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Date:   Tue, 13 Dec 2022 08:23:29 +0100
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Subject: Re: [PATCH linux-next v2] x86/xen/time: prefer tsc as clocksource
 when it is invariant
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To:     Krister Johansen <kjlx@templeofstupid.com>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@linux.intel.com>, x86@kernel.org,
        "H. Peter Anvin" <hpa@zytor.com>, xen-devel@lists.xenproject.org,
        linux-kernel@vger.kernel.org,
        Marcelo Tosatti <mtosatti@redhat.com>,
        Anthony Liguori <aliguori@amazon.com>,
        David Reaver <me@davidreaver.com>,
        Brendan Gregg <brendan@intel.com>,
        Juergen Gross <jgross@suse.com>,
        Boris Ostrovsky <boris.ostrovsky@oracle.com>
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From:   Jan Beulich <jbeulich@suse.com>
In-Reply-To: <20221212220519.GA1935@templeofstupid.com>
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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