Re: [PATCH v4 5/8] RISC-V: cpufeature: Put vendor_id to work

Conor Dooley
2023-02-09T19:05:10Z

[-- Attachment #1: Type: text/plain, Size: 3587 bytes --]

Hey Drew,

On Thu, Feb 09, 2023 at 04:26:25PM +0100, Andrew Jones wrote:
> When [ab]using alternatives as cpufeature "static keys", which can
> be used in assembly, also put vendor_id to work as application-
> specific data. This will be initially used in Zicboz's application to
> clear_page(), as Zicboz's block size must also be considered. In that
> case, vendor_id's role will be to convey the maximum block size which
> the Zicboz clear_page() implementation supports.
>
> cpufeature alternative applications 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/kernel/cpufeature.c | 10 ++++++++--
> 1 file changed, 8 insertions(+), 2 deletions(-)
>
> diff --git a/arch/riscv/kernel/cpufeature.c b/arch/riscv/kernel/cpufeature.c
> index 0d2db03cf167..74736b4f0624 100644
> --- a/arch/riscv/kernel/cpufeature.c
> +++ b/arch/riscv/kernel/cpufeature.c
> @@ -278,6 +278,11 @@ void __init riscv_fill_hwcap(void)
> }
>
> #ifdef CONFIG_RISCV_ALTERNATIVE

I think a comment here about what "application check" means would be
nice.
That wording just feels clunky & the meaning is not immediately
graspable?

/*
* riscv_cpufeature_application_check() - Check if a cpufeature applies.
* The presence of a cpufeature does not mean it is necessarily
* useable. This function is used to apply the alternative on a
* case-by-case basis.
*/

Dunno, does something like that convey the intent?

> +static bool riscv_cpufeature_application_check(u32 feature, u16 data)
> +{
> + return data == 0;
> +}
> +
> void __init_or_module riscv_cpufeature_patch_func(struct alt_entry *begin,
> struct alt_entry *end,
> unsigned int stage)
> @@ -289,8 +294,6 @@ void __init_or_module riscv_cpufeature_patch_func(struct alt_entry *begin,
> return;
>
> for (alt = begin; alt < end; alt++) {
> - if (alt->vendor_id != 0)
> - continue;

Can you remind me what makes this "safe"?
My understanding was that a vendor_id of zero was safe, as zero is
reserved in JEDEC.
What is stopping someone stuffing this with a given value and
colliding with a real vendor's errata?

for (alt = begin; alt < end; alt++) {
if (alt->vendor_id != A_VENDOR_ID)
continue;
if (alt->errata_id >= ERRATA_A_NUMBER)
continue;

tmp = (1U << alt->errata_id);
if (cpu_req_errata & tmp) {
oldptr = ALT_OLD_PTR(alt);
altptr = ALT_ALT_PTR(alt);

/* On vm-alternatives, the mmu isn't running yet */
if (stage == RISCV_ALTERNATIVES_EARLY_BOOT)
memcpy((void *)__pa_symbol(oldptr),
(void *)__pa_symbol(altptr),
alt->alt_len);
else
patch_text_nosync(oldptr, altptr, alt->alt_len);
}
}

I've probably just missing something, my brain swapped out alternatives
the other week. Hopefully whatever I missed isn't embarrassingly obvious!

Cheers,
Conor.

> if (alt->errata_id >= RISCV_ISA_EXT_MAX) {
> WARN(1, "This extension id:%d is not in ISA extension list",
> alt->errata_id);
> @@ -300,6 +303,9 @@ void __init_or_module riscv_cpufeature_patch_func(struct alt_entry *begin,
> if (!__riscv_isa_extension_available(NULL, alt->errata_id))
> continue;
>
> + if (!riscv_cpufeature_application_check(alt->errata_id, alt->vendor_id))
> + continue;
> +
> oldptr = ALT_OLD_PTR(alt);
> altptr = ALT_ALT_PTR(alt);
> patch_text_nosync(oldptr, altptr, alt->alt_len);
> --
> 2.39.1
>

[-- Attachment #2: signature.asc --]
[-- Type: application/pgp-signature, Size: 228 bytes --]