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

[Boris Ostrovsky]

On 12/12/22 5:09 PM, Krister Johansen wrote:
> On Mon, Dec 12, 2022 at 01:48:24PM -0500, Boris Ostrovsky wrote:
> > 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.
> Ok, should I split this into a separate patch on the linux side too?


Yes. Once the Xen patch has been accepted you will either submit the same patch for Linux or sync Linux file with Xen (if there are more differences).


> > > +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?
> I was under the impression that if the mode was 0 (default) it would be
> possible for the tsc to become emulated in the future, perhaps after a
> migration.  The presence of the tsc_mode noemulate meant that we could
> count on the falseneess of the XEN_CPUID_TSC_EMULATED check remaining
> constant.


This will filter out most modern processors with TSC scaling support where in default mode we don't intercept RDTCS after migration. But I don't think we have proper interface to determine this so we don't have much choice but to indeed make this check.


-boris