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

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Message-ID: <03ac8bac-c8d1-4a3b-a07f-2bbf04e726b6@intel.com>
Date: Fri, 22 Aug 2025 09:46:48 +0800
From: Xiaoyao Li <xiaoyao.li@...el.com>
To: Sohil Mehta <sohil.mehta@...el.com>, 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, Xin Li <xin@...or.com>
Subject: Re: [PATCH v3 13/15] x86/cpu/intel: Bound the non-architectural
 constant_tsc model checks

On 8/22/2025 3:43 AM, Sohil Mehta wrote:
> 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.

yeah. It's the intentional behavior of QEMU.

Invariant TSC is ummigratable unless users explicitly configures the TSC 
frequency, e.g., "-cpu host,tsc-frequency=xxx". Because the TSC 
frequency is by default the host's frequency if no "tsc-frequency" 
specified, and it will change when the VM is migrated to a host with a 
different TSC frequency.

It's the specific behavior/rule of QEMU. We just need to keep it in 
mind. If we want to expose invariant TSC to the guest with QEMU's "-cpu 
host", we can either:
1) explicitly configure the "tsc-frequency", or
2) explicitly turn off "migratable"