Messages in this thread | | | Date | Tue, 27 Feb 2024 11:01:54 +0530 | Subject | Re: [PATCH] arm64/hw_breakpoint: Determine lengths from generic perf breakpoint macros | From | Anshuman Khandual <> |
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On 2/26/24 16:34, Mark Rutland wrote: > 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@kernel.org> >>>> Cc: Mark Rutland <mark.rutland@arm.com> >>>> Cc: Catalin Marinas <catalin.marinas@arm.com> >>>> Cc: linux-arm-kernel@lists.infradead.org >>>> Cc: linux-kernel@vger.kernel.org >>>> Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.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.
Fair enough, but just wondering how about deriving len_in_bytes from hweight_long(ARM_BREAKPOINT_LEN_*) instead ? This also drops the hard coding using the platform macros itself, without going to user ABI.
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