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

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Message-ID: <6046a6d0-bae3-41bb-9960-82c403f62476@suse.com>
Date: Tue, 22 Apr 2025 11:57:57 +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,
 linux-pm@...r.kernel.org, linux-edac@...r.kernel.org,
 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,
 andrew.cooper3@...rix.com, peterz@...radead.org, namhyung@...nel.org,
 mark.rutland@....com, alexander.shishkin@...ux.intel.com, jolsa@...nel.org,
 irogers@...gle.com, adrian.hunter@...el.com, kan.liang@...ux.intel.com,
 wei.liu@...nel.org, ajay.kaher@...adcom.com,
 bcm-kernel-feedback-list@...adcom.com, tony.luck@...el.com,
 pbonzini@...hat.com, vkuznets@...hat.com, seanjc@...gle.com,
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 haiyangz@...rosoft.com, decui@...rosoft.com
Subject: Re: [RFC PATCH v2 22/34] x86/msr: Utilize the alternatives mechanism
 to read MSR

On 22.04.25 11:20, Xin Li wrote:
> 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.

Please see my response to the previous patch.


Juergen

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