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Message-ID: <ZdxwTkUALQfqjagf@FVFF77S0Q05N>
Date: Mon, 26 Feb 2024 11:04:46 +0000
From: Mark Rutland <mark.rutland@....com>
To: Anshuman Khandual <anshuman.khandual@....com>
Cc: Will Deacon <will@...nel.org>, linux-arm-kernel@...ts.infradead.org,
broonie@...nel.org, Catalin Marinas <catalin.marinas@....com>,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH] arm64/hw_breakpoint: Determine lengths from generic perf
breakpoint macros
On Mon, Feb 26, 2024 at 08:19:39AM +0530, Anshuman Khandual wrote:
> On 2/23/24 18:22, Will Deacon wrote:
> > On Fri, Feb 23, 2024 at 05:01:02PM +0530, Anshuman Khandual wrote:
> >> Both platform i.e ARM_BREAKPOINT_LEN_X and generic i.e HW_BREAKPOINT_LEN_X
> >> macros are used interchangeably to convert event->attr.bp_len and platform
> >> breakpoint control arch_hw_breakpoint_ctrl->len. Let's be consistent while
> >> deriving one from the other. This does not cause any functional changes.
> >>
> >> Cc: Will Deacon <will@...nel.org>
> >> Cc: Mark Rutland <mark.rutland@....com>
> >> Cc: Catalin Marinas <catalin.marinas@....com>
> >> Cc: linux-arm-kernel@...ts.infradead.org
> >> Cc: linux-kernel@...r.kernel.org
> >> Signed-off-by: Anshuman Khandual <anshuman.khandual@....com>
> >> ---
> >> This applies on v6.8-rc5
> >>
> >> arch/arm64/kernel/hw_breakpoint.c | 16 ++++++++--------
> >> 1 file changed, 8 insertions(+), 8 deletions(-)
> >>
> >> diff --git a/arch/arm64/kernel/hw_breakpoint.c b/arch/arm64/kernel/hw_breakpoint.c
> >> index 35225632d70a..1ab9fc865ddd 100644
> >> --- a/arch/arm64/kernel/hw_breakpoint.c
> >> +++ b/arch/arm64/kernel/hw_breakpoint.c
> >> @@ -301,28 +301,28 @@ static int get_hbp_len(u8 hbp_len)
> >>
> >> switch (hbp_len) {
> >> case ARM_BREAKPOINT_LEN_1:
> >> - len_in_bytes = 1;
> >> + len_in_bytes = HW_BREAKPOINT_LEN_1;
> >
> > I don't think we should do this. The HW_BREAKPOINT_LEN_* definitions are
> > part of the user ABI and, although they correspond to the length in bytes,
> > that's not necessarily something we should rely on.
>
> Why should not we rely on the user ABI macros if these byte lengths were
> initially derived from them.
Why should we change the clear:
len_in_bytes = 1;
.. to the longer, and less clear:
len_in_bytes = HW_BREAKPOINT_LEN_1;
.. ?
> But also there are similar conversions in arch_bp_generic_fields().
Those are specifically for converting from the rch_hw_breakpoint_ctrl encodings
to the perf_event_attr encodings. There we don't care about the specific value
of the byte, just that we're using the correct encoding.
> These hard coded raw byte length numbers seems cryptic, where as in reality
> these are just inter converted from generic HW breakpoints lengths.
There are three distinct concepts here:
1. The length in bytes, as returned above by get_hbp_len()
2. The length as encoded in the ARM_BREAKPOINT_LEN_* encoding
3. The length as encoded in the HW_BREAKPOINT_LEN_* encoding.
I think you're arguing that since 1 and 3 happen to have the values we should
treat them as the same thing. I think that Will and I believe that they should
be kept distinct because they are distinct concepts.
I don't think this needs to change, and can be left as-is.
Mark.
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