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From:   Bhupesh Sharma <bhsharma@redhat.com>
To:     linux-arm-kernel@lists.infradead.org
Cc:     linux-kernel@vger.kernel.org, bhsharma@redhat.com,
        bhupesh.linux@gmail.com, Mark Rutland <mark.rutland@arm.com>,
        Will Deacon <will@kernel.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>
Subject: [PATCH 2/2] perf/arm64: Allow per-task kernel breakpoints
Date:   Tue,  4 Feb 2020 03:56:24 +0530
Message-Id: <1580768784-25868-3-git-send-email-bhsharma@redhat.com>
In-Reply-To: <1580768784-25868-1-git-send-email-bhsharma@redhat.com>
References: <1580768784-25868-1-git-send-email-bhsharma@redhat.com>
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commit 478fcb2cdb23 ("arm64: Debugging support") disallowed per-task
kernel breakpoints on arm64 since these would have potentially
complicated the stepping code.

However, we now have several use-cases (for e.g. perf) which require
per-task address execution h/w breakpoint to be exercised/set on arm64:

For e.g. we can set address execution h/w breakpoints, using the
format prescribed by 'perf-list' command:
mem:<addr>[/len][:access]                          [Hardware breakpoint]

Without this patch, currently 'perf stat -e' reports that per-task
address execution h/w breakpoints are 'not supported' on arm64. See
below:

$ TEST_FUNC="vfs_read"
$ ADDR=0x`cat /proc/kallsyms | grep -P "\\s$TEST_FUNC\$" | cut -f1 -d' '`
$ perf stat -e mem:$ADDR:x -x';' -- cat /proc/cpuinfo > /dev/null

<not supported>;;mem:0xffff00001031dd68:x;0;100.00;;

After this patch, this use-case can be supported:

$ TEST_FUNC="vfs_read"
$ ADDR=0x`cat /proc/kallsyms | grep -P "\\s$TEST_FUNC\$" | cut -f1 -d' '`
$ perf stat -e mem:$ADDR:x -x';' -- cat /proc/cpuinfo > /dev/null

5;;mem:0xfffffe0010361d20:x;912200;100.00;;

Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Will Deacon <will@kernel.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Bhupesh Sharma <bhsharma@redhat.com>
---
 arch/arm64/kernel/hw_breakpoint.c | 7 -------
 1 file changed, 7 deletions(-)

diff --git a/arch/arm64/kernel/hw_breakpoint.c b/arch/arm64/kernel/hw_breakpoint.c
index 0b727edf4104..c28f04e02845 100644
--- a/arch/arm64/kernel/hw_breakpoint.c
+++ b/arch/arm64/kernel/hw_breakpoint.c
@@ -562,13 +562,6 @@ int hw_breakpoint_arch_parse(struct perf_event *bp,
 	hw->address &= ~alignment_mask;
 	hw->ctrl.len <<= offset;
 
-	/*
-	 * Disallow per-task kernel breakpoints since these would
-	 * complicate the stepping code.
-	 */
-	if (hw->ctrl.privilege == AARCH64_BREAKPOINT_EL1 && bp->hw.target)
-		return -EINVAL;
-
 	return 0;
 }
 
-- 
2.7.4