linux-kernel - Re: [RFC PATCH v2 22/34] x86/msr: Utilize the alternatives mechanism to read MSR

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Message-ID: <281505d5-7459-4903-887e-dc78a4c1fce4@suse.com>
Date: Wed, 23 Apr 2025 18:11:10 +0200
From: Jürgen Groß <jgross@...e.com>
To: Xin Li <xin@...or.com>, linux-kernel@...r.kernel.org,
 kvm@...r.kernel.org, linux-perf-users@...r.kernel.org,
 linux-hyperv@...r.kernel.org, virtualization@...ts.linux.dev,
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 xen-devel@...ts.xenproject.org, linux-acpi@...r.kernel.org,
 linux-hwmon@...r.kernel.org, netdev@...r.kernel.org,
 platform-driver-x86@...r.kernel.org
Cc: tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
 dave.hansen@...ux.intel.com, x86@...nel.org, hpa@...or.com, acme@...nel.org,
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Subject: Re: [RFC PATCH v2 22/34] x86/msr: Utilize the alternatives mechanism
 to read MSR

On 23.04.25 11:03, Xin Li wrote:
> On 4/22/2025 4:12 AM, Jürgen Groß wrote:
>>> +
>>> +static __always_inline bool __rdmsrq(u32 msr, u64 *val, int type)
>>> +{
>>> +    bool ret;
>>> +
>>> +#ifdef CONFIG_XEN_PV
>>> +    if (cpu_feature_enabled(X86_FEATURE_XENPV))
>>> +        return __xenpv_rdmsrq(msr, val, type);
>>
>> I don't think this will work for the Xen PV case.
> 
> Well, I have been testing the code on xen-4.17 coming with Ubuntu
> 24.04.2 LTS :)

Hmm, seems that the accessed MSR(s) are the ones falling back to the
native_rdmsr() calls. At least on the hardware you tested on.

>> X86_FEATURE_XENPV is set only after the first MSR is being read.
> 
> No matter whether the code works or not, good catch!
> 
>>
>> This can be fixed by setting the feature earlier, but it shows that the
>> paravirt feature has its benefits in such cases.
> 
> See my other reply to let Xen handle all the details.
> 
> Plus the code actually works, I would actually argue the opposite :-P

BTW, it was in kernel 6.12 I had to change the MSR read emulation for
Xen-PV the last time (fix some problems with changed x86 topology
detection). Things like that won't be easily put into the hypervisor,
which needs to serve other OS-es, too.


Juergen

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