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[2603:800c:3202:ffa7:497b:6ae4:953c:7ad1]) by smtp.gmail.com with ESMTPSA id j13sm468980pgp.29.2021.07.26.11.33.54 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 26 Jul 2021 11:33:55 -0700 (PDT) Subject: Re: [PATCH v2 3/3] target/arm: Add sve-default-vector-length cpu property To: Andrew Jones References: <20210723203344.968563-1-richard.henderson@linaro.org> <20210723203344.968563-4-richard.henderson@linaro.org> <20210726145951.g3ywwhcfstmsi7vi@gator> From: Richard Henderson Message-ID: Date: Mon, 26 Jul 2021 08:33:52 -1000 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.11.0 MIME-Version: 1.0 In-Reply-To: <20210726145951.g3ywwhcfstmsi7vi@gator> 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::1035; envelope-from=richard.henderson@linaro.org; helo=mail-pj1-x1035.google.com X-Spam_score_int: -24 X-Spam_score: -2.5 X-Spam_bar: -- X-Spam_report: (-2.5 / 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=-0.438, RCVD_IN_DNSWL_NONE=-0.0001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: qemu-arm@nongnu.org, qemu-devel@nongnu.org Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 7/26/21 4:59 AM, Andrew Jones wrote: >> +SVE User-mode Default Vector Length Property >> +-------------------------------------------- >> + >> +For qemu-aarch64, the cpu property `sve-default-vector-length=N` is >> +defined to mirror the Linux kernel parameter file >> +`/proc/sys/abi/sve_default_vector_length`. The default length, `N`, >> +is in units of bytes and must be between 16 and 8192. > > Hmm. If a user inputs anything greater than 256, then won't it get > silently reduced to 256? Yes. >> +If not specified, the default vector length is 64. >> + >> +If the default length is larger than the maximum vector length enabled >> +with `sve` properties, the actual vector length will be reduced. > > Here it's pointed out that the default may be reduced, but it implies that > that only happens if an sve property is also given. Won't users wonder > why they're only getting vectors that are 256 bytes large even when they > ask for more? I guess adding that 256 is the maximum length supported by qemu should be sufficient? r~