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SubjectRe: [PATCH v2 1/2] KVM: X86: Add per-VM no-HLT-exiting capability
On 05/02/2018 07:57, Wanpeng Li wrote:
> From: Wanpeng Li <>
> If host CPUs are dedicated to a VM, we can avoid VM exits on HLT.
> This patch adds the per-VM non-HLT-exiting capability.
> Cc: Paolo Bonzini <>
> Cc: Radim Krčmář <>
> Signed-off-by: Wanpeng Li <>
> ---
> v1 -> v2:
> * vmx_clear_hlt() around INIT handling
> * vmx_clear_hlt() upon SMI and implement auto halt restart

Hi Wanpeng,

sorry I could not answer before.

We do not need to implement AutoHalt. It's a messy functionality and
the way it works is much simpler: on RSM the microcode reads AutoHALT's
bit 0 and... decrements RIP if it is 1. All you need to do however is
clear the activity state. Guests should expect anyway that "CLI;HLT"
can be interrupted by an NMI and follow it with a JMP.

Second, I would prefer to implement at the same time MWAIT and PAUSE
passthrough, as in

> The three capabilities are more or less all doing the same thing.
> Perhaps it would make some sense to only leave PAUSE spin loops in
> guest, but not HLT/MWAIT; but apart from that I think users would
> probably enable all of them. So I think we should put in the
> documentation that blindly passing the KVM_CHECK_EXTENSION result to
> KVM_ENABLE_CAP is a valid thing to do when vCPUs are associated to
> dedicated physical CPUs.
> Let's get rid of KVM_CAP_X86_GUEST_MWAIT altogether and
> add a new capability. But let's use just one.

Thanks again for your work, and sorry for slightly contradicting Radim's
review. I've rebased and applied patch 2.


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