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B21C3C433C7; Tue, 2 Apr 2024 19:00:48 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1712084449; bh=LAtbIzYyxLFCGtNL9bIPT1JIqLRx2Xsuxm7vJ6doThY=; h=From:To:Cc:Subject:In-Reply-To:References:Date:From; b=DZ4bwmeyU0UKQkMQsDplzrpVduUBrEcwHrOmJLCrxxwVSSz7gljEEa+sexxuDhkek Q0GOstLz6J79g/c0cVcSX17twHaTIKdjTixPFXRS8JbIvVrzjKSzQBNsMOUi8Orv+0 eTOFSIzOFqF54mss/lkSeoVUjD3eE7sj6Qbo4R/C+DsOhInYQujzr9TRTIyvF42SYs 2DcamE9zwT9rnfb/YDlyGyJL43Nv7KXKmrPPOR0it/KuuLjKQKuQeuXFrLszqT2l7r UqR7rgnQaJFyPmmM9Hba7oykBUvPCPbFT7r/p+Kd+6Vy38Y16qbQUGNcbEbcfly4gU y88HfgZbdXtTw== From: =?utf-8?B?QmrDtnJuIFTDtnBlbA==?= To: Conor Dooley Cc: Pu Lehui , Stefan O'Rear , bpf@vger.kernel.org, linux-riscv@lists.infradead.org, netdev@vger.kernel.org, Alexei Starovoitov , Daniel Borkmann , Andrii Nakryiko , Martin KaFai Lau , Eduard Zingerman , Song Liu , Yonghong Song , John Fastabend , KP Singh , Stanislav Fomichev , Hao Luo , Jiri Olsa , Mykola Lysenko , Manu Bretelle , Pu Lehui Subject: Re: [PATCH bpf-next 2/5] riscv, bpf: Relax restrictions on Zbb instructions In-Reply-To: <20240402-ample-preview-c84edb69db1b@spud> References: <20240328124916.293173-1-pulehui@huaweicloud.com> <20240328124916.293173-3-pulehui@huaweicloud.com> <3ed9fe94-2610-41eb-8a00-a9f37fcf2b1a@app.fastmail.com> <20240328-ferocity-repose-c554f75a676c@spud> <87cyr7rgdn.fsf@all.your.base.are.belong.to.us> <20240402-ample-preview-c84edb69db1b@spud> Date: Tue, 02 Apr 2024 21:00:45 +0200 Message-ID: <871q7nr3mq.fsf@all.your.base.are.belong.to.us> MIME-Version: 1.0 X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20240402_120050_181804_B7930EB6 X-CRM114-Status: GOOD ( 28.39 ) X-BeenThere: linux-riscv@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: base64 Sender: "linux-riscv" 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19:00:48 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1712084449; bh=LAtbIzYyxLFCGtNL9bIPT1JIqLRx2Xsuxm7vJ6doThY=; h=From:To:Cc:Subject:In-Reply-To:References:Date:From; b=DZ4bwmeyU0UKQkMQsDplzrpVduUBrEcwHrOmJLCrxxwVSSz7gljEEa+sexxuDhkek Q0GOstLz6J79g/c0cVcSX17twHaTIKdjTixPFXRS8JbIvVrzjKSzQBNsMOUi8Orv+0 eTOFSIzOFqF54mss/lkSeoVUjD3eE7sj6Qbo4R/C+DsOhInYQujzr9TRTIyvF42SYs 2DcamE9zwT9rnfb/YDlyGyJL43Nv7KXKmrPPOR0it/KuuLjKQKuQeuXFrLszqT2l7r UqR7rgnQaJFyPmmM9Hba7oykBUvPCPbFT7r/p+Kd+6Vy38Y16qbQUGNcbEbcfly4gU y88HfgZbdXtTw== From: =?utf-8?B?QmrDtnJuIFTDtnBlbA==?= To: Conor Dooley Cc: Pu Lehui , Stefan O'Rear , bpf@vger.kernel.org, linux-riscv@lists.infradead.org, netdev@vger.kernel.org, Alexei Starovoitov , Daniel Borkmann , Andrii Nakryiko , Martin KaFai Lau , Eduard Zingerman , Song Liu , Yonghong Song , John Fastabend , KP Singh , Stanislav Fomichev , Hao Luo , Jiri Olsa , Mykola Lysenko , Manu Bretelle , Pu Lehui Subject: Re: [PATCH bpf-next 2/5] riscv, bpf: Relax restrictions on Zbb instructions In-Reply-To: <20240402-ample-preview-c84edb69db1b@spud> References: <20240328124916.293173-1-pulehui@huaweicloud.com> <20240328124916.293173-3-pulehui@huaweicloud.com> <3ed9fe94-2610-41eb-8a00-a9f37fcf2b1a@app.fastmail.com> <20240328-ferocity-repose-c554f75a676c@spud> <87cyr7rgdn.fsf@all.your.base.are.belong.to.us> <20240402-ample-preview-c84edb69db1b@spud> Date: Tue, 02 Apr 2024 21:00:45 +0200 Message-ID: <871q7nr3mq.fsf@all.your.base.are.belong.to.us> Precedence: bulk X-Mailing-List: bpf@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable Hey Conor! Conor Dooley writes: > On Tue, Apr 02, 2024 at 04:25:24PM +0200, Bj=C3=B6rn T=C3=B6pel wrote: >> Pu Lehui writes: >>=20 >> > On 2024/3/29 6:07, Conor Dooley wrote: >> >> On Thu, Mar 28, 2024 at 03:34:31PM -0400, Stefan O'Rear wrote: >> >>> On Thu, Mar 28, 2024, at 8:49 AM, Pu Lehui wrote: >> >>>> From: Pu Lehui >> >>>> >> >>>> This patch relaxes the restrictions on the Zbb instructions. The ha= rdware >> >>>> is capable of recognizing the Zbb instructions independently, elimi= nating >> >>>> the need for reliance on kernel compile configurations. >> >>> >> >>> This doesn't make sense to me. >> >>=20 >> >> It doesn't make sense to me either. Of course the hardware's capabili= ty >> >> to understand an instruction is independent of whether or not a >> >> toolchain is capable of actually emitting the instruction. >> >>=20 >> >>> RISCV_ISA_ZBB is defined as: >> >>> >> >>> Adds support to dynamically detect the presence of the Z= BB >> >>> extension (basic bit manipulation) and enable its usage. >> >>> >> >>> In other words, RISCV_ISA_ZBB=3Dn should disable everything that att= empts >> >>> to detect Zbb at runtime. It is mostly relevant for code size reduct= ion, >> >>> which is relevant for BPF since if RISCV_ISA_ZBB=3Dn all rvzbb_enabl= ed() >> >>> checks can be constant-folded. >> > >> > Thanks for review. My initial thought was the same as yours, but after= =20 >> > discussions [0] and test verifications, the hardware can indeed=20 >> > recognize the zbb instruction even if the kernel has not enabled=20 >> > CONFIG_RISCV_ISA_ZBB. As Conor mentioned, we are just acting as a JIT = to=20 >> > emit zbb instruction here. Maybe is_hw_zbb_capable() will be better? >>=20 >> I still think Lehui's patch is correct; Building a kernel that can boot >> on multiple platforms (w/ or w/o Zbb support) and not having Zbb insn in >> the kernel proper, and iff Zbb is available at run-time the BPF JIT will >> emit Zbb. > > This sentence is -ENOPARSE to me, did you accidentally omit some words? > Additionally he config option has nothing to do with building kernels that > boot on multiple platforms, it only controls whether optimisations for Zbb > are built so that if Zbb is detected they can be used. Ugh, sorry about that! I'm probably confused myself. >> For these kind of optimizations, (IMO) it's better to let the BPF JIT >> decide at run-time. > > Why is bpf a different case to any other user in this regard? > I think that the commit message is misleading and needs to be changed, > because the point "the hardware is capable of recognising the Zbb > instructions independently..." is completely unrelated to the purpose > of the config option. Of course the hardware understanding the option > has nothing to do with kernel configuration. The commit message needs to > explain why bpf is a special case and is exempt from an=20 > > I totally understand any point about bpf being different in terms of > needing toolchain support, but IIRC it was I who pointed out up-thread. > The part of the conversation that you're replying to here is about the > semantics of the Kconfig option and the original patch never mentioned > trying to avoid a dependency on toolchains at all, just kernel > configurations. The toolchain requirements I don't think are even super > hard to fulfill either - the last 3 versions of ld and lld all meet the > criteria. Thanks for making it more clear, and I agree that the toolchain requirements are not hard to fulfull. My view has been that "BPF is like userland", but I realize now that's odd. Let's make BPF similar to the rest of the RV kernel. If ZBB=3Dn, then the BPF JIT doesn't know about emitting Zbb. Bj=C3=B6rn