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[37.24.206.209]) by smtp.gmail.com with ESMTPSA id f23-20020a170906049700b00a4588098c5esm803943eja.132.2024.03.14.08.33.29 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 14 Mar 2024 08:33:29 -0700 (PDT) X-BeenThere: xen-devel@lists.xenproject.org List-Id: Xen developer discussion List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Errors-To: xen-devel-bounces@lists.xenproject.org Precedence: list Sender: "Xen-devel" X-Inumbo-ID: 35951c5f-e218-11ee-a1ee-f123f15fe8a2 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.com; s=google; t=1710430410; x=1711035210; darn=lists.xenproject.org; h=content-transfer-encoding:in-reply-to:autocrypt:from:references:cc :to:content-language:subject:user-agent:mime-version:date:message-id :from:to:cc:subject:date:message-id:reply-to; bh=QY+LJPNp7m6PeIjTFExpYUxA+SrCAjz9mhvCE9G7UGk=; b=Xvnz4n9PDjikuHFo6zmv92l0IDetndd6Gjk1oHts6F1YIZqywHvTXT0OlHWUS1i6pW rPN7afjwHoBORPU7qiHRdmdKQ+jrDUUjYrrrsIM/4QKAngoZZYTrpMMSCm1W71oAEzuN YSFVEQGVfcBn7hoI8KThzjvorz7tKUDbmv2RVRwCfnBvBw2wqo0WOhXTeT/PL5kui2Gi MxAM63498ga7HeGnJx42yBEeRxkQX0/AgKORrNyDzAN76wsnHk13iz5PKtGJM1jVkdE3 ZSiHWX16zd2hyX9Zdi+RDVSpfK+NLiTmLk8dZsXqRKlM1Y5nmx4Pd0zUE/5lpImnmsUh VQBg== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1710430410; x=1711035210; h=content-transfer-encoding:in-reply-to:autocrypt:from:references:cc :to:content-language:subject:user-agent:mime-version:date:message-id :x-gm-message-state:from:to:cc:subject:date:message-id:reply-to; bh=QY+LJPNp7m6PeIjTFExpYUxA+SrCAjz9mhvCE9G7UGk=; b=Bb4OdzAtv90jNQp0YwW2blvZAA61uptHpsY4KYl2v/70iVX2wpK6tfgpI1y2pEyW9m yEbo2lCUaGlgnnuU/Caq3NzMRFD6XRBPkbtF5xDkf6EmoTaLEXAzTilUFpWSqzdj2pm7 zJpgY9DMqic/sfs147bWXYmx2Qlq3xl5daFahBXdIEq6uaR8TYHcYwMC2k5rcbPHUPYP 61lYvCcLJX/5wGh2JCOf5RYFNt6QTYsF3d6RQEH7vtbdYpihHRANYTt1vUBBA0farz+Z X+nnTUDhPss0uEQEQVyoRIQ56IJ2dL06+l0hoR9A09I/UcUH44IMyGXcQMMvF6sksZQi nJFg== X-Forwarded-Encrypted: i=1; AJvYcCUcd2xb2dd2Z7Rg1EdB6vBYB3+uSf+zG6MH/KQ49Uvc/i836bTTAhWuFN5xIPR9q/i2KG29zVgEKMxfvO8JGXsxHsrkreHH8lIQi+5RrYI= X-Gm-Message-State: AOJu0YwdNEl6w1kCD6S8fMUbY1xTRaEVlK0lwFsus/lyiAo7wDVn11qL i4ok+3eGU+jWYGGYircjSmSW2OnA/VhjRAK3YUx53mqkdVjgOtgMGP4zoOOO8w== X-Google-Smtp-Source: AGHT+IGj2cW4rbPrS21BAX30Hty+57JXFKO7Whn4B06BftR1b529siPQV812q3cQsy/3K5AOGqql1g== X-Received: by 2002:a17:906:7fc8:b0:a46:2760:3c9b with SMTP id r8-20020a1709067fc800b00a4627603c9bmr1326410ejs.34.1710430409734; Thu, 14 Mar 2024 08:33:29 -0700 (PDT) Message-ID: Date: Thu, 14 Mar 2024 16:33:28 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RISCV] [PATCH 0/7] xen/bitops: Reduce the mess, starting with ffs() Content-Language: en-US To: Andrew Cooper Cc: =?UTF-8?Q?Roger_Pau_Monn=C3=A9?= , Wei Liu , Stefano Stabellini , Julien Grall , Volodymyr Babchuk , Bertrand Marquis , Michal Orzel , Oleksii Kurochko , Shawn Anastasio , "consulting @ bugseng . com" , Simone Ballarin , Federico Serafini , Nicola Vetrini , Xen-devel References: <20240313172716.2325427-1-andrew.cooper3@citrix.com> <1805b028-e21f-448f-bf94-2156092e40a5@citrix.com> From: Jan Beulich Autocrypt: addr=jbeulich@suse.com; keydata= xsDiBFk3nEQRBADAEaSw6zC/EJkiwGPXbWtPxl2xCdSoeepS07jW8UgcHNurfHvUzogEq5xk hu507c3BarVjyWCJOylMNR98Yd8VqD9UfmX0Hb8/BrA+Hl6/DB/eqGptrf4BSRwcZQM32aZK 7Pj2XbGWIUrZrd70x1eAP9QE3P79Y2oLrsCgbZJfEwCgvz9JjGmQqQkRiTVzlZVCJYcyGGsD /0tbFCzD2h20ahe8rC1gbb3K3qk+LpBtvjBu1RY9drYk0NymiGbJWZgab6t1jM7sk2vuf0Py O9Hf9XBmK0uE9IgMaiCpc32XV9oASz6UJebwkX+zF2jG5I1BfnO9g7KlotcA/v5ClMjgo6Gl MDY4HxoSRu3i1cqqSDtVlt+AOVBJBACrZcnHAUSuCXBPy0jOlBhxPqRWv6ND4c9PH1xjQ3NP nxJuMBS8rnNg22uyfAgmBKNLpLgAGVRMZGaGoJObGf72s6TeIqKJo/LtggAS9qAUiuKVnygo 3wjfkS9A3DRO+SpU7JqWdsveeIQyeyEJ/8PTowmSQLakF+3fote9ybzd880fSmFuIEJldWxp Y2ggPGpiZXVsaWNoQHN1c2UuY29tPsJgBBMRAgAgBQJZN5xEAhsDBgsJCAcDAgQVAggDBBYC AwECHgECF4AACgkQoDSui/t3IH4J+wCfQ5jHdEjCRHj23O/5ttg9r9OIruwAn3103WUITZee e7Sbg12UgcQ5lv7SzsFNBFk3nEQQCACCuTjCjFOUdi5Nm244F+78kLghRcin/awv+IrTcIWF hUpSs1Y91iQQ7KItirz5uwCPlwejSJDQJLIS+QtJHaXDXeV6NI0Uef1hP20+y8qydDiVkv6l IreXjTb7DvksRgJNvCkWtYnlS3mYvQ9NzS9PhyALWbXnH6sIJd2O9lKS1Mrfq+y0IXCP10eS FFGg+Av3IQeFatkJAyju0PPthyTqxSI4lZYuJVPknzgaeuJv/2NccrPvmeDg6Coe7ZIeQ8Yj t0ARxu2xytAkkLCel1Lz1WLmwLstV30g80nkgZf/wr+/BXJW/oIvRlonUkxv+IbBM3dX2OV8 AmRv1ySWPTP7AAMFB/9PQK/VtlNUJvg8GXj9ootzrteGfVZVVT4XBJkfwBcpC/XcPzldjv+3 HYudvpdNK3lLujXeA5fLOH+Z/G9WBc5pFVSMocI71I8bT8lIAzreg0WvkWg5V2WZsUMlnDL9 mpwIGFhlbM3gfDMs7MPMu8YQRFVdUvtSpaAs8OFfGQ0ia3LGZcjA6Ik2+xcqscEJzNH+qh8V m5jjp28yZgaqTaRbg3M/+MTbMpicpZuqF4rnB0AQD12/3BNWDR6bmh+EkYSMcEIpQmBM51qM EKYTQGybRCjpnKHGOxG0rfFY1085mBDZCH5Kx0cl0HVJuQKC+dV2ZY5AqjcKwAxpE75MLFkr wkkEGBECAAkFAlk3nEQCGwwACgkQoDSui/t3IH7nnwCfcJWUDUFKdCsBH/E5d+0ZnMQi+G0A nAuWpQkjM1ASeQwSHEeAWPgskBQL In-Reply-To: <1805b028-e21f-448f-bf94-2156092e40a5@citrix.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit On 14.03.2024 15:45, Andrew Cooper wrote: > On 13/03/2024 5:27 pm, Andrew Cooper wrote: >> Start cleaning it up with ffs() and friends. Across the board, this adds: >> >> * Functioning bitops without arch-specific asm > > It turns out that RISC-V doesn't have a CLZ instruction in the base > ISA.  As a consequence, __builtin_ffs() emits a library call to ffs() on > GCC, or a de Bruijn sequence on Clang. > > The optional Zbb extension adds a CLZ instruction, after which > __builtin_ffs() emits a very simple sequence. > > This leaves us with several options. > > 1) Put generic_ffs() back in, although if we do this then it's going to > be out-of-line in lib/ where it can be mostly ignored. > > 2) Require Zbb for Xen. > > 3) Alternative it up with Zbb or generic_ffs(). > > > I've got half a mind to do 1) irrespective.  It's mostly just shuffling > logic out of bitops.h into lib/. Yes. Might also help with the bi-sectability issue you faced. > I also think we should do option 2 for RISCV.  Given the instruction > groups that H does mandate, it's unrealistic to expect that such a chip > wouldn't support Zbb/etc. I'm not so sure here. > Also, getting full alternatives working is yet-more work that's not > trivial at this point in RISCV's development.  I think it is entirely > reasonable to avoid this work for now, and make it a problem for anyone > who has an H-capable Zbb-incapable system.  (with a strong implication > that this is work that probably never needs to be done.) That's definitely for later. Jan