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

[Krister Johansen <kjlx@templeofstupid.com>]

On Wed, Dec 14, 2022 at 04:46:10PM -0500, Boris Ostrovsky wrote:
> 
> On 12/14/22 1:01 PM, Krister Johansen wrote:
> > On Tue, Dec 13, 2022 at 04:25:32PM -0500, Boris Ostrovsky wrote:
> > > 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).
> > Thanks.  Based upon the feedback I received from you and Jan, I may try
> > to shrink the check in xen_tsc_safe_clocksource() down a bit.  In that
> > case, I may only need to refer to a single field in the leaf that
> > provides this information.  In that case, are you alright with dropping
> > the change to the header and referring to the value directly, or would
> > you prefer that I proceed with adding these to the public API?
> 
> It would certainly be appreciated if you updated the header files but it's up to maintainers to decide whether it's required.

Sure, I'm just trying to avoid generating extra work for the maintainers
if this patch isn't likely to make it in.  I'm cutting a v3 that doesn't
reference the header.  If it's acceptable, and this looks otherwise
unobjectionable, then I'll go ahead and put together the pieces for the
public API, if that's still a desireable change.

> > > > > > +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.
> > Yes, if this remains a single boot-time check, I'm not sure that knowing
> > whether the processor supports tsc scaling helps us.  If tsc_mode is
> > default, there's always a possibility of the tsc becoming emulated later
> > on as part of migration, correct?
> 
> If the processor supports TSC scaling I don't think it's possible (it can happen in theory) but if it doesn't and you migrate to a CPU running at different frequency then yes, hypervisor will start emulating RDTSC.

Yes, I wondered whether it's reasonable to expect that users migrate
between hardware that is pretty similar, or if this case of moving from
a CPU that supports tsc scaling to one that doesn't is likely to happen
in practice.

-K