[PATCH v2 3/2] RISC-V: sbi: remove sbi_ecall tracepoints
Radim Krčmář
rkrcmar at ventanamicro.com
Thu Jun 26 01:10:48 PDT 2025
2025-06-25T09:34:15+01:00, David Laight <david.laight.linux at gmail.com>:
> On Wed, 25 Jun 2025 09:51:45 +0200
> Radim Krčmář <rkrcmar at ventanamicro.com> wrote:
>> 2025-06-24T15:09:09+02:00, Radim Krčmář <rkrcmar at ventanamicro.com>:
>> > For another example, let's have the following function:
>> >
>> > struct sbiret some_sbi_ecall(uintptr_t a0, uintptr_t a1)
>> > {
>> > return sbi_ecall(123, 456, a0, a1);
>> > }
>> >
> ...
>>
>> GCC 15.1 still leaves "mv" outside the branch, but at least seems to be
>> on the right track (undesired overhead is marked with leading stars):
>>
>> 0xffffffff800236e8 <+0>: addi sp,sp,-48
>> 0xffffffff800236ea <+2>: sd s0,32(sp)
>> 0xffffffff800236ec <+4>: sd ra,40(sp)
>> 0xffffffff800236ee <+6>: addi s0,sp,48
>> * 0xffffffff800236f0 <+8>: mv a4,a0
>> * 0xffffffff800236f2 <+10>: mv a5,a1
>> 0xffffffff800236f4 <+12>: nop
>> * 0xffffffff800236f8 <+16>: mv a0,a4
>> * 0xffffffff800236fa <+18>: mv a1,a5
>> 0xffffffff800236fc <+20>: li a7,123
>> 0xffffffff80023700 <+24>: li a6,456
>> 0xffffffff80023704 <+28>: ecall
>> * 0xffffffff80023708 <+32>: mv a5,a0
>> * 0xffffffff8002370a <+34>: mv a2,a1
>> 0xffffffff8002370c <+36>: nop
>> 0xffffffff80023710 <+40>: ld ra,40(sp)
>> 0xffffffff80023712 <+42>: ld s0,32(sp)
>> * 0xffffffff80023714 <+44>: mv a0,a5
>> * 0xffffffff80023716 <+46>: mv a1,a2
>> 0xffffffff80023718 <+48>: addi sp,sp,48
>> 0xffffffff8002371a <+50>: ret
>> [Tracing goes to +126]
>
> How much do a few register moves/spills matter compared to the
> cost of the called code?
I didn't do any serious analysis... In general, simpler functions are
going to suffer a higher ratio of overhead from adding tracepoints, and
the constant overhead per added tracepoint increases with the number of
its arguments.
For a trap to kernel mode that passes through a dozen tracepoint sites,
we could save under a hundred instructions if disabled tracepoints had
the minimal overhead. I don't have a good idea how much of the total
trap execution-time/instruction-count/entropy-increase that actually is.
> There will but much worse things out there if you look.
Definitely, I am trying my best not to look, but I sometimes happen to
stumble upon something, and try to understand it.
Waiting till the tracepoint overhead resolves itself sounds fine to me.
Thanks.
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