Messages in this thread | | | Subject | Re: [PATCH 1/2] x86,kvm: move qemu/guest FPU switching out to vcpu_run | From | Quan Xu <> | Date | Thu, 16 Nov 2017 20:12:58 +0800 |
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On 2017-11-16 18:21, Paolo Bonzini wrote: > On 16/11/2017 06:06, Quan Xu wrote: >> when vcpu thread is scheduled out, the pkru value in >> current->thread.fpu.state may be the host pkru value, instead of >> guest pkru value (of course, this _assumes_ that the pkru is in >> current->thread.fpu.state as well). in this way, the pkru may be a >> coner case. > Rik may correct me, but I think this is not possible. Preemption is > disabled all the time while PKRU = guest_pkru (which is only during > vmx_vcpu_run).
refer to the following code,
vcpu_enter_guest() { ... preempt_disable() ... kvm_x86_ops->run(vcpu); (actually it is vmx_vcpu_run()) ... ... preempt_enable(); }
when preempt_disable before to run kvm_x86_ops->run.. in kvm_x86_ops->run, the pkru is restored with host_pkru (IF guest_pkru != host_pkru). all this happened under preempt_disable().
it's not true that preemtion is disable all the time while PKRU = guest_pkru..
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."
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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