Re: [PATCH linux-next v2] x86/xen/time: prefer tsc as clocksource when it is invariant

[Boris Ostrovsky]

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