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Date:   Mon, 12 Dec 2022 13:48:24 -0500
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To:     Krister Johansen <kjlx@templeofstupid.com>,
        Juergen Gross <jgross@suse.com>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@linux.intel.com>, x86@kernel.org,
        "H. Peter Anvin" <hpa@zytor.com>, xen-devel@lists.xenproject.org,
        linux-kernel@vger.kernel.org,
        Marcelo Tosatti <mtosatti@redhat.com>,
        Anthony Liguori <aliguori@amazon.com>,
        David Reaver <me@davidreaver.com>,
        Brendan Gregg <brendan@intel.com>
References: <20221208163650.GA3225@templeofstupid.com>
 <1e6c1b08-d573-fba9-61fd-d40a74427d46@oracle.com>
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 <20221212160524.GB1973@templeofstupid.com>
From:   Boris Ostrovsky <boris.ostrovsky@oracle.com>
Subject: Re: [PATCH linux-next v2] x86/xen/time: prefer tsc as clocksource
 when it is invariant
In-Reply-To: <20221212160524.GB1973@templeofstupid.com>
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On 12/12/22 11:05 AM, Krister Johansen wrote:
>
> diff --git a/arch/x86/include/asm/xen/cpuid.h b/arch/x86/include/asm/xen/cpuid.h
> index 6daa9b0c8d11..d9d7432481e9 100644
> --- a/arch/x86/include/asm/xen/cpuid.h
> +++ b/arch/x86/include/asm/xen/cpuid.h
> @@ -88,6 +88,12 @@
>    *             EDX: shift amount for tsc->ns conversion
>    * Sub-leaf 2: EAX: host tsc frequency in kHz
>    */
> +#define XEN_CPUID_TSC_EMULATED       (1u << 0)
> +#define XEN_CPUID_HOST_TSC_RELIABLE  (1u << 1)
> +#define XEN_CPUID_RDTSCP_INSTR_AVAIL (1u << 2)
> +#define XEN_CPUID_TSC_MODE_DEFAULT   (0)
> +#define XEN_CPUID_TSC_MODE_EMULATE   (1u)
> +#define XEN_CPUID_TSC_MODE_NOEMULATE (2u)


This file is a copy of Xen public interface so this change should go to Xen first.


>   
> +static int __init xen_tsc_safe_clocksource(void)
> +{
> +	u32 eax, ebx, ecx, edx;
> +
> +	if (!(xen_hvm_domain() || xen_pvh_domain()))
> +		return 0;
> +
> +	if (!(boot_cpu_has(X86_FEATURE_CONSTANT_TSC)))
> +		return 0;
> +
> +	if (!(boot_cpu_has(X86_FEATURE_NONSTOP_TSC)))
> +		return 0;
> +
> +	if (check_tsc_unstable())
> +		return 0;
> +
> +	cpuid(xen_cpuid_base() + 3, &eax, &ebx, &ecx, &edx);
> +
> +	if (eax & XEN_CPUID_TSC_EMULATED)
> +		return 0;
> +
> +	if (ebx != XEN_CPUID_TSC_MODE_NOEMULATE)
> +		return 0;


Why is the last test needed?


-boris