Re: [PATCH 1/2] x86,kvm: move qemu/guest FPU switching out to vcpu_run

[Quan Xu <quan.xu0@xxxxxxxxx>]

On 2017-11-16 20:18, Paolo Bonzini wrote:
> On 16/11/2017 13:12, Quan Xu wrote:
> > However it seems there is still some gap..
> >
> > as Rik said, "at context switch time, the context switch code will save
> > the guest FPU state to current->thread.fpu when the VCPU thread is scheduled out."
> By "guest FPU state" Rik means "guest FPU with host PKRU".
  :( actually it is host_pkru, just with different names..

> Guest PKRU is always stored in vcpu->arch.pkru rather than in the guest
> FPU state, so guest PKRU will never be in current->thread.fpu.state either.
>
> KVM_GET_XSAVE will the guest FPU state with vcpu->arch.pkru and
> migration will work properly.
  agreed, I fix it.. that's why I concern.. there are so much methods to
  write PKRU with host_pkru/guest_pkru..

 after migration, KVM_GET|SET_XSAVE restore the right pkru.. but we 
introduce
 another method:

      -- When the VCPU thread is scheduled back in, the context
         switch code will restore current->thread.fpu to the FPU
         registers.


there is still a window to restore current->thread.fpu to the FPU 
registers before enter guest mode and

preempt_disable().

on target machine, after migration, the pkru value is source_host_pkru 
in current->thread.fpu.

in case,

            source_host_pkru_value != guest_pkru_value,
            target_host_pkru_value == guest_pkru_value..

source_host_pkru_value may be restored to PKRU.. make pkru status inconsistent..


thanks

Quan
Alibaba Cloud
> Thanks,
>
> Paolo
>
> > after preempt_enable() in vcpu_enter_guest(), the vcpu thread is
> > scheduled out,
> > in kvm_x86_ops->run, the PKRU has been restored with host_pkru (IF
> > guest_pkru != host_pkru)..
> > instead of guest_pkru..
> >
> > then the PKRU is host_pkru, how to save guest_pkru current->thread.fpu?
> >
> > as mentioned, all this _assumes_ that the pkru is in
> > current->thread.fpu.state as well.
> >
> >
> > thanks,
> >
> > Quan
> > Alibaba Cloud
> >
> > > Context switching will only happen in vcpu_enter_guest() after
> > > preempt_enable() for a preemptible kernel, or in vcpu_run via
> > > cond_resched() for a non-preemptible kernel.
> > >
> > > Thanks,
> > >
> > > Paolo
> > >
> > > > VM migration again, in case,
> > > >              source_host_pkru_value != guest_pkru_value,
> > > >              target_host_pkru_value == guest_pkru_value..
> > > >
> > > > the pkru status would be inconsistent..