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([2a01:41e1:61f0:7000:3e31:ed2e:ad9:c4fc]) by smtp.gmail.com with ESMTPSA id a640c23a62f3a-c11fbbaa4cbsm784610366b.3.2026.06.29.08.14.12 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 29 Jun 2026 08:14:13 -0700 (PDT) Message-ID: <759ad7207f201e9b81e1d4d9c5ba3d6375db44ec.camel@redhat.com> Subject: Re: [PATCH v2 1/2] x86/msr: Document I/O-like MSR semantics in /dev/cpu/*/msr driver From: Tim Wiederhake To: "H. Peter Anvin" , Thomas Gleixner , Ingo Molnar , Borislav Petkov , Dave Hansen , x86@kernel.org, linux-kernel@vger.kernel.org Date: Mon, 29 Jun 2026 17:14:12 +0200 In-Reply-To: <0239cda7-b638-44df-9739-746a7173a577@zytor.com> References: <20260626174037.1128563-1-twiederh@redhat.com> <20260626202323.1169433-2-twiederh@redhat.com> <03245373bd3912bb7421b93c383c9e07454aadd2.camel@redhat.com> <0239cda7-b638-44df-9739-746a7173a577@zytor.com> Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable User-Agent: Evolution 3.58.2 (3.58.2-1.fc43) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 On Fri, 2026-06-26 at 14:42 -0700, H. Peter Anvin wrote: > On 2026-06-26 13:33, Tim Wiederhake wrote: > >=20 > > This version splits the change in two: > >=20 > > =C2=A0 1/2 adds a comment explaining why the driver loops over the same > > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 register, so future readers don't have t= o rediscover the > > rationale. > > =C2=A0 2/2 removes only the read loop, where repeated access to the sam= e > > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 register is less obviously useful. > >=20 > The same applies to the read loop, although the use case(s) are > obviously > different. >=20 > Either way, you risk breaking working tools for no reason. >=20 > -hpa While looking into this I also noticed that /dev/cpu/*/msr is the only register-addressed character device that doesn't advance ppos on read. /dev/cpu/*/cpuid, /dev/port, and /dev/nvram do. That causes ftell() and lseek(SEEK_CUR, 0) to disagree after an fread(). I don't know how important that is though, and whether that is more of a glibc issue. Can you please point me to a tool that uses the multiple-read behavior? I want to mention that in the file comment, but turbostat, rdmsr, cpupower, coreboot's inteltool, all read exactly 8 bytes. Thanks for the review, Tim