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

[Jan Beulich <jbeulich@xxxxxxxx>]

On 12.12.2022 23:05, Krister Johansen wrote:
> On Mon, Dec 12, 2022 at 05:46:29PM +0100, Jan Beulich wrote:
>> On 12.12.2022 17:05, Krister Johansen wrote:
>>> Both the Intel SDM[4] and the Xen tsc documentation explain that marking
>>> a tsc as invariant means that it should be considered stable by the OS
>>> and is elibile to be used as a wall clock source.  The Xen documentation
>>> further clarifies that this is only reliable on HVM and PVH because PV
>>> cannot intercept a cpuid instruction.
>>
>> Without meaning to express a view on the argumentation as a whole, this
>> PV aspect is suspicious. Unless you open-code a use of the CPUID insn
>> in the kernel, all uses of CPUID are going to be processed by Xen by
>> virtue of the respective pvops hook. Documentation says what it says
>> for environments where this might not be the case.
> 
> Thanks, appreciate the clarification here. Just restating this for my
> own understanding: your advice would be to drop this check below?

No, I'm unconvinced of if/where this transformation is really appropriate.
My comment was merely to indicate that the justification for ...

>>> +   if (!(xen_hvm_domain() || xen_pvh_domain()))
>>> +           return 0;

... this isn't really correct.

> And then update the commit message to dispense with the distinction
> between HVM, PV, and PVH?
> 
>>> +   cpuid(xen_cpuid_base() + 3, &eax, &ebx, &ecx, &edx);
>>
>> Xen leaf 3 has sub-leaves, so I think you need to set ecx to zero before
>> this call.
> 
> The cpuid() inline in arch/x86/include/asm/processor.h assigns zero to
> ecx prior to calling __cpuid.  In arch/x86/boot/cpuflags.c the macros
> are a little different, but it looks like there too, the macro passes 0
> as an input argument to cpuid_count which ends up being %ecx.  Happy to
> fix this up if I'm looking at the wrong cpuid functions, though.

Oh, I didn't expect cpuid() to be more than a trivial wrapper around the
the pvops hook, and I merely looked at native_cpuid() and xen_cpuid().
I'm sorry for the noise then. Yet still, with there being sub-leaves, I'd
recommend switching to cpuid_count() just for clarity.

Jan