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

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

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.


> > > > > +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.


> The other thing that might be possible here is to add a background
> timer that periodically checks if the tsc is still not emulated, and if
> it suddenly becomes so, change the rating again to prefer the xen
> clocksource.  I had written this off initially as an impractical
> solution, since it seemed like a lot more mechanism and because it meant
> the performance characteristics of the system would change without user
> intervention.  However, if this seems like a good idea, I'm not opposed
> to giving it a try.

I don't think we should do it. Having the kernel suddenly change clocksource 
will probably be somewhat of a surprise to users.


-boris