Re: [PATCH v5 5/8] riscv: cpufeatures: Put the upper 16 bits of patch ID to work

Andrew Jones
2023-02-23T12:53:28Z

On Wed, Feb 22, 2023 at 05:27:47PM +0000, Conor Dooley wrote:
> On Tue, Feb 21, 2023 at 08:09:13PM +0100, Andrew Jones wrote:
> > cpufeature IDs are consecutive integers starting at 26, so a 32-bit
> > patch ID allows an aircraft carrier load of feature IDs. Repurposing
> > the upper 16 bits still leaves a boat load of feature IDs and gains
> > 16 bits which may be used to control patching on a per patch-site
> > basis.
> >
> > This will be initially used in Zicboz's application to clear_page(),
> > as Zicboz's block size must also be considered. In that case, the
> > upper 16-bit value's role will be to convey the maximum block size
> > which the Zicboz clear_page() implementation supports.
> >
> > cpufeature patch sites which need to check for the existence or
> > absence of other cpufeatures may also be able to make use of this.
> >
> > Signed-off-by: Andrew Jones <ajones@ventanamicro.com>
> > ---
> > arch/riscv/include/asm/alternative.h | 3 +++
> > arch/riscv/kernel/cpufeature.c | 37 +++++++++++++++++++++++++---
> > 2 files changed, 36 insertions(+), 4 deletions(-)
> >
> > diff --git a/arch/riscv/include/asm/alternative.h b/arch/riscv/include/asm/alternative.h
> > index 8f39d4e8598d..f2cb543b0bd2 100644
> > --- a/arch/riscv/include/asm/alternative.h
> > +++ b/arch/riscv/include/asm/alternative.h
> > @@ -17,6 +17,9 @@
> > #include <linux/stddef.h>
> > #include <asm/hwcap.h>
> >
> > +#define PATCH_ID_CPUFEATURE_ID(p) ((u16)((u32)(p) & 0xffff))
> > +#define PATCH_ID_CPUFEATURE_VALUE(p) ((u16)(((u32)(p) >> 16) & 0xffff))
>
> I was just fiddling around a bit with macros, I think these do the same
> thing:
> #define PATCH_ID_CPUFEATURE_ID(p) ((p) & GENMASK(15, 0))
> #define PATCH_ID_CPUFEATURE_VALUE(p) FIELD_GET(GENMASK(31, 16), (p))
> Although without the same care about types - is there a specific reason
> you were casting like that?

Just to be pedantic, but I just remembered we already have
upper/lower_16_bits() in linux/kernel.h. I'll use those.

>
> Either way, I think I prefer this approach to the vendor_id stuffing!
> If we do end up needing to fit an aircraft carrier, we can come back and
> revisit another parameter in the alternatives I suppose...
>
> I don't really know if the macros do anything to help with
> understandability, so with or without trying to use macros:
> Reviewed-by: Conor Dooley <conor.dooley@microchip.com>

Thanks,
drew

>
> Thanks,
> Conor.
>
> > #define RISCV_ALTERNATIVES_BOOT 0 /* alternatives applied during regular boot */
> > #define RISCV_ALTERNATIVES_MODULE 1 /* alternatives applied during module-init */
> > #define RISCV_ALTERNATIVES_EARLY_BOOT 2 /* alternatives applied before mmu start */
> > diff --git a/arch/riscv/kernel/cpufeature.c b/arch/riscv/kernel/cpufeature.c
> > index 6102b6bb5db3..0594989ead63 100644
> > --- a/arch/riscv/kernel/cpufeature.c
> > +++ b/arch/riscv/kernel/cpufeature.c
> > @@ -273,12 +273,35 @@ void __init riscv_fill_hwcap(void)
> > }
> >
> > #ifdef CONFIG_RISCV_ALTERNATIVE
> > +/*
> > + * Alternative patch sites consider 48 bits when determining when to patch
> > + * the old instruction sequence with the new. These bits are broken into a
> > + * 16-bit vendor ID and a 32-bit patch ID. A non-zero vendor ID means the
> > + * patch site is for an erratum, identified by the 32-bit patch ID. When
> > + * the vendor ID is zero, the patch site is for a cpufeature. cpufeatures
> > + * further break down patch ID into two 16-bit numbers. The lower 16 bits
> > + * are the cpufeature ID and the upper 16 bits are used for a value specific
> > + * to the cpufeature and patch site. If the upper 16 bits are zero, then it
> > + * implies no specific value is specified. cpufeatures that want to control
> > + * patching on a per-site basis will provide non-zero values and implement
> > + * checks here. The checks return true when patching should be done, and
> > + * false otherwise.
> > + */
> > +static bool riscv_cpufeature_patch_check(u16 id, u16 value)
> > +{
> > + if (!value)
> > + return true;
> > +
> > + return false;
> > +}
> > +
> > void __init_or_module riscv_cpufeature_patch_func(struct alt_entry *begin,
> > struct alt_entry *end,
> > unsigned int stage)
> > {
> > struct alt_entry *alt;
> > void *oldptr, *altptr;
> > + u16 id, value;
> >
> > if (stage == RISCV_ALTERNATIVES_EARLY_BOOT)
> > return;
> > @@ -286,13 +309,19 @@ void __init_or_module riscv_cpufeature_patch_func(struct alt_entry *begin,
> > for (alt = begin; alt < end; alt++) {
> > if (alt->vendor_id != 0)
> > continue;
> > - if (alt->patch_id >= RISCV_ISA_EXT_MAX) {
> > - WARN(1, "This extension id:%d is not in ISA extension list",
> > - alt->patch_id);
> > +
> > + id = PATCH_ID_CPUFEATURE_ID(alt->patch_id);
> > +
> > + if (id >= RISCV_ISA_EXT_MAX) {
> > + WARN(1, "This extension id:%d is not in ISA extension list", id);
> > continue;
> > }
> >
> > - if (!__riscv_isa_extension_available(NULL, alt->patch_id))
> > + if (!__riscv_isa_extension_available(NULL, id))
> > + continue;
> > +
> > + value = PATCH_ID_CPUFEATURE_VALUE(alt->patch_id);
> > + if (!riscv_cpufeature_patch_check(id, value))
> > continue;
> >
> > oldptr = ALT_OLD_PTR(alt);
> > --
> > 2.39.1
> >