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

[Wanpeng Li]

2017-12-12 11:36 GMT+08:00 Peter Xu <peterx@xxxxxxxxxx>:
> On Tue, Dec 12, 2017 at 05:51:26AM +0800, Wanpeng Li wrote:
>> 2017-12-12 4:48 GMT+08:00 David Hildenbrand <david@xxxxxxxxxx>:
>> > On 10.12.2017 22:44, Wanpeng Li wrote:
>> >> From: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>
>> >>
>> >> ------------[ 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@xxxxxxxxxx>
>> >> Cc: Radim KrÄmÃÅ <rkrcmar@xxxxxxxxxx>
>> >> Cc: Rik van Riel <riel@xxxxxxxxxx>
>> >> Cc: stable@xxxxxxxxxxxxxxx
>> >> Fixes: f775b13eedee (x86,kvm: move qemu/guest FPU switching out to vcpu_run)
>> >> Signed-off-by: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>
>> >> ---
>> >>  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.

Regards,
Wanpeng Li