Re: [PATCH -next v19 03/24] riscv: hwprobe: Add support for RISCV_HWPROBE_BASE_BEHAVIOR_V

Evan Green
2023-05-09T18:30:17Z

On Tue, May 9, 2023 at 10:59 AM Palmer Dabbelt <palmer@dabbelt.com> wrote:
>
> On Tue, 09 May 2023 10:32:03 PDT (-0700), Evan Green wrote:
> > On Tue, May 9, 2023 at 9:41 AM Andy Chiu <andy.chiu@sifive.com> wrote:
> >>
> >> On Tue, May 9, 2023 at 7:05 PM Heiko Stübner <heiko@sntech.de> wrote:
> >> >
> >> > Am Dienstag, 9. Mai 2023, 12:30:12 CEST schrieb Andy Chiu:
> >> > > Probing kernel support for Vector extension is available now.
> >> > >
> >> > > Signed-off-by: Andy Chiu <andy.chiu@sifive.com>
> >> > > ---
> >> > > Documentation/riscv/hwprobe.rst | 10 ++++++++++
> >> > > arch/riscv/include/asm/hwprobe.h | 2 +-
> >> > > arch/riscv/include/uapi/asm/hwprobe.h | 3 +++
> >> > > arch/riscv/kernel/sys_riscv.c | 9 +++++++++
> >> > > 4 files changed, 23 insertions(+), 1 deletion(-)
> >> > >
> >> > > diff --git a/Documentation/riscv/hwprobe.rst b/Documentation/riscv/hwprobe.rst
> >> > > index 9f0dd62dcb5d..b8755e180fbf 100644
> >> > > --- a/Documentation/riscv/hwprobe.rst
> >> > > +++ b/Documentation/riscv/hwprobe.rst
> >> > > @@ -53,6 +53,9 @@ The following keys are defined:
> >> > > programs (it may still be executed in userspace via a
> >> > > kernel-controlled mechanism such as the vDSO).
> >> > >
> >> > > + * :c:macro:`RISCV_HWPROBE_BASE_BEHAVIOR_V`: Support for Vector extension, as
> >> > > + defined by verion 1.0 of the RISC-V Vector extension.
> >> >
> >> > ^^ version [missing the S]
> >> >
> >> > > +
> >> > > * :c:macro:`RISCV_HWPROBE_KEY_IMA_EXT_0`: A bitmask containing the extensions
> >> > > that are compatible with the :c:macro:`RISCV_HWPROBE_BASE_BEHAVIOR_IMA`:
> >> > > base system behavior.
> >> > > @@ -64,6 +67,13 @@ The following keys are defined:
> >> > > * :c:macro:`RISCV_HWPROBE_IMA_C`: The C extension is supported, as defined
> >> > > by version 2.2 of the RISC-V ISA manual.
> >> > >
> >> > > +* :c:macro:`RISCV_HWPROBE_KEY_V_EXT_0`: A bitmask containing the extensions
> >> > > + that are compatible with the :c:macro:`RISCV_HWPROBE_BASE_BEHAVIOR_V`: base
> >> > > + system behavior.
> >> > > +
> >> > > + * :c:macro:`RISCV_HWPROBE_V`: The V extension is supported, as defined by
> >> > > + version 1.0 of the RISC-V Vector extension manual.
> >> > > +
> >> >
> >> > this seems to be doubling the RISCV_HWPROBE_BASE_BEHAVIOR_V state without
> >> > adding additional information? Both essentially tell the system that
> >> > V extension "defined by verion 1.0 of the RISC-V Vector extension" is supported.
> >>
> >> I was thinking that RISCV_HWPROBE_BASE_BEHAVIOR_V indicates the kernel
> >> has a probe for vector (just like RISCV_HWPROBE_BASE_BEHAVIOR_IMA) and
> >> RISCV_HWPROBE_KEY_V_EXT_0 is where the kernel reports what exactly the
> >> extension is. This maps to the condition matching of F,D, and C in
> >> IMA. If that is not the case then I think there is no need for this
> >> entry.
> >>
> >> >
> >> > I don't question that we'll probably need a key for deeper vector-
> >> > specifics but I guess I'd the commit message should definitly explain
> >> > why there is a duplication here.
> >>
> >> I suppose something like Zvfh should fall into the category of
> >> RISCV_HWPROBE_KEY_V_EXT_0. I will add this example into the commit
> >> message if you agree that is a good example.
> >>
> >> >
> >> >
> >> > > * :c:macro:`RISCV_HWPROBE_KEY_CPUPERF_0`: A bitmask that contains performance
> >> > > information about the selected set of processors.
> >> > >
> >> > > diff --git a/arch/riscv/include/asm/hwprobe.h b/arch/riscv/include/asm/hwprobe.h
> >> > > index 78936f4ff513..39df8604fea1 100644
> >> > > --- a/arch/riscv/include/asm/hwprobe.h
> >> > > +++ b/arch/riscv/include/asm/hwprobe.h
> >> > > @@ -8,6 +8,6 @@
> >> > >
> >> > > #include <uapi/asm/hwprobe.h>
> >> > >
> >> > > -#define RISCV_HWPROBE_MAX_KEY 5
> >> > > +#define RISCV_HWPROBE_MAX_KEY 6
> >> > >
> >> > > #endif
> >> > > diff --git a/arch/riscv/include/uapi/asm/hwprobe.h b/arch/riscv/include/uapi/asm/hwprobe.h
> >> > > index 8d745a4ad8a2..93a7fd3fd341 100644
> >> > > --- a/arch/riscv/include/uapi/asm/hwprobe.h
> >> > > +++ b/arch/riscv/include/uapi/asm/hwprobe.h
> >> > > @@ -22,6 +22,7 @@ struct riscv_hwprobe {
> >> > > #define RISCV_HWPROBE_KEY_MIMPID 2
> >> > > #define RISCV_HWPROBE_KEY_BASE_BEHAVIOR 3
> >> > > #define RISCV_HWPROBE_BASE_BEHAVIOR_IMA (1 << 0)
> >> > > +#define RISCV_HWPROBE_BASE_BEHAVIOR_V (1 << 1)
> >> > > #define RISCV_HWPROBE_KEY_IMA_EXT_0 4
> >> > > #define RISCV_HWPROBE_IMA_FD (1 << 0)
> >> > > #define RISCV_HWPROBE_IMA_C (1 << 1)
> >> > > @@ -32,6 +33,8 @@ struct riscv_hwprobe {
> >> > > #define RISCV_HWPROBE_MISALIGNED_FAST (3 << 0)
> >> > > #define RISCV_HWPROBE_MISALIGNED_UNSUPPORTED (4 << 0)
> >> > > #define RISCV_HWPROBE_MISALIGNED_MASK (7 << 0)
> >> > > +#define RISCV_HWPROBE_KEY_V_EXT_0 6
> >> > > +#define RISCV_HWPROBE_V (1 << 0)
> >> > > /* Increase RISCV_HWPROBE_MAX_KEY when adding items. */
> >> > >
> >> > > #endif
> >> > > diff --git a/arch/riscv/kernel/sys_riscv.c b/arch/riscv/kernel/sys_riscv.c
> >> > > index 5db29683ebee..6280a7f778b3 100644
> >> > > --- a/arch/riscv/kernel/sys_riscv.c
> >> > > +++ b/arch/riscv/kernel/sys_riscv.c
> >> > > @@ -10,6 +10,7 @@
> >> > > #include <asm/cpufeature.h>
> >> > > #include <asm/hwprobe.h>
> >> > > #include <asm/sbi.h>
> >> > > +#include <asm/vector.h>
> >> > > #include <asm/switch_to.h>
> >> > > #include <asm/uaccess.h>
> >> > > #include <asm/unistd.h>
> >> > > @@ -161,6 +162,7 @@ static void hwprobe_one_pair(struct riscv_hwprobe *pair,
> >> > > */
> >> > > case RISCV_HWPROBE_KEY_BASE_BEHAVIOR:
> >> > > pair->value = RISCV_HWPROBE_BASE_BEHAVIOR_IMA;
> >> > > + pair->value |= RISCV_HWPROBE_BASE_BEHAVIOR_V;
> >> >
> >> > Doesn't this also need a
> >> > if (has_vector())
> >> >
> >>
> >> If the RISCV_HWPROBE_KEY_BASE_BEHAVIOR part just tells whether hwprobe
> >> supports probing of a set of extensions then I think we should not add
> >> the if statement here, but maybe I misunderstood something..
> >
> > The intention was to show that the I, M, and A extensions are actually
> > present on this machine, not that the other probe keys exist. Usermode
> > is allowed to query any hwprobe key, they just get back the key set to
> > -1 and value set to 0 on unknown keys. We "cheated" a bit for
> > determining I, M, and A exist since they're already prerequisites of
> > Linux, which is why there's no conditional there.
>
> We should probably add a comment so it doesn't trip someone up again.

There is one there, it just got clipped in the context diff. It looks
like this (after gmail mangles it):

/*
* The kernel already assumes that the base single-letter ISA
* extensions are supported on all harts, and only supports the
* IMA base, so just cheat a bit here and tell that to
* userspace.
*/
case RISCV_HWPROBE_KEY_BASE_BEHAVIOR:
pair->value = RISCV_HWPROBE_BASE_BEHAVIOR_IMA;
break;