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[31.30.173.16]) by smtp.gmail.com with ESMTPSA id p11-20020a5d68cb000000b003247d3e5d99sm8235575wrw.55.2023.10.23.10.35.17 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 23 Oct 2023 10:35:17 -0700 (PDT) Date: Mon, 23 Oct 2023 19:35:16 +0200 From: Andrew Jones To: Daniel Henrique Barboza Cc: qemu-devel@nongnu.org, qemu-riscv@nongnu.org, alistair.francis@wdc.com, bmeng@tinylab.org, liweiwei@iscas.ac.cn, zhiwei_liu@linux.alibaba.com, palmer@rivosinc.com Subject: Re: [PATCH v3 3/6] target/riscv/tcg: add user flag for profile support Message-ID: <20231023-b0eb8f3478a61875a22de747@orel> References: <20231020223951.357513-1-dbarboza@ventanamicro.com> <20231020223951.357513-4-dbarboza@ventanamicro.com> <20231023-2018025adea3ffaebbbefe23@orel> <0e66af36-bd36-4b42-b901-ed726af207b7@ventanamicro.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <0e66af36-bd36-4b42-b901-ed726af207b7@ventanamicro.com> Received-SPF: pass client-ip=2a00:1450:4864:20::42a; envelope-from=ajones@ventanamicro.com; helo=mail-wr1-x42a.google.com X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 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, RCVD_IN_DNSWL_NONE=-0.0001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=unavailable autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org On Mon, Oct 23, 2023 at 02:00:00PM -0300, Daniel Henrique Barboza wrote: > > > On 10/23/23 05:16, Andrew Jones wrote: > > On Fri, Oct 20, 2023 at 07:39:48PM -0300, Daniel Henrique Barboza wrote: > > > The TCG emulation implements all the extensions described in the > > > RVA22U64 profile, both mandatory and optional. The mandatory extensions > > > will be enabled via the profile flag. We'll leave the optional > > > extensions to be enabled by hand. > > > > > > Given that this is the first profile we're implementing in TCG we'll > > > need some ground work first: > > > > > > - all profiles declared in riscv_profiles[] will be exposed to users. > > > TCG is the main accelerator we're considering when adding profile > > > support in QEMU, so for now it's safe to assume that all profiles in > > > riscv_profiles[] will be relevant to TCG; > > > > > > - we'll not support user profile settings for vendor CPUs. The flags > > > will still be exposed but users won't be able to change them. The idea > > > is that vendor CPUs in the future can enable profiles internally in > > > their cpu_init() functions, showing to the external world that the CPU > > > supports a certain profile. But users won't be able to enable/disable > > > it; > > > > > > - Setting a profile to 'true' means 'enable all mandatory extensions of > > > this profile, setting it to 'false' means disabling all its mandatory > > > extensions. Disabling a profile is discouraged for regular use and will > > > issue an user warning. User choices for individual extensions will take > > > precedence, i.e. enabling a profile will not enable extensions that the > > > user set to 'false', and vice-versa. This will make us independent of > > > left-to-right ordering in the QEMU command line, i.e. the following QEMU > > > command lines: > > > > > > -cpu rv64,zicbom=false,rva22u64=true,Zifencei=false > > > -cpu rv64,zicbom=false,Zifencei=false,rva22u64=true > > > -cpu rv64,rva22u64=true,zicbom=false,Zifencei=false > > > > > > They mean the same thing: "enable all mandatory extensions of the > > > rva22u64 profile while keeping zicbom and Zifencei disabled". > > > > Hmm, I'm not sure I agree with special-casing profiles like this. I think > > the left-to-right processing should be consistent for all. I'm also not > > sure we should always warn when disabling a profile. For example, if a > > user does > > > > -cpu rv64,rva22u64=true,rva22u64=false > > > > then they'll get a warning, even though all they're doing is restoring the > > cpu model. While that looks like an odd thing to do, a script may be > > adding the rva22u64=true and the rva22u64=false is the user input which > > undoes what the script did. > > QEMU options do not work with a "the user enabled then disabled the same option, > thus it'll count as nothing happened" logic. The last instance of the option will > overwrite all previous instances. In the example you mentioned above the user would > disable all mandatory extensions of rva22u64 in the CPU, doesn't matter if the > same profile was enabled beforehand. Yup, I'm aware, but I keep thinking that we'll only be using profiles with a base cpu type. If you start with nothing (a base) and then add a profile and take the same one away, you shouldn't be taking away anything else. I agree that if you use a profile on some cpu type that already enabled a bunch of stuff itself, then disabling a profile would potentially remove some of those too, but mixing cpu types that have their own extensions and profiles seems like a great way to confuse oneself as to what extensions will be present. IOW, we should be adding a base cpu type at the same time we're adding these profiles. > > Sure, the can put code in place to make this happen, but then this would make > profiles act different than regular extensions. "-cpu rv64,zicbom=true -cpu rv64,zicbom=false" > will disable zicbom, it will not preserve the original 'zicbom' rv64 default. If > we're going to keep left-to-right ordering consistent, this behavior should also > be consistent as well. It will be consistent if we always override whatever was on the left with what's on the right, which means with -cpu rv64,rva22u64=true -cpu rv64,zicbom=false zicbom will be disabled, but with -cpu rv64,zicbom=false -cpu rv64,rva22u64=true it will be enabled. The same goes if the properties are given to the same -cpu parameter. > > > As for warnings, I agree that we'll throw warnings even when nothing of notice happened. > For example: > > -cpu rv64,rva22u64=false -cpu rv64,rva22u64=true > > This will throw a warning even though the user ended up enabling the extension > in the end. > > > We can fix it by postponing warnings to realize(). > > > > > > As far as warnings go, it'd be nice to warn when mandatory profile > > extensions are disabled from an enabled profile. Doing that might be > > useful for debug, but users which do it without being aware they're > > "breaking" the profile may learn from that warning. Note, the warning > > should only come when the profile is actually enabled and when the > > extension would actually be disabled, i.e. > > > > -cpu rv64,rva22u64=true,c=off > > > > should warn > > > > -cpu rv64,c=off,rva22u64=true > > > > should not warn (rva22u64 overrides c=off since it's to the right) > > > > -cpu rv64,rva22u64=true,rva22u64=false,c=off > > > > should not warn (rva22u64 is not enabled) > > Ack for all the above. > > > > > And, > > > > -cpu rv64,rva22u64=true,rva24u64=false > > > > should warn for each extension which is mandatory in both profiles. > > The way I'm imagining this happening is to cycle through all profiles during realize(), > see which ones are enabled, and then warn if the user disabled their mandatory > extensions. In this example we would warn for all rva22 mandatory extensions > that were disabled because we disabled rva24, but we won't emit any warnings for > rva24 mandatory extensions given that the profile is marked as disabled. Yup, sounds good. Thanks, drew