Re: [PATCH 1/2] x86/hyperv: Allow guests to enable InvariantTSC

From: Vitaly Kuznetsov <vkuznets@redhat.com>
To: Andrea Parri <parri.andrea@gmail.com>,
	linux-kernel@vger.kernel.org, linux-hyperv@vger.kernel.org,
	x86@kernel.org
Cc: "K . Y . Srinivasan" <kys@microsoft.com>,
	Haiyang Zhang <haiyangz@microsoft.com>,
	Stephen Hemminger <sthemmin@microsoft.com>,
	Sasha Levin <sashal@kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	"H . Peter Anvin" <hpa@zytor.com>,
	Michael Kelley <mikelley@microsoft.com>,
	Andrea Parri <parri.andrea@gmail.com>
Subject: Re: [PATCH 1/2] x86/hyperv: Allow guests to enable InvariantTSC
Date: Fri, 04 Oct 2019 18:57:14 +0200	[thread overview]
Message-ID: <87k19k1mad.fsf@vitty.brq.redhat.com> (raw)
In-Reply-To: <20191003155200.22022-1-parri.andrea@gmail.com>

Andrea Parri <parri.andrea@gmail.com> writes:

> If the hardware supports TSC scaling, Hyper-V will set bit 15 of the
> HV_PARTITION_PRIVILEGE_MASK in guest VMs with a compatible Hyper-V
> configuration version.  Bit 15 corresponds to the
> AccessTscInvariantControls privilege.  If this privilege bit is set,
> guests can access the HvSyntheticInvariantTscControl MSR: guests can
> set bit 0 of this synthetic MSR to enable the InvariantTSC feature.
> After setting the synthetic MSR, CPUID will enumerate support for
> InvariantTSC.

I tried getting more information from TLFS but as of 5.0C this feature
is not described there. I'm really interested in why this additional
interface is needed, e.g. why can't Hyper-V just set InvariantTSC
unconditionally when TSC scaling is supported?

>
> Signed-off-by: Andrea Parri <parri.andrea@gmail.com>
> ---
>  arch/x86/include/asm/hyperv-tlfs.h | 5 +++++
>  arch/x86/kernel/cpu/mshyperv.c     | 7 ++++++-
>  2 files changed, 11 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/include/asm/hyperv-tlfs.h b/arch/x86/include/asm/hyperv-tlfs.h
> index 7741e211f7f51..5f10f7f2098db 100644
> --- a/arch/x86/include/asm/hyperv-tlfs.h
> +++ b/arch/x86/include/asm/hyperv-tlfs.h
> @@ -86,6 +86,8 @@
>  #define HV_X64_ACCESS_FREQUENCY_MSRS		BIT(11)
>  /* AccessReenlightenmentControls privilege */
>  #define HV_X64_ACCESS_REENLIGHTENMENT		BIT(13)
> +/* AccessTscInvariantControls privilege */
> +#define HV_X64_ACCESS_TSC_INVARIANT		BIT(15)
>  
>  /*
>   * Feature identification: indicates which flags were specified at partition
> @@ -278,6 +280,9 @@
>  #define HV_X64_MSR_TSC_EMULATION_CONTROL	0x40000107
>  #define HV_X64_MSR_TSC_EMULATION_STATUS		0x40000108
>  
> +/* TSC invariant control */
> +#define HV_X64_MSR_TSC_INVARIANT_CONTROL	0x40000118
> +
>  /*
>   * Declare the MSR used to setup pages used to communicate with the hypervisor.
>   */
> diff --git a/arch/x86/kernel/cpu/mshyperv.c b/arch/x86/kernel/cpu/mshyperv.c
> index 267daad8c0360..105844d542e5c 100644
> --- a/arch/x86/kernel/cpu/mshyperv.c
> +++ b/arch/x86/kernel/cpu/mshyperv.c
> @@ -286,7 +286,12 @@ static void __init ms_hyperv_init_platform(void)
>  	machine_ops.shutdown = hv_machine_shutdown;
>  	machine_ops.crash_shutdown = hv_machine_crash_shutdown;
>  #endif
> -	mark_tsc_unstable("running on Hyper-V");
> +	if (ms_hyperv.features & HV_X64_ACCESS_TSC_INVARIANT) {
> +		wrmsrl(HV_X64_MSR_TSC_INVARIANT_CONTROL, 0x1);
> +		setup_force_cpu_cap(X86_FEATURE_TSC_RELIABLE);
> +	} else {
> +		mark_tsc_unstable("running on Hyper-V");
> +	}
>  
>  	/*
>  	 * Generation 2 instances don't support reading the NMI status from

-- 
Vitaly