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

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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..





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