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

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Message-ID: <91f9217a-cc2d-4a6e-bada-290312f73d82@suse.com>
Date: Tue, 22 Apr 2025 10:59:50 +0200
From: Jürgen Groß <jgross@...e.com>
To: "Xin Li (Intel)" <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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 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 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.


Juergen

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