Re: [PATCH v2 1/3] KVM: x86: add support for user wait instructions

[Wanpeng Li]

On Tue, 28 May 2019 at 13:16, Tao Xu <tao3.xu@xxxxxxxxx> wrote:
>
>
> On 27/05/2019 18:30, Peter Zijlstra wrote:
> > On Fri, May 24, 2019 at 03:56:35PM +0800, Tao Xu wrote:
> >> This patch adds support for UMONITOR, UMWAIT and TPAUSE instructions
> >> in kvm, and by default dont't expose it to kvm and provide a capability
> >> to enable it.
> >
> > I'm thinking this should be conditional on the guest being a 1:1 guest,
> > and I also seem to remember we have bits for that already -- they were
> > used to disable paravirt spinlocks for example.
> >
>
> Hi Peter,
>
> I am wondering if "1:1 guest" means different guests in the same host
> should have different settings on user wait instructions?
>
> User wait instructions(UMONITOR, UMWAIT and TPAUSE) can use in guest
> only when the VMCS Secondary Processor-Based VM-Execution Control bit 26
> is 1, otherwise any execution of TPAUSE, UMONITOR, or UMWAIT causes a #UD.
>
> So with a capability to enable it, we use qemu kvm_vm_ioctl_enable_cap()
> to enable it. The qemu link is blew:
> https://lists.gnu.org/archive/html/qemu-devel/2019-05/msg05810.html
>
> By using different QEMU parameters, different guests in the same host
> would have different features with or without user wait instructions.
>
> About "disable paravirt spinlocks" case, I am wondering if it uses

Please refer to a4429e53c9 (KVM: Introduce paravirtualization hints
and KVM_HINTS_DEDICATED) and b2798ba0b87 (KVM: X86: Choose qspinlock
when dedicated physical CPUs are available).

> kernel parameters? If it uses kernel parameters, different guests in the
> same host may have same settings on user wait instructions.
>
> Or when we uses kernel parameters to disable user wait instructions, for
> a host chooses to enable user wait instructions, we should do some work
> on QEMU to choose disable or enable user wait instructions?
>
> Thanks
>
> Tao