Re: [PATCH 1/3] RISC-V: Framework for vendor extensions

Charlie Jenkins
2023-07-06T19:51:09Z

On Thu, Jul 06, 2023 at 06:15:57PM +0100, Conor Dooley wrote:
> Hey Charlie,
>
> On Wed, Jul 05, 2023 at 08:30:17PM -0700, Charlie Jenkins wrote:
> > Create Kconfig files, Makefiles, and functions to enable vendors to
> > provide information via the riscv_hwprobe syscall about which vendor
> > extensions are available.
>
> This is all apparently from reading the diff, you don't need to tell us
> what files you have created etc. Please just stick with explaining the
> rationale for your changes (especially anything that might make someone
> reading it go "huh").
>
> >
> > Signed-off-by: Charlie Jenkins <charlie@rivosinc.com>
> > ---
> > arch/riscv/Kbuild | 1 +
> > arch/riscv/Kconfig | 1 +
> > arch/riscv/Kconfig.vendor | 3 +++
> > arch/riscv/include/asm/hwprobe.h | 1 +
> > arch/riscv/kernel/sys_riscv.c | 40 ++++++++++++++++++++++++++++++++---
> > arch/riscv/vendor_extensions/Makefile | 3 +++
> > 6 files changed, 46 insertions(+), 3 deletions(-)
>
> > diff --git a/arch/riscv/Kbuild b/arch/riscv/Kbuild
> > index afa83e307a2e..bea38010d9db 100644
> > --- a/arch/riscv/Kbuild
> > +++ b/arch/riscv/Kbuild
> > @@ -3,6 +3,7 @@
> > obj-y += kernel/ mm/ net/
> > obj-$(CONFIG_BUILTIN_DTB) += boot/dts/
> > obj-y += errata/
> > +obj-y += vendor_extensions/
> > obj-$(CONFIG_KVM) += kvm/
> >
> > obj-$(CONFIG_ARCH_HAS_KEXEC_PURGATORY) += purgatory/
> > diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
> > index c1505c7729ec..19404ede0ee3 100644
> > --- a/arch/riscv/Kconfig
> > +++ b/arch/riscv/Kconfig
> > @@ -276,6 +276,7 @@ config AS_HAS_OPTION_ARCH
> >
> > source "arch/riscv/Kconfig.socs"
> > source "arch/riscv/Kconfig.errata"
> > +source "arch/riscv/Kconfig.vendor"
> >
> > menu "Platform type"
> >
> > diff --git a/arch/riscv/Kconfig.vendor b/arch/riscv/Kconfig.vendor
> > new file mode 100644
> > index 000000000000..213ac3e6fed5
> > --- /dev/null
> > +++ b/arch/riscv/Kconfig.vendor
> > @@ -0,0 +1,3 @@
> > +menu "Vendor extensions selection"
> > +
> > +endmenu # "Vendor extensions selection"
>
> These files don't do anything, don't add them until you need to.
I wasn't sure if it was more clear to split up the vendor extension
framework from the T-Head specific calls since the main goal of this
series is to propose a vendor extension framework. I can merge this with
the following patch.
>
> > diff --git a/arch/riscv/include/asm/hwprobe.h b/arch/riscv/include/asm/hwprobe.h
> > index 78936f4ff513..fadb38b83243 100644
> > --- a/arch/riscv/include/asm/hwprobe.h
> > +++ b/arch/riscv/include/asm/hwprobe.h
> > @@ -9,5 +9,6 @@
> > #include <uapi/asm/hwprobe.h>
> >
> > #define RISCV_HWPROBE_MAX_KEY 5
> > +#define RISCV_HWPROBE_VENDOR_EXTENSION_SPACE (UL(1)<<63)
>
> Should this not be BIT_ULL(63)? Although I am not sure that we can
> actually do this, more on that front later.
>
> >
> > #endif
> > diff --git a/arch/riscv/kernel/sys_riscv.c b/arch/riscv/kernel/sys_riscv.c
> > index 26ef5526bfb4..2351a5f7b8b1 100644
> > --- a/arch/riscv/kernel/sys_riscv.c
> > +++ b/arch/riscv/kernel/sys_riscv.c
> > @@ -188,9 +188,35 @@ static u64 hwprobe_misaligned(const struct cpumask *cpus)
> > return perf;
> > }
> >
> > +static int hwprobe_vendor(__u64 mvendorid, struct riscv_hwprobe *pair,
> > + const struct cpumask *cpus)
> > +{
> > + switch (mvendorid) {
> > + default:
> > + return -1;
> > + }
> > +
> > + return 0;
> > +}
> > +
> > static void hwprobe_one_pair(struct riscv_hwprobe *pair,
> > const struct cpumask *cpus)
> > {
> > + int err;
> > +
> > + if (((unsigned long) pair->key) >= RISCV_HWPROBE_VENDOR_EXTENSION_SPACE) {
>
> Hopefully Bjorn or someone that actually knows a thing or two about uapi
> stuff can chime in here, but I think what you are doing here (where the
> vendor space sets the MSB) really muddies the api. These keys are defined
> as signed 64 bit numbers & -1 is the value set when a key is not valid.
> I'd much rather we kept the negative space off-limits, and used the 62nd
> bit instead, avoiding using negative numbers for valid keys.
>
Yeah that makes sense, I can change this up.
> > + struct riscv_hwprobe mvendorid = {
> > + .key = RISCV_HWPROBE_KEY_MVENDORID,
> > + .value = 0
> > + };
> > +
> > + hwprobe_arch_id(&mvendorid, cpus);
>
> I think this needs a comment explaining why you do this hwprobe call,
> > + if (mvendorid.value != -1ULL)
> > + err = hwprobe_vendor(mvendorid.value, pair, cpus);
> > + else
> > + err = -1;
> > + }
>
> I don't really understand the control flow here. Why are you continuing
> on to the switch statement, if you have either a) already ran
> hwprobe_vendor() or b) noticed that mvendorid.value is not valid?
>
The purpose of this was to consolidate the error handling to a single
spot at the bottom of the file. It would fall through the switch
statement and set the values appropriately. I guess it does seem a bit
awkward.
> > switch (pair->key) {
> > case RISCV_HWPROBE_KEY_MVENDORID:
> > case RISCV_HWPROBE_KEY_MARCHID:
> > @@ -217,13 +243,21 @@ static void hwprobe_one_pair(struct riscv_hwprobe *pair,
> >
> > /*
> > * For forward compatibility, unknown keys don't fail the whole
> > - * call, but get their element key set to -1 and value set to 0
> > - * indicating they're unrecognized.
> > + * call, instead an error is raised to indicate the element key
> > + * is unrecognized.
> > */
> > default:
> > + err = -1;
> > + break;
> > + }
> > +
> > + /*
> > + * Setting the element key to -1 and value to 0 indicates that
> > + * hwprobe was unable to find the requested key.
> > + */
> > + if (err != 0) {
> > pair->key = -1;
> > pair->value = 0;
> > - break;
> > }
> > }
> >
> > diff --git a/arch/riscv/vendor_extensions/Makefile b/arch/riscv/vendor_extensions/Makefile
> > new file mode 100644
> > index 000000000000..e815895e9372
> > --- /dev/null
> > +++ b/arch/riscv/vendor_extensions/Makefile
> > @@ -0,0 +1,3 @@
> > +ifdef CONFIG_RELOCATABLE
> > +KBUILD_CFLAGS += -fno-pie
> > +endif
>
> There are no files in this directory, why do you need to do a dance
> about relocatable kernels?
>
This is framework for the following patch in the series. I saw these
lines in the errata Makefile so I created this Makefile to set up the
following patch in the series. I can merge this patch with the following
patch to make this series more clear.
> Cheers,
> Conor.