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

[Paolo Bonzini]

On 24/05/19 09:56, Tao Xu wrote:
> +7.19 KVM_CAP_ENABLE_USR_WAIT_PAUSE
> +
> +Architectures: x86
> +Parameters: args[0] whether feature should be enabled or not
> +
> +With this capability enabled, a VM can use UMONITOR, UMWAIT and TPAUSE
> +instructions. If the instruction causes a delay, the amount of
> +time delayed is called here the physical delay. The physical delay is
> +first computed by determining the virtual delay (the time to delay
> +relative to the VMâs timestamp counter). Otherwise, UMONITOR, UMWAIT
> +and TPAUSE cause an invalid-opcode exception(#UD).
> +

There is no need to make it a capability.  You can just check the guest
CPUID and see if it includes X86_FEATURE_WAITPKG.

Paolo