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Tue, 17 Nov 2020 19:20:54 -0800 (PST) Date: Tue, 17 Nov 2020 19:20:51 -0800 Message-Id: <20201118032051.1405907-1-pcc@google.com> Mime-Version: 1.0 X-Mailer: git-send-email 2.29.2.299.gdc1121823c-goog Subject: [PATCH] arm64: mte: optimize asynchronous tag check fault flag check From: Peter Collingbourne To: Catalin Marinas X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20201117_222058_520548_54DBCECB X-CRM114-Status: GOOD ( 17.12 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Peter Collingbourne , Linux ARM Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org We don't need to check for MTE support before checking the flag because it can only be set if the hardware supports MTE. As a result we can unconditionally check the flag bit which is expected to be in a register and therefore the check can be done in a single instruction instead of first needing to load the hwcaps. On a DragonBoard 845c with a kernel built with CONFIG_ARM64_MTE=y with the powersave governor this reduces the cost of a kernel entry/exit (invalid syscall) from 465.1ns to 463.8ns. Signed-off-by: Peter Collingbourne Link: https://linux-review.googlesource.com/id/If4dc3501fd4e4f287322f17805509613cfe47d24 --- arch/arm64/kernel/syscall.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/arch/arm64/kernel/syscall.c b/arch/arm64/kernel/syscall.c index e4c0dadf0d92..f533e83fd722 100644 --- a/arch/arm64/kernel/syscall.c +++ b/arch/arm64/kernel/syscall.c @@ -123,7 +123,7 @@ static void el0_svc_common(struct pt_regs *regs, int scno, int sc_nr, local_daif_restore(DAIF_PROCCTX); user_exit(); - if (system_supports_mte() && (flags & _TIF_MTE_ASYNC_FAULT)) { + if (IS_ENABLED(CONFIG_ARM64_MTE) && (flags & _TIF_MTE_ASYNC_FAULT)) { /* * Process the asynchronous tag check fault before the actual * syscall. do_notify_resume() will send a signal to userspace