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

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Message-ID: <7fea6158-9b7d-4bfb-b709-729266803c32@zytor.com>
Date: Tue, 22 Apr 2025 02:20:32 -0700
From: Xin Li <xin@...or.com>
To: Jürgen Groß <jgross@...e.com>,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        linux-perf-users@...r.kernel.org, linux-hyperv@...r.kernel.org,
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        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,
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Subject: Re: [RFC PATCH v2 22/34] x86/msr: Utilize the alternatives mechanism
 to read MSR

On 4/22/2025 1:59 AM, Jürgen Groß wrote:
> On 22.04.25 10:22, Xin Li (Intel) wrote:
>> To eliminate the indirect call overhead introduced by the pv_ops API,
>> utilize the alternatives mechanism to read MSR:
>>
>>      1) When built with !CONFIG_XEN_PV, X86_FEATURE_XENPV becomes a
>>         disabled feature, preventing the Xen code from being built
>>         and ensuring the native code is executed unconditionally.
>>
>>      2) When built with CONFIG_XEN_PV:
>>
>>         2.1) If not running on the Xen hypervisor (!X86_FEATURE_XENPV),
>>              the kernel runtime binary is patched to unconditionally
>>              jump to the native MSR read code.
>>
>>         2.2) If running on the Xen hypervisor (X86_FEATURE_XENPV), the
>>              kernel runtime binary is patched to unconditionally jump
>>              to the Xen MSR read code.
>>
>> The alternatives mechanism is also used to choose the new immediate
>> form MSR read instruction when it's available.
>>
>> Consequently, remove the pv_ops MSR read APIs and the Xen callbacks.
> 
> Same as the comment to patch 5: there is no indirect call overhead after
> the system has come up.
> 

Please check https://lore.kernel.org/lkml/87y1h81ht4.ffs@tglx/.

And it's was also mentioned in the previous patch:

https://lore.kernel.org/lkml/20250422082216.1954310-22-xin@zytor.com/

Please let me know what I have missed.

Thanks!
     Xin