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

[Boris Ostrovsky <boris.ostrovsky@oracle.com>]

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