0x1c0/0x240 [kvm]
>>> >>   kvm_arch_vcpu_ioctl_run+0x1658/0x2fb0 [kvm]
>>> >>   kvm_vcpu_ioctl+0x479/0x880 [kvm]
>>> >>   do_vfs_ioctl+0x142/0x9a0
>>> >>   SyS_ioctl+0x74/0x80
>>> >>   do_syscall_64+0x15f/0x600
>>> >>
>>> >> This can be reproduced by running any testcase in kvm-unit-tests since
>>> >> the qemu userspace FPU context is not initialized, which results in the
>>> >> init path from kvm_apic_accept_events() will load/put qemu userspace
>>> >> FPU context w/o initialized. In addition, w/o this splatting we still
>>> >> should initialize vcpu->arch.user_fpu instead of current->thread.fpu.
>>> >> This patch fixes it by initializing qemu user FPU context if it is
>>> >> uninitialized before KVM_RUN.
>>> >>
>>> >> Cc: Paolo Bonzini <pbonzini@...hat.com>
>>> >> Cc: Radim Krčmář <rkrcmar@...hat.com>
>>> >> Cc: Rik van Riel <riel@...hat.com>
>>> >> Cc: stable@...r.kernel.org
>>> >> Fixes: f775b13eedee (x86,kvm: move qemu/guest FPU switching out to vcpu_run)
>>> >> Signed-off-by: Wanpeng Li <wanpeng.li@...mail.com>
>>> >> ---
>>> >>  arch/x86/kvm/x86.c | 7 +++++--
>>> >>  1 file changed, 5 insertions(+), 2 deletions(-)
>>> >>
>>> >> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>>> >> index a92b22f..063a643 100644
>>> >> --- a/arch/x86/kvm/x86.c
>>> >> +++ b/arch/x86/kvm/x86.c
>>> >> @@ -7273,10 +7273,13 @@ static int complete_emulated_mmio(struct kvm_vcpu *vcpu)
>>> >>
>>> >>  int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu, struct kvm_run *kvm_run)
>>> >>  {
>>> >> -     struct fpu *fpu = &current->thread.fpu;
>>> >> +     struct fpu *fpu = &vcpu->arch.user_fpu;
>>> >>       int r;
>>> >>
>>> >> -     fpu__initialize(fpu);
>>> >> +     if (!fpu->initialized) {
>>> >> +             fpstate_init(&fpu->state);
>>> >> +             fpu->initialized = 1;
>>> >> +     }
>>> >
>>> > Is there a chance of keeping using fpu__initialize() ? Duplicating the
>>> > code is ugly.
>>>
>>> There is a warning in fpu__initialize() which results in just
>>> current->thread.fpu can take advantage of.
>>>
>>> >
>>> > E.g. can't we simply initialize that in kvm_load_guest_fpu?
>>>
>>> We still miss to initialize qemu user FPU context for the above calltrace.
>>
>> IMHO we should not really init the user FPU since we should always
>> load FPU then put FPU.  The problem now is that for vcpus that with
>> vcpu_id>1 we'll first put the FPU before loading it. So, instead how
>> about we make sure we load the FPU first even for non-bootstrap vcpus?
>> And we can actually drop fpu__initialize() call, like:
>
> It will introduce extra overhead for all the cases which can't enter
> into vcpu_run(), I think move
> fpstate_init(&vcpu->arch.user_fpu.state); to fx_init() is better.

Paolo, which method you prefer?

Regards,
Wanpeng Li