From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail.zytor.com (terminus.zytor.com [198.137.202.136]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 4C23B1EFFBF; Tue, 22 Apr 2025 09:13:54 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=198.137.202.136 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1745313237; cv=none; b=sJGXbxdeKArJJgrV/yT88cXl9SmZRQ3mwOgdXiCRYgSRa0Uh5uWA4kwIaQDP6e05tIoIRdkE3VjdXkBdf2rU2u89uW4a1MSVouBk2QegyRPXZpriAQ46PZHbzTyYf6VEcCfQwM+wEzfqszGWsYplsA7umwHTRd6QvFHJzfl2AHg= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1745313237; c=relaxed/simple; bh=8t87NXRHmH+x11mIfycM4fyvuVR/eKMQUHrwiHhPu78=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=ZqRSsyIK2jm7yc24LVHKX8b7QMSP93cmnmSZmjGw0Jb2uyi9tIhq3VynnSgryOhP6EpNYEBScfisyxM1Hn+/7VIrIYk0R/mlV4Hr9QAGzKFxAnIurqXH1nBb3yK+TopRfuhdN9KnD7dx4xjKPg7HpLVEfaE2Fhaf3mQXjxcSQqk= ARC-Authentication-Results:i=1; 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h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=ZPeuqs2QxCZr+2CQbzBLOCpW3Aawds6/1cwg0KOGkN4s7Xzb3PZq6E2KiAUhdSklS 6bmAZ9hG+CfiogKMARVhoVzL3oriBG5dyHPHGBto0/mSKUTbfZ4aFLYUysVlh6JFC/ ZfI2lvkhOZHiUCZh+IkdkYywF31eKyF7de+lHzDs4b9fjhHs57HIRf++LaLMNtLPKY QczCdjB0+4sEGoXJlmUjHhZjwRqeUafNM/3rfcGnb2CJu8EO98o0Ia9j/fEpQcfhQG lCV0OoOhQjfirIbZQCPq86OrgvY0dVY0/wQxoL36KQ5giQ3vDEnESbBhySaQesbEM7 0Bz5uP6uYbAzw== Message-ID: Date: Tue, 22 Apr 2025 02:12:46 -0700 Precedence: bulk X-Mailing-List: linux-hwmon@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC PATCH v2 06/34] x86/msr: Use the alternatives mechanism to read PMC To: =?UTF-8?B?SsO8cmdlbiBHcm/Dnw==?= , linux-kernel@vger.kernel.org, kvm@vger.kernel.org, linux-perf-users@vger.kernel.org, linux-hyperv@vger.kernel.org, virtualization@lists.linux.dev, linux-pm@vger.kernel.org, linux-edac@vger.kernel.org, xen-devel@lists.xenproject.org, linux-acpi@vger.kernel.org, linux-hwmon@vger.kernel.org, netdev@vger.kernel.org, platform-driver-x86@vger.kernel.org Cc: tglx@linutronix.de, mingo@redhat.com, bp@alien8.de, dave.hansen@linux.intel.com, x86@kernel.org, hpa@zytor.com, acme@kernel.org, andrew.cooper3@citrix.com, peterz@infradead.org, namhyung@kernel.org, mark.rutland@arm.com, alexander.shishkin@linux.intel.com, jolsa@kernel.org, irogers@google.com, adrian.hunter@intel.com, kan.liang@linux.intel.com, wei.liu@kernel.org, ajay.kaher@broadcom.com, bcm-kernel-feedback-list@broadcom.com, tony.luck@intel.com, pbonzini@redhat.com, vkuznets@redhat.com, seanjc@google.com, luto@kernel.org, boris.ostrovsky@oracle.com, kys@microsoft.com, haiyangz@microsoft.com, decui@microsoft.com References: <20250422082216.1954310-1-xin@zytor.com> <20250422082216.1954310-7-xin@zytor.com> Content-Language: en-US From: Xin Li Autocrypt: addr=xin@zytor.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 4/22/2025 1:38 AM, Jürgen Groß wrote: > On 22.04.25 10:21, Xin Li (Intel) wrote: >> To eliminate the indirect call overhead introduced by the pv_ops API, >> use the alternatives mechanism to read PMC: > > Which indirect call overhead? The indirect call is patched via the > alternative mechanism to a direct one. > See below. >> >>      1) When built with !CONFIG_XEN_PV, X86_FEATURE_XENPV becomes a >>         disabled feature, preventing the Xen PMC read code from being >>         built and ensuring the native code is executed unconditionally. > > Without CONFIG_XEN_PV CONFIG_PARAVIRT_XXL is not selected, resulting in > native code anyway. Yes, this is kept in this patch, but in a little different way. > >> >>      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 PMC 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 PMC read code. >> >> Consequently, remove the pv_ops PMC read API. > > I don't see the value of this patch. > > It adds more #ifdef and code lines without any real gain. > > In case the x86 maintainers think it is still worth it, I won't object. I think we want to totally bypass pv_ops in the case 2.1). Do you mean the indirect call is patched to call native code *directly* for 2.1? I don't know it, can you please elaborate? AFAIK, Xen PV has been the sole user of pv_ops for nearly 20 years. This raises significant doubts about whether pv_ops provides Linux with the value of being a well-abstracted "CPU" or "Platform". And the x86 maintainers have said that it's a maintenance nightmare. Thanks! Xin