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From:   Vitaly Kuznetsov <vkuznets@redhat.com>
To:     Ani Sinha <ani@anisinha.ca>, linux-kernel@vger.kernel.org
Cc:     anirban.sinha@nokia.com, mikelley@microsoft.com,
        Ani Sinha <ani@anisinha.ca>,
        "K. Y. Srinivasan" <kys@microsoft.com>,
        Haiyang Zhang <haiyangz@microsoft.com>,
        Stephen Hemminger <sthemmin@microsoft.com>,
        Wei Liu <wei.liu@kernel.org>, Dexuan Cui <decui@microsoft.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>,
        linux-hyperv@vger.kernel.org
Subject: Re: [PATCH] x86/hyperv: add comment describing
 TSC_INVARIANT_CONTROL MSR setting bit 0
In-Reply-To: <20210716063341.2865562-1-ani@anisinha.ca>
References: <20210716063341.2865562-1-ani@anisinha.ca>
Date:   Fri, 16 Jul 2021 15:25:55 +0200
Message-ID: <8735se1jbw.fsf@vitty.brq.redhat.com>
MIME-Version: 1.0
Content-Type: text/plain
Precedence: bulk

Ani Sinha <ani@anisinha.ca> writes:

> Commit dce7cd62754b5 ("x86/hyperv: Allow guests to enable InvariantTSC")
> added the support for HV_X64_MSR_TSC_INVARIANT_CONTROL. Setting bit 0
> of this synthetic MSR will allow hyper-v guests to report invariant TSC
> CPU feature through CPUID. This comment adds this explanation to the code
> and mentions where the Intel's generic platform init code reads this
> feature bit from CPUID. The comment will help developers understand how
> the two parts of the initialization (hyperV specific and non-hyperV
> specific generic hw init) are related.
>
> Signed-off-by: Ani Sinha <ani@anisinha.ca>
> ---
>  arch/x86/kernel/cpu/mshyperv.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
>
> diff --git a/arch/x86/kernel/cpu/mshyperv.c b/arch/x86/kernel/cpu/mshyperv.c
> index 715458b7729a..d2429748170d 100644
> --- a/arch/x86/kernel/cpu/mshyperv.c
> +++ b/arch/x86/kernel/cpu/mshyperv.c
> @@ -368,6 +368,14 @@ static void __init ms_hyperv_init_platform(void)
>  	machine_ops.crash_shutdown = hv_machine_crash_shutdown;
>  #endif
>  	if (ms_hyperv.features & HV_ACCESS_TSC_INVARIANT) {
> +		/*
> +		 * Setting bit 0 of the synthetic MSR 0x40000118 enables
> +		 * guests to report invariant TSC feature through CPUID
> +		 * instruction, CPUID 0x800000007/EDX, bit 8. See code in
> +		 * early_init_intel() where this bit is examined. The
> +		 * setting of this MSR bit should happen before init_intel()
> +		 * is called.

It should probably be emphasized, that write to 0x40000118
updates/changes guest visible CPUIDs. This may not be clear as CPUIDs
are generally considered 'static'. 

> +		 */
>  		wrmsrl(HV_X64_MSR_TSC_INVARIANT_CONTROL, 0x1);
>  		setup_force_cpu_cap(X86_FEATURE_TSC_RELIABLE);
>  	}

-- 
Vitaly