Re: [PATCH] riscv: errata: thead: use riscv_nonstd_cache_ops for CMO

Guo Ren
2023-09-13T00:07:13Z

On Wed, Sep 13, 2023 at 3:00 AM Jessica Clarke <jrtc27@jrtc27.com> wrote:
>
> On 12 Sep 2023, at 11:53, Guo Ren <guoren@kernel.org> wrote:
> >
> > On Tue, Sep 12, 2023 at 03:27:40PM +0800, Jisheng Zhang wrote:
> >> Previously, we use alternative mechanism to dynamically patch
> >> the CMO operations for THEAD C906/C910 during boot for performance
> >> reason. But as pointed out by Arnd, "there is already a significant
> >> cost in accessing the invalidated cache lines afterwards, which is
> >> likely going to be much higher than the cost of an indirect branch".
> >> And indeed, there's no performance difference with GMAC and EMMC per
> >> my test on Sipeed Lichee Pi 4A board.
> >>
> >> Use riscv_nonstd_cache_ops for THEAD C906/C910 CMO to simplify
> >> the alternative code, and to acchieve Arnd's goal -- "I think
> >> moving the THEAD ops at the same level as all nonstandard operations
> >> makes sense, but I'd still leave CMO as an explicit fast path that
> >> avoids the indirect branch. This seems like the right thing to do both
> >> for readability and for platforms on which the indirect branch has a
> >> noticeable overhead."
> >>
> >> Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
> >> ---
> >> arch/riscv/Kconfig.errata | 1 +
> >> arch/riscv/errata/thead/errata.c | 76 +++++++++++++++++++++++++++-
> >> arch/riscv/include/asm/errata_list.h | 50 +++---------------
> >> 3 files changed, 81 insertions(+), 46 deletions(-)
> >>
> >> diff --git a/arch/riscv/Kconfig.errata b/arch/riscv/Kconfig.errata
> >> index 566bcefeab50..d7972914f9de 100644
> >> --- a/arch/riscv/Kconfig.errata
> >> +++ b/arch/riscv/Kconfig.errata
> >> @@ -78,6 +78,7 @@ config ERRATA_THEAD_CMO
> >> bool "Apply T-Head cache management errata"
> >> depends on ERRATA_THEAD && MMU
> >> select RISCV_DMA_NONCOHERENT
> >> + select RISCV_NONSTANDARD_CACHE_OPS
> >> default y
> >> help
> >> This will apply the cache management errata to handle the
> >> diff --git a/arch/riscv/errata/thead/errata.c b/arch/riscv/errata/thead/errata.c
> >> index 0554ed4bf087..1c320abfe446 100644
> >> --- a/arch/riscv/errata/thead/errata.c
> >> +++ b/arch/riscv/errata/thead/errata.c
> >> @@ -12,8 +12,10 @@
> >> #include <asm/alternative.h>
> >> #include <asm/cacheflush.h>
> >> #include <asm/cpufeature.h>
> >> +#include <asm/dma-noncoherent.h>
> >> #include <asm/errata_list.h>
> >> #include <asm/hwprobe.h>
> >> +#include <asm/io.h>
> >> #include <asm/patch.h>
> >> #include <asm/vendorid_list.h>
> >>
> >> @@ -33,6 +35,75 @@ static bool errata_probe_pbmt(unsigned int stage,
> >> return false;
> >> }
> >>
> >> +/*
> >> + * dcache.ipa rs1 (invalidate, physical address)
> >> + * | 31 - 25 | 24 - 20 | 19 - 15 | 14 - 12 | 11 - 7 | 6 - 0 |
> >> + * 0000001 01010 rs1 000 00000 0001011
> >> + * dache.iva rs1 (invalida, virtual address)
> >> + * 0000001 00110 rs1 000 00000 0001011
> > Remove dache.iva rs1 ...
> >
> >> + *
> >> + * dcache.cpa rs1 (clean, physical address)
> >> + * | 31 - 25 | 24 - 20 | 19 - 15 | 14 - 12 | 11 - 7 | 6 - 0 |
> >> + * 0000001 01001 rs1 000 00000 0001011
> > Remove dcache.cpa rs1 ...
> >
> >> + * dcache.cva rs1 (clean, virtual address)
> >> + * 0000001 00101 rs1 000 00000 0001011
> > Remove dcache.cva rs1 ...
> >
> >> + *
> >> + * dcache.cipa rs1 (clean then invalidate, physical address)
> >> + * | 31 - 25 | 24 - 20 | 19 - 15 | 14 - 12 | 11 - 7 | 6 - 0 |
> >> + * 0000001 01011 rs1 000 00000 0001011
> >> + * dcache.civa rs1 (... virtual address)
> >> + * 0000001 00111 rs1 000 00000 0001011
> > Remove dcache.civa rs1 ...
> >
> >> + *
> >> + * sync.s (make sure all cache operations finished)
> >> + * | 31 - 25 | 24 - 20 | 19 - 15 | 14 - 12 | 11 - 7 | 6 - 0 |
> >> + * 0000000 11001 00000 000 00000 0001011
> >> + */
> >> +#define THEAD_inval_A0 ".long 0x0265000b"
> >> +#define THEAD_clean_A0 ".long 0x0255000b"
> >> +#define THEAD_flush_A0 ".long 0x0275000b"
> >> +#define THEAD_SYNC_S ".long 0x0190000b"
> >> +
> >> +#define THEAD_CMO_OP(_op, _start, _size, _cachesize) \
> >> +asm volatile("mv a0, %1\n\t" \
> >> + "j 2f\n\t" \
> >> + "3:\n\t" \
> >> + THEAD_##_op##_A0 "\n\t" \
> >> + "add a0, a0, %0\n\t" \
> >> + "2:\n\t" \
> >> + "bltu a0, %2, 3b\n\t" \
> >> + THEAD_SYNC_S \
> >> + : : "r"(_cachesize), \
> >> + "r"((unsigned long)(_start) & ~((_cachesize) - 1UL)), \
> >> + "r"((unsigned long)(_start) + (_size)) \
> >> + : "a0")
> >> +
> >> +static void thead_errata_cache_inv(phys_addr_t paddr, size_t size)
> >> +{
> >> + void *vaddr = phys_to_virt(paddr);
> > No need to phys_to_virt, and we could use paddr directly (dcache.ipa
> > rs1).
> >
> >> +
> >> + THEAD_CMO_OP(inval, vaddr, size, riscv_cbom_block_size);
> >> +}
> >> +
> >> +static void thead_errata_cache_wback(phys_addr_t paddr, size_t size)
> >> +{
> >> + void *vaddr = phys_to_virt(paddr);
> >> +
> >> + THEAD_CMO_OP(clean, vaddr, size, riscv_cbom_block_size);
> >> +}
> > Please remove the thead_errata_cache_wback because T-HEAD processors
> > would prioritize using an invalid cacheline instead of evicting an
> > existing cacheline. When we do dcache clean, the following operations
> > are to let other interconnect masters read. So, keeping wback_inv for
> > T-HEAD processors is the best choice, and maybe some other processors'
> > vendor has a different idea, but please use the wback_inv instead of
> > wback_only for the T-HEAD processors.
>
> Unless you can demonstrate that your cores have significantly worse
> performance when using wback instead of wback_inv I do not think the
> non-standard implementation should deviate from the semantics of the
> standard one. There are efforts to unify the implemented semantics of
> the operations across architectures and this would obstruct those.
I'm afraid I have to disagree with the view that this obstructs
"unifying the implemented semantics of the operations across
architectures."

static const struct riscv_nonstd_cache_ops thead_errata_cmo_ops = {
- .wback = &thead_errata_cache_wback,
+ .wback = &thead_errata_cache_wback_inv,
.inv = &thead_errata_cache_inv,
.wback_inv = &thead_errata_cache_wback_inv,

I don't see how the above patch obstructs unifying. On the contrary,
it decreases the custom function, which could help unify. Could you
give the least respect for the vendor's choice?

>
> Jess
>
> >> +
> >> +static void thead_errata_cache_wback_inv(phys_addr_t paddr, size_t size)
> >> +{
> >> + void *vaddr = phys_to_virt(paddr);
> > Ditto.
> >
> >> +
> >> + THEAD_CMO_OP(flush, vaddr, size, riscv_cbom_block_size);
> >> +}
> >> +
> >> +static const struct riscv_nonstd_cache_ops thead_errata_cmo_ops = {
> >> + .wback = &thead_errata_cache_wback,
> > Please .wback = &thead_errata_cache_wback_inv,
> >
> >
> >> + .inv = &thead_errata_cache_inv,
> >> + .wback_inv = &thead_errata_cache_wback_inv,
> >> +};
> >> +
> >> static bool errata_probe_cmo(unsigned int stage,
> >> unsigned long arch_id, unsigned long impid)
> >> {
> >> @@ -48,6 +119,8 @@ static bool errata_probe_cmo(unsigned int stage,
> >> if (stage == RISCV_ALTERNATIVES_BOOT) {
> >> riscv_cbom_block_size = L1_CACHE_BYTES;
> >> riscv_noncoherent_supported();
> >> + if (IS_ENABLED(CONFIG_RISCV_NONSTANDARD_CACHE_OPS))
> >> + riscv_noncoherent_register_cache_ops(&thead_errata_cmo_ops);
> >> }
> >>
> >> return true;
> >> @@ -77,8 +150,7 @@ static u32 thead_errata_probe(unsigned int stage,
> >> if (errata_probe_pbmt(stage, archid, impid))
> >> cpu_req_errata |= BIT(ERRATA_THEAD_PBMT);
> >>
> >> - if (errata_probe_cmo(stage, archid, impid))
> >> - cpu_req_errata |= BIT(ERRATA_THEAD_CMO);
> >> + errata_probe_cmo(stage, archid, impid);
> >>
> >> if (errata_probe_pmu(stage, archid, impid))
> >> cpu_req_errata |= BIT(ERRATA_THEAD_PMU);
> >> diff --git a/arch/riscv/include/asm/errata_list.h b/arch/riscv/include/asm/errata_list.h
> >> index b55b434f0059..ea33288f8a25 100644
> >> --- a/arch/riscv/include/asm/errata_list.h
> >> +++ b/arch/riscv/include/asm/errata_list.h
> >> @@ -24,9 +24,8 @@
> >>
> >> #ifdef CONFIG_ERRATA_THEAD
> >> #define ERRATA_THEAD_PBMT 0
> >> -#define ERRATA_THEAD_CMO 1
> >> -#define ERRATA_THEAD_PMU 2
> >> -#define ERRATA_THEAD_NUMBER 3
> >> +#define ERRATA_THEAD_PMU 1
> >> +#define ERRATA_THEAD_NUMBER 2
> >> #endif
> >>
> >> #ifdef __ASSEMBLY__
> >> @@ -94,54 +93,17 @@ asm volatile(ALTERNATIVE( \
> >> #define ALT_THEAD_PMA(_val)
> >> #endif
> >>
> >> -/*
> >> - * dcache.ipa rs1 (invalidate, physical address)
> >> - * | 31 - 25 | 24 - 20 | 19 - 15 | 14 - 12 | 11 - 7 | 6 - 0 |
> >> - * 0000001 01010 rs1 000 00000 0001011
> >> - * dache.iva rs1 (invalida, virtual address)
> >> - * 0000001 00110 rs1 000 00000 0001011
> >> - *
> >> - * dcache.cpa rs1 (clean, physical address)
> >> - * | 31 - 25 | 24 - 20 | 19 - 15 | 14 - 12 | 11 - 7 | 6 - 0 |
> >> - * 0000001 01001 rs1 000 00000 0001011
> >> - * dcache.cva rs1 (clean, virtual address)
> >> - * 0000001 00101 rs1 000 00000 0001011
> >> - *
> >> - * dcache.cipa rs1 (clean then invalidate, physical address)
> >> - * | 31 - 25 | 24 - 20 | 19 - 15 | 14 - 12 | 11 - 7 | 6 - 0 |
> >> - * 0000001 01011 rs1 000 00000 0001011
> >> - * dcache.civa rs1 (... virtual address)
> >> - * 0000001 00111 rs1 000 00000 0001011
> >> - *
> >> - * sync.s (make sure all cache operations finished)
> >> - * | 31 - 25 | 24 - 20 | 19 - 15 | 14 - 12 | 11 - 7 | 6 - 0 |
> >> - * 0000000 11001 00000 000 00000 0001011
> >> - */
> >> -#define THEAD_inval_A0 ".long 0x0265000b"
> >> -#define THEAD_clean_A0 ".long 0x0255000b"
> >> -#define THEAD_flush_A0 ".long 0x0275000b"
> >> -#define THEAD_SYNC_S ".long 0x0190000b"
> >> -
> >> #define ALT_CMO_OP(_op, _start, _size, _cachesize) \
> >> -asm volatile(ALTERNATIVE_2( \
> >> - __nops(6), \
> >> +asm volatile(ALTERNATIVE( \
> >> + __nops(5), \
> >> "mv a0, %1\n\t" \
> >> "j 2f\n\t" \
> >> "3:\n\t" \
> >> CBO_##_op(a0) \
> >> "add a0, a0, %0\n\t" \
> >> "2:\n\t" \
> >> - "bltu a0, %2, 3b\n\t" \
> >> - "nop", 0, RISCV_ISA_EXT_ZICBOM, CONFIG_RISCV_ISA_ZICBOM, \
> >> - "mv a0, %1\n\t" \
> >> - "j 2f\n\t" \
> >> - "3:\n\t" \
> >> - THEAD_##_op##_A0 "\n\t" \
> >> - "add a0, a0, %0\n\t" \
> >> - "2:\n\t" \
> >> - "bltu a0, %2, 3b\n\t" \
> >> - THEAD_SYNC_S, THEAD_VENDOR_ID, \
> >> - ERRATA_THEAD_CMO, CONFIG_ERRATA_THEAD_CMO) \
> >> + "bltu a0, %2, 3b\n\t", \
> >> + 0, RISCV_ISA_EXT_ZICBOM, CONFIG_RISCV_ISA_ZICBOM) \
> >> : : "r"(_cachesize), \
> >> "r"((unsigned long)(_start) & ~((_cachesize) - 1UL)), \
> >> "r"((unsigned long)(_start) + (_size)) \
> >> --
> >> 2.40.1
> >>
> >>
> >> _______________________________________________
> >> linux-riscv mailing list
> >> linux-riscv@lists.infradead.org
> >> http://lists.infradead.org/mailman/listinfo/linux-riscv
> >>
> >
> > _______________________________________________
> > linux-riscv mailing list
> > linux-riscv@lists.infradead.org
> > http://lists.infradead.org/mailman/listinfo/linux-riscv
>
>

--
Best Regards
Guo Ren