linux-kernel - Re: [PATCH v3 13/15] x86/cpu/intel: Bound the non-architectural constant

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Message-ID: <5f5f1230-f373-469c-b0d9-abc80199886e@intel.com>
Date: Thu, 21 Aug 2025 12:43:21 -0700
From: Sohil Mehta <sohil.mehta@...el.com>
To: David Woodhouse <dwmw2@...radead.org>, <x86@...nel.org>, Dave Hansen
	<dave.hansen@...ux.intel.com>, Tony Luck <tony.luck@...el.com>,
	Jürgen Gross <jgross@...e.com>, Boris Ostrovsky
	<boris.ostrovsky@...cle.com>, xen-devel <xen-devel@...ts.xenproject.org>
CC: Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>,
	Arnaldo Carvalho de Melo <acme@...nel.org>, Namhyung Kim
	<namhyung@...nel.org>, Mark Rutland <mark.rutland@....com>, "Alexander
 Shishkin" <alexander.shishkin@...ux.intel.com>, Jiri Olsa <jolsa@...nel.org>,
	Ian Rogers <irogers@...gle.com>, Adrian Hunter <adrian.hunter@...el.com>,
	"Kan Liang" <kan.liang@...ux.intel.com>, Thomas Gleixner
	<tglx@...utronix.de>, Borislav Petkov <bp@...en8.de>, "H . Peter Anvin"
	<hpa@...or.com>, "Rafael J . Wysocki" <rafael@...nel.org>, Len Brown
	<lenb@...nel.org>, Andy Lutomirski <luto@...nel.org>, Viresh Kumar
	<viresh.kumar@...aro.org>, Jean Delvare <jdelvare@...e.com>, Guenter Roeck
	<linux@...ck-us.net>, Zhang Rui <rui.zhang@...el.com>, Andrew Cooper
	<andrew.cooper3@...rix.com>, "David Laight" <david.laight.linux@...il.com>,
	Dapeng Mi <dapeng1.mi@...ux.intel.com>, <linux-perf-users@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>, <linux-acpi@...r.kernel.org>,
	<linux-pm@...r.kernel.org>, <kvm@...r.kernel.org>, <xiaoyao.li@...el.com>,
	Xin Li <xin@...or.com>
Subject: Re: [PATCH v3 13/15] x86/cpu/intel: Bound the non-architectural
 constant_tsc model checks

On 8/21/2025 12:34 PM, Sohil Mehta wrote:
> On 8/21/2025 6:15 AM, David Woodhouse wrote:
> 
>> Hm. My test host is INTEL_HASWELL_X (0x63f). For reasons which are
>> unclear to me, QEMU doesn't set bit 8 of 0x80000007 EDX unless I
>> explicitly append ',+invtsc' to the existing '-cpu host' on its command
>> line. So now my guest doesn't think it has X86_FEATURE_CONSTANT_TSC.
>>
> 
> Haswell should have X86_FEATURE_CONSTANT_TSC, so I would have expected
> the guest bit to be set. Until now, X86_FEATURE_CONSTANT_TSC was set
> based on the Family-model instead of the CPUID enumeration which may
> have hid the issue.
> 

Correction:
s/instead/as well as

> From my initial look at the QEMU implementation, this seems intentional.
> 
> QEMU considers Invariant TSC as un-migratable which prevents it from
> being exposed to migratable guests (default).
> target/i386/cpu.c:
> [FEAT_8000_0007_EDX]
>          .unmigratable_flags = CPUID_APM_INVTSC,
> 
> Can you please try '-cpu host,migratable=off'?

This is mainly to verify. If confirmed, I am not sure what the long term
solution should be.