linux-kernel - Re: [PATCH 1/3] KVM: x86: expose UMWAIT/UMONITOR/TPAUSE CPU features into VM.

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Message-ID: <8605faeb-c46e-d54c-cc3a-d01b23405705@redhat.com>
Date:   Mon, 20 Aug 2018 12:16:01 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Jingqi Liu <jingqi.liu@...el.com>, rkrcmar@...hat.com,
        tglx@...utronix.de, mingo@...hat.com, hpa@...or.com
Cc:     x86@...nel.org, kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        wei.w.wang@...el.com
Subject: Re: [PATCH 1/3] KVM: x86: expose UMWAIT/UMONITOR/TPAUSE CPU features
 into VM.

On 10/07/2018 10:54, Jingqi Liu wrote:
> UMONITOR, UMWAIT, and TPAUSE are a set of user wait instructions.
> 
> UMONITOR arms address monitoring hardware using an address. A store
> to an address within the specified address range triggers the
> monitoring hardware to wake up the processor waiting in umwait.
> 
> UMWAIT instructs the processor to enter an implementation-dependent
> optimized state while monitoring a range of addresses. The optimized
> state may be either a light-weight power/performance optimized state
> (c0.1 state) or an improved power/performance optimized state
> (c0.2 state).
> 
> TPAUSE instructs the processor to enter an implementation-dependent
> optimized state c0.1 or c0.2 state and wake up when time-stamp counter
> reaches specified timeout.
> 
> Availability of the user wait instructions is indicated by the presence
> of the CPUID feature flag WAITPKG CPUID.0x07.0x0:ECX[5].
> 
> This patch exposes the umonitor,umwait, and tpause features to the guest.
> 
> The release document ref below link:
> https://software.intel.com/sites/default/files/managed/c5/15/\
> architecture-instruction-set-extensions-programming-reference.pdf
> This patch has a dependency on https://lkml.org/lkml/2018/6/15/657.
> 
> Signed-off-by: Jingqi Liu <jingqi.liu@...el.com>

Hi Jingqi, you also need to track and save/restore IA32_UMWAIT_CONTROL.

Thanks,

Paolo

> ---
>  arch/x86/kvm/cpuid.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index 7e042e3..e16c05e 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -406,7 +406,7 @@ static inline int __do_cpuid_ent(struct kvm_cpuid_entry2 *entry, u32 function,
>  		F(AVX512VBMI) | F(LA57) | F(PKU) | 0 /*OSPKE*/ |
>  		F(AVX512_VPOPCNTDQ) | F(UMIP) | F(AVX512_VBMI2) | F(GFNI) |
>  		F(VAES) | F(VPCLMULQDQ) | F(AVX512_VNNI) | F(AVX512_BITALG) |
> -		F(CLDEMOTE);
> +		F(CLDEMOTE) | F(WAITPKG);
>  
>  	/* cpuid 7.0.edx*/
>  	const u32 kvm_cpuid_7_0_edx_x86_features =
>