From: Krister Johansen <kjlx@templeofstupid.com>
To: Jan Beulich <jbeulich@suse.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>
Subject: Re: [PATCH linux-next v2] x86/xen/time: prefer tsc as clocksource when it is invariant
Date: Mon, 12 Dec 2022 14:05:19 -0800 [thread overview]
Message-ID: <20221212220519.GA1935@templeofstupid.com> (raw)
In-Reply-To: <1eb6048b-bf23-78a0-9c3c-54bbd12c3864@suse.com>
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?
> > + if (!(xen_hvm_domain() || xen_pvh_domain()))
> > + return 0;
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.
-K
next prev parent reply other threads:[~2022-12-12 22:05 UTC|newest]
Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-12-08 16:36 [PATCH linux-next] x86/xen/time: prefer tsc as clocksource when it is invariant Krister Johansen
2022-12-09 19:32 ` Boris Ostrovsky
2022-12-12 15:57 ` Krister Johansen
2022-12-12 16:05 ` [PATCH linux-next v2] " Krister Johansen
2022-12-12 16:46 ` Jan Beulich
2022-12-12 22:05 ` Krister Johansen [this message]
2022-12-13 7:23 ` Jan Beulich
2022-12-13 18:58 ` Krister Johansen
2022-12-14 8:17 ` Jan Beulich
2022-12-14 18:01 ` Krister Johansen
2022-12-12 18:48 ` Boris Ostrovsky
2022-12-12 22:09 ` Krister Johansen
2022-12-13 21:25 ` Boris Ostrovsky
2022-12-14 18:01 ` Krister Johansen
2022-12-14 21:46 ` Boris Ostrovsky
2022-12-16 16:20 ` Krister Johansen
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