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[60.242.181.102]) by smtp.gmail.com with ESMTPSA id u13sm1893006pfh.123.2021.08.04.01.53.52 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 04 Aug 2021 01:53:52 -0700 (PDT) Date: Wed, 04 Aug 2021 18:53:47 +1000 From: Nicholas Piggin Subject: Re: [PATCH] powerpc: Remove MSR_PR check in interrupt_exit_{user/kernel}_prepare() To: Benjamin Herrenschmidt , Christophe Leroy , Michael Ellerman , Paul Mackerras Cc: linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org References: <1628064412.48kzr1eula.astroid@bobo.none> In-Reply-To: MIME-Version: 1.0 Message-Id: <1628067212.yli25q0lwj.astroid@bobo.none> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Excerpts from Christophe Leroy's message of August 4, 2021 6:37 pm: >=20 >=20 > Le 04/08/2021 =C3=A0 10:08, Nicholas Piggin a =C3=A9crit=C2=A0: >> Excerpts from Christophe Leroy's message of August 4, 2021 3:27 pm: >>> In those hot functions that are called at every interrupt, any saved >>> cycle is worth it. >>> >>> interrupt_exit_user_prepare() and interrupt_exit_kernel_prepare() are >>> called from three places: >>> - From entry_32.S >>> - From interrupt_64.S >>> - From interrupt_exit_user_restart() and interrupt_exit_kernel_restart(= ) >>> >>> In entry_32.S, there are inambiguously called based on MSR_PR: >>> >>> interrupt_return: >>> lwz r4,_MSR(r1) >>> addi r3,r1,STACK_FRAME_OVERHEAD >>> andi. r0,r4,MSR_PR >>> beq .Lkernel_interrupt_return >>> bl interrupt_exit_user_prepare >>> ... >>> .Lkernel_interrupt_return: >>> bl interrupt_exit_kernel_prepare >>> >>> In interrupt_64.S, that's similar: >>> >>> interrupt_return_\srr\(): >>> ld r4,_MSR(r1) >>> andi. r0,r4,MSR_PR >>> beq interrupt_return_\srr\()_kernel >>> interrupt_return_\srr\()_user: /* make backtraces match the _kernel va= riant */ >>> addi r3,r1,STACK_FRAME_OVERHEAD >>> bl interrupt_exit_user_prepare >>> ... >>> interrupt_return_\srr\()_kernel: >>> addi r3,r1,STACK_FRAME_OVERHEAD >>> bl interrupt_exit_kernel_prepare >>> >>> In interrupt_exit_user_restart() and interrupt_exit_kernel_restart(), >>> MSR_PR is verified respectively by BUG_ON(!user_mode(regs)) and >>> BUG_ON(user_mode(regs)) prior to calling interrupt_exit_user_prepare() >>> and interrupt_exit_kernel_prepare(). >>> >>> The verification in interrupt_exit_user_prepare() and >>> interrupt_exit_kernel_prepare() are therefore useless and can be remove= d. >>> >>> Signed-off-by: Christophe Leroy >>=20 >> Probably okay to do now things are ironing out. >>=20 >> Unless we want to make a new define for interrupt handler debug and put >> a bunch of these asserts under it. There's quite a lot more here, and >> in asm/interrupt.h, etc. >=20 > But that one is so trivial that I'm not sure there is any point in keepin= g it even as a kind of=20 > additional DEBUG level, unless you want those BUG_ONs because you don't t= rust the compiler. Fair point. Acked-by: Nicholas Piggin