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([2804:7f0:b401:e1f2:e374:8883:e6bd:fa0b]) by smtp.gmail.com with ESMTPSA id u7-20020a17090ac88700b0028b89520c7asm10072888pjt.9.2024.01.22.13.20.59 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 22 Jan 2024 13:21:00 -0800 (PST) Subject: Re: [PATCH] docs/system/arm/virt.rst: Add note on CPU features off by default To: Peter Maydell Cc: qemu-trivial@nongnu.org References: <20240122134334.91411-1-gustavo.romero@linaro.org> From: Gustavo Romero Message-ID: Date: Mon, 22 Jan 2024 18:20:58 -0300 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.6.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit Received-SPF: pass client-ip=2607:f8b0:4864:20::1033; envelope-from=gustavo.romero@linaro.org; helo=mail-pj1-x1033.google.com X-Spam_score_int: -32 X-Spam_score: -3.3 X-Spam_bar: --- X-Spam_report: (-3.3 / 5.0 requ) BAYES_00=-1.9, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, NICE_REPLY_A=-1.22, RCVD_IN_DNSWL_NONE=-0.0001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001, T_SCC_BODY_TEXT_LINE=-0.01 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-trivial@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Mon, 22 Jan 2024 21:21:07 -0000 On 1/22/24 1:31 PM, Peter Maydell wrote: > On Mon, 22 Jan 2024 at 13:43, Gustavo Romero wrote: >> >> Add a note on CPU features off by default on `virt` machines. These CPU >> features will remain off even if `-cpu max` is given so the user must >> turn on them explicitly on `virt` machines. >> >> Signed-off-by: Gustavo Romero > > Hi -- it looks like you forgot to cc qemu-devel on this; > could you do that on the next version, please? (All our > automated patch tooling looks at qemu-devel only. The > idea is that for other mailing lists you cc the other > list plus qemu-devel.) Hi Peter! Oh, I missed it indeed :-) Did that for the v2. Thanks! >> --- a/docs/system/arm/virt.rst >> +++ b/docs/system/arm/virt.rst >> @@ -69,6 +69,17 @@ Supported guest CPU types: >> Note that the default is ``cortex-a15``, so for an AArch64 guest you must >> specify a CPU type. >> >> +Also, please note that passing ``max`` CPU (i.e. ``-cpu max``) won't >> +enable all the CPU features for a given ``virt`` machine. Some CPU >> +features are ``off`` by default (e.g. MTE) -- see machine-specific list >> +below or check them for a given machine passing ``help``. For instance, >> +for the ``virt`` machine pass ``-machine virt,help`` to QEMU. >> + >> +For example, because MTE CPU feature is ``off`` by default it's >> +necessary to turn MTE ``on`` explicitly to have this CPU feature >> +available on the ``virt`` machine, even when ``max`` CPU is specified: >> +``-machine virt,mte=on -cpu max`` . > > I think I would phrase this as: > > Where a CPU architectural feature requires support in > both the CPU itself and in the wider system, it may > not be enabled by default, but instead require a machine > option to enable it. For instance MTE support must be > enabled with ``-machine virt,mte=on``, as well as by > selecting an MTE-capable CPU with the ``-cpu`` option. > See the list of machine-specific options below. Got it. Sure, that version is much better due to the explanation of why some feature are off. I kept the initial part about -cpu max not being sufficient to select the feature because I believe some users might get confused by selecting the max cpu and not getting all the CPU features, like it happened to me :) Thanks! Cheers, Gustavo