Re: [PATCH v4 09/10] tools: lib: perf: Implement riscv mmap support

[Ian Rogers]

On Mon, Jul 31, 2023 at 9:06 AM Alexandre Ghiti <alexghiti@xxxxxxxxxxxx> wrote:
>
> On Mon, Jul 31, 2023 at 5:10 PM Ian Rogers <irogers@xxxxxxxxxx> wrote:
> >
> > On Mon, Jul 31, 2023 at 3:27 AM Alexandre Ghiti <alexghiti@xxxxxxxxxxxx> wrote:
> > >
> > > On Mon, Jul 31, 2023 at 12:15 PM Alexandre Ghiti <alexghiti@xxxxxxxxxxxx> wrote:
> > > >
> > > > Hi Ian,
> > > >
> > > > On Fri, Jul 28, 2023 at 7:53 PM Ian Rogers <irogers@xxxxxxxxxx> wrote:
> > > > >
> > > > > On Thu, Jul 27, 2023 at 7:28 AM Alexandre Ghiti <alexghiti@xxxxxxxxxxxx> wrote:
> > > > > >
> > > > > > riscv now supports mmaping hardware counters so add what's needed to
> > > > > > take advantage of that in libperf.
> > > > > >
> > > > > > Signed-off-by: Alexandre Ghiti <alexghiti@xxxxxxxxxxxx>
> > > > > > Reviewed-by: Andrew Jones <ajones@xxxxxxxxxxxxxxxx>
> > > > > > Reviewed-by: Atish Patra <atishp@xxxxxxxxxxxx>
> > > > > > ---
> > > > > >  tools/lib/perf/mmap.c | 65 +++++++++++++++++++++++++++++++++++++++++++
> > > > > >  1 file changed, 65 insertions(+)
> > > > > >
> > > > > > diff --git a/tools/lib/perf/mmap.c b/tools/lib/perf/mmap.c
> > > > > > index 0d1634cedf44..378a163f0554 100644
> > > > > > --- a/tools/lib/perf/mmap.c
> > > > > > +++ b/tools/lib/perf/mmap.c
> > > > > > @@ -392,6 +392,71 @@ static u64 read_perf_counter(unsigned int counter)
> > > > > >
> > > > > >  static u64 read_timestamp(void) { return read_sysreg(cntvct_el0); }
> > > > > >
> > > > > > +#elif __riscv_xlen == 64
> > > > >
> > > > > This is something of an odd guard, perhaps:
> > > > > #elif defined(__riscv) && __riscv_xlen == 64
> > > > >
> > > > > That way it is more intention revealing that this is riscv code. Could
> > > > > you add a comment relating to the __riscv_xlen ?
> > > >
> > > > I guess Andrew answered that already.
> > > >
> >
> > Not sure. I still think it looks weird:
> > ...
> > #if defined(__i386__) || defined(__x86_64__)
> > ...
> > #elif defined(__aarch64__)
> > ...
> > #elif __riscv_xlen == 64
> > ...
> > #else
> > static u64 read_perf_counter(unsigned int counter __maybe_unused) { return 0; }
> > static u64 read_timestamp(void) { return 0; }
> > #endif
> >
> > The first two are clearly #ifdef-ing architecture specific assembly
> > code, under what conditions I get RISC-V code  ¯\(ツ)/¯ At least worth
> > a comment like "csrr is only available when you have xlens of 64
> > because ..."
>
> __riscv_xlen indicates riscv64, which is the only target of this
> patchset. But if you prefer, I don't mind adding back the
> defined(__riscv) if I re-spin a new version.

This kind of begs the question as to why there is no __riscv64 ifdef.
The issue with xlen is it isn't intention revealing so for regular
people trying to understand the code it would be nice to document it.

> >
> > > > >
> > > > > > +
> > > > > > +/* TODO: implement rv32 support */
> > > > > > +
> > > > > > +#define CSR_CYCLE      0xc00
> > > > > > +#define CSR_TIME       0xc01
> > > > > > +
> > > > > > +#define csr_read(csr)                                          \
> > > > > > +({                                                             \
> > > > > > +       register unsigned long __v;                             \
> > > > > > +               __asm__ __volatile__ ("csrr %0, " #csr          \
> > > > > > +                : "=r" (__v) :                                 \
> > > > > > +                : "memory");                                   \
> > > > >
> > > > > To avoid the macro pasting that could potentially go weird, could this be:
> > > > >
> > > > > __asm__ __volatile__ ("csrr %0, %1",
> > > > >   : "=r"(__v) /* outputs */
> > > > >   : "i"(csr) /* inputs */
> > > > >   : "memory" /* clobbers */)
> > >
> > > Forgot to answer this one: it compiles, but I have to admit that I
> > > don't understand the difference and if that's correct (all macros in
> > > arch/riscv/include/asm/csr.h use # to do this) and what benefits it
> > > brings. Can you elaborate more on things that could "go weird"?
> >
> > So rather than use an input constraint for the asm block you are using
> > the C preprocessor to paste in the csr argument. If csr is something
> > like "1" then it looks good and you'll get "csrr %0,1". If you pass
> > something like "1 << 31" then that will be pasted as "csrr %0, 1 <<
> > 31" and that starts to get weird in the context of being in the
> > assembler where it is unlikely the C operators work. Using the input
> > constraint avoids this, causes the C compiler to check the type of the
> > argument and you'll probably get more intelligible error messages as a
> > consequence.
> >
>
> Thanks. So if I'm not mistaken, in this exact context, given we only
> use csr_read() through the csr_read_num() function, it seems ok right?

So you've formed a cargo cult and the justification is not wanting to
stop a copy-paste chain from somewhere else. This code itself will be
copy-pasted and we go to some ends to encourage that by placing parts
of it in include/uapi/linux/perf_event.h. It seems better to catch
this issue early rather than propagate it.

> > >
> > > Thanks again,
> > >
> > > Alex
> > >
> > > > >
> > > > > Also, why is this clobbering memory? Worth adding a comment.
> > > >
> > > > No idea, I see that it is also done this way in
> > > > arch/riscv/include/asm/csr.h. @Atish Kumar Patra , @Palmer Dabbelt ?
> >
> > It would seem to make sense then not to have a memory constraint until
> > we know why we're doing it?
> >
>
> I have just had the answer internally (thanks to @Brendan Sweeney):
> csr modifications can alter how the memory is accessed (satp which
> changes the address space, sum which allows/disallows userspace
> access...), so we need the memory barrier to make sure the compiler
> does not reorder the memory accesses.

The conditions you mention shouldn't apply here though? Even if you
add a memory barrier for the compiler what is stopping the hardware
reordering loads and stores? If it absolutely has to be there then a
comment something like "There is a bug is riscv where the csrr
instruction can clobber memory breaking future reads and some how this
constraint fixes it by ... ".

Thanks,
Ian

> Thanks,
>
> Alex
>
> > Thanks,
> > Ian
> >
> > > >
> > > > Thanks for your comments!
> > > >
> > > > Alex
> > > >
> > > > >
> > > > > Thanks,
> > > > > Ian
> > > > >
> > > > > > +                __v;                                           \
> > > > > > +})
> > > > > > +
> > > > > > +static unsigned long csr_read_num(int csr_num)
> > > > > > +{
> > > > > > +#define switchcase_csr_read(__csr_num, __val)           {\
> > > > > > +       case __csr_num:                                 \
> > > > > > +               __val = csr_read(__csr_num);            \
> > > > > > +               break; }
> > > > > > +#define switchcase_csr_read_2(__csr_num, __val)         {\
> > > > > > +       switchcase_csr_read(__csr_num + 0, __val)        \
> > > > > > +       switchcase_csr_read(__csr_num + 1, __val)}
> > > > > > +#define switchcase_csr_read_4(__csr_num, __val)         {\
> > > > > > +       switchcase_csr_read_2(__csr_num + 0, __val)      \
> > > > > > +       switchcase_csr_read_2(__csr_num + 2, __val)}
> > > > > > +#define switchcase_csr_read_8(__csr_num, __val)         {\
> > > > > > +       switchcase_csr_read_4(__csr_num + 0, __val)      \
> > > > > > +       switchcase_csr_read_4(__csr_num + 4, __val)}
> > > > > > +#define switchcase_csr_read_16(__csr_num, __val)        {\
> > > > > > +       switchcase_csr_read_8(__csr_num + 0, __val)      \
> > > > > > +       switchcase_csr_read_8(__csr_num + 8, __val)}
> > > > > > +#define switchcase_csr_read_32(__csr_num, __val)        {\
> > > > > > +       switchcase_csr_read_16(__csr_num + 0, __val)     \
> > > > > > +       switchcase_csr_read_16(__csr_num + 16, __val)}
> > > > > > +
> > > > > > +       unsigned long ret = 0;
> > > > > > +
> > > > > > +       switch (csr_num) {
> > > > > > +       switchcase_csr_read_32(CSR_CYCLE, ret)
> > > > > > +       default:
> > > > > > +               break;
> > > > > > +       }
> > > > > > +
> > > > > > +       return ret;
> > > > > > +#undef switchcase_csr_read_32
> > > > > > +#undef switchcase_csr_read_16
> > > > > > +#undef switchcase_csr_read_8
> > > > > > +#undef switchcase_csr_read_4
> > > > > > +#undef switchcase_csr_read_2
> > > > > > +#undef switchcase_csr_read
> > > > > > +}
> > > > > > +
> > > > > > +static u64 read_perf_counter(unsigned int counter)
> > > > > > +{
> > > > > > +       return csr_read_num(CSR_CYCLE + counter);
> > > > > > +}
> > > > > > +
> > > > > > +static u64 read_timestamp(void)
> > > > > > +{
> > > > > > +       return csr_read_num(CSR_TIME);
> > > > > > +}
> > > > > > +
> > > > > >  #else
> > > > > >  static u64 read_perf_counter(unsigned int counter __maybe_unused) { return 0; }
> > > > > >  static u64 read_timestamp(void) { return 0; }
> > > > > > --
> > > > > > 2.39.2
> > > > > >