Re: [PATCH 2/3] riscv: Implement Zicbom-based cache management operations

Samuel Holland
2022-06-12T19:15:12Z

On 6/9/22 7:43 PM, Heiko Stuebner wrote:
> The Zicbom ISA-extension was ratified in november 2021
> and introduces instructions for dcache invalidate, clean
> and flush operations.
>
> Implement cache management operations based on them.
>
> Of course not all cores will support this, so implement an
> alternative-based mechanism that replaces empty instructions
> with ones done around Zicbom instructions.
>
> We're using prebuild instructions for the Zicbom instructions
> for now, to not require a bleeding-edge compiler (gcc-12)
> for these somewhat simple instructions.
>
> Signed-off-by: Heiko Stuebner <heiko@sntech.de>
> Reviewed-by: Anup Patel <anup@brainfault.org>
> Reviewed-by: Guo Ren <guoren@kernel.org>
> Cc: Christoph Hellwig <hch@lst.de>
> Cc: Atish Patra <atish.patra@wdc.com>
> Cc: Guo Ren <guoren@kernel.org>
> ---
> arch/riscv/Kconfig | 15 +++++
> arch/riscv/include/asm/cacheflush.h | 6 ++
> arch/riscv/include/asm/errata_list.h | 36 ++++++++++-
> arch/riscv/include/asm/hwcap.h | 1 +
> arch/riscv/kernel/cpu.c | 1 +
> arch/riscv/kernel/cpufeature.c | 18 ++++++
> arch/riscv/kernel/setup.c | 2 +
> arch/riscv/mm/Makefile | 1 +
> arch/riscv/mm/dma-noncoherent.c | 93 ++++++++++++++++++++++++++++
> 9 files changed, 172 insertions(+), 1 deletion(-)
> create mode 100644 arch/riscv/mm/dma-noncoherent.c
>
> diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
> index 32ffef9f6e5b..384d0c15f2b6 100644
> --- a/arch/riscv/Kconfig
> +++ b/arch/riscv/Kconfig
> @@ -376,6 +376,21 @@ config RISCV_ISA_SVPBMT
>
> If you don't know what to do here, say Y.
>
> +config RISCV_ISA_ZICBOM
> + bool "Zicbom extension support for non-coherent dma operation"
> + select ARCH_HAS_DMA_PREP_COHERENT
> + select ARCH_HAS_SYNC_DMA_FOR_DEVICE
> + select ARCH_HAS_SYNC_DMA_FOR_CPU
> + select ARCH_HAS_SETUP_DMA_OPS

ARCH_HAS_SETUP_DMA_OPS needs to be separate from the non-coherent DMA option,
because iommu_setup_dma_ops() will need to be called from arch_setup_dma_ops()
even on a fully-coherent system. (But this change is not strictly necessary for
this series.)

> + select DMA_DIRECT_REMAP
> + select RISCV_ALTERNATIVE
> + default y
> + help
> + Adds support to dynamically detect the presence of the ZICBOM extension
> + (Cache Block Management Operations) and enable its usage.
> +
> + If you don't know what to do here, say Y.
> +
> config FPU
> bool "FPU support"
> default y
> diff --git a/arch/riscv/include/asm/cacheflush.h b/arch/riscv/include/asm/cacheflush.h
> index 23ff70350992..eb12d014b158 100644
> --- a/arch/riscv/include/asm/cacheflush.h
> +++ b/arch/riscv/include/asm/cacheflush.h
> @@ -42,6 +42,12 @@ void flush_icache_mm(struct mm_struct *mm, bool local);
>
> #endif /* CONFIG_SMP */
>
> +#ifdef CONFIG_RISCV_ISA_ZICBOM
> +void riscv_init_cbom_blocksize(void);
> +#else
> +static inline void riscv_init_cbom_blocksize(void) { }
> +#endif
> +
> /*
> * Bits in sys_riscv_flush_icache()'s flags argument.
> */
> diff --git a/arch/riscv/include/asm/errata_list.h b/arch/riscv/include/asm/errata_list.h
> index 398e351e7002..2e80a75b5241 100644
> --- a/arch/riscv/include/asm/errata_list.h
> +++ b/arch/riscv/include/asm/errata_list.h
> @@ -20,7 +20,8 @@
> #endif
>
> #define CPUFEATURE_SVPBMT 0
> -#define CPUFEATURE_NUMBER 1
> +#define CPUFEATURE_CMO 1

This seems like it should be CPUFEATURE_ZICBOM for consistency.

> +#define CPUFEATURE_NUMBER 2
>
> #ifdef __ASSEMBLY__
>
> @@ -87,6 +88,39 @@ asm volatile(ALTERNATIVE( \
> #define ALT_THEAD_PMA(_val)
> #endif
>
> +/*
> + * cbo.clean rs1
> + * | 31 - 20 | 19 - 15 | 14 - 12 | 11 - 7 | 6 - 0 |
> + * 0...01 rs1 010 00000 0001111
> + *
> + * cbo.flush rs1
> + * | 31 - 20 | 19 - 15 | 14 - 12 | 11 - 7 | 6 - 0 |
> + * 0...10 rs1 010 00000 0001111
> + *
> + * cbo.inval rs1
> + * | 31 - 20 | 19 - 15 | 14 - 12 | 11 - 7 | 6 - 0 |
> + * 0...00 rs1 010 00000 0001111
> + */
> +#define CBO_INVAL_A0 ".long 0x15200F"
> +#define CBO_CLEAN_A0 ".long 0x25200F"
> +#define CBO_FLUSH_A0 ".long 0x05200F"
> +
> +#define ALT_CMO_OP(_op, _start, _size, _cachesize) \
> +asm volatile(ALTERNATIVE( \
> + __nops(5), \
> + "mv a0, %1\n\t" \
> + "j 2f\n\t" \
> + "3:\n\t" \
> + CBO_##_op##_A0 "\n\t" \
> + "add a0, a0, %0\n\t" \
> + "2:\n\t" \
> + "bltu a0, %2, 3b\n\t", 0, \
> + CPUFEATURE_CMO, CONFIG_RISCV_ISA_ZICBOM) \
> + : : "r"(_cachesize), \
> + "r"(ALIGN((_start), (_cachesize))), \
> + "r"(ALIGN((_start) + (_size), (_cachesize))) \

Here, the starting address needs to be truncated, not aligned upward. Otherwise,
the first partial cache line will be missed. And since the starting address is
known to be a multiple of the cache line size, the ending address does not need
to be rounded up either:

- "r"(ALIGN((_start), (_cachesize))), \
- "r"(ALIGN((_start) + (_size), (_cachesize))) \
+ "r"((_start) & ~((_cachesize) - 1UL)), \
+ "r"((_start) + (_size)) \

Then to prevent causing any collateral damage, you need to make sure slab
allocations can not share a cache block:

--- a/arch/riscv/include/asm/cache.h
+++ b/arch/riscv/include/asm/cache.h
@@ -11,6 +11,8 @@

#define L1_CACHE_BYTES (1 << L1_CACHE_SHIFT)

+#define ARCH_DMA_MINALIGN L1_CACHE_BYTES
+
/*
* RISC-V requires the stack pointer to be 16-byte aligned, so ensure that
* the flat loader aligns it accordingly.

> + : "a0")
> +
> #endif /* __ASSEMBLY__ */
>
> #endif
> diff --git a/arch/riscv/include/asm/hwcap.h b/arch/riscv/include/asm/hwcap.h
> index 4e2486881840..6044e402003d 100644
> --- a/arch/riscv/include/asm/hwcap.h
> +++ b/arch/riscv/include/asm/hwcap.h
> @@ -53,6 +53,7 @@ extern unsigned long elf_hwcap;
> enum riscv_isa_ext_id {
> RISCV_ISA_EXT_SSCOFPMF = RISCV_ISA_EXT_BASE,
> RISCV_ISA_EXT_SVPBMT,
> + RISCV_ISA_EXT_ZICBOM,
> RISCV_ISA_EXT_ID_MAX = RISCV_ISA_EXT_MAX,
> };
>
> diff --git a/arch/riscv/kernel/cpu.c b/arch/riscv/kernel/cpu.c
> index fba9e9f46a8c..0365557f7122 100644
> --- a/arch/riscv/kernel/cpu.c
> +++ b/arch/riscv/kernel/cpu.c
> @@ -89,6 +89,7 @@ int riscv_of_parent_hartid(struct device_node *node)
> static struct riscv_isa_ext_data isa_ext_arr[] = {
> __RISCV_ISA_EXT_DATA(sscofpmf, RISCV_ISA_EXT_SSCOFPMF),
> __RISCV_ISA_EXT_DATA(svpbmt, RISCV_ISA_EXT_SVPBMT),
> + __RISCV_ISA_EXT_DATA(zicbom, RISCV_ISA_EXT_ZICBOM),
> __RISCV_ISA_EXT_DATA("", RISCV_ISA_EXT_MAX),
> };
>
> diff --git a/arch/riscv/kernel/cpufeature.c b/arch/riscv/kernel/cpufeature.c
> index 6a40cb8134bd..e956f4d763ec 100644
> --- a/arch/riscv/kernel/cpufeature.c
> +++ b/arch/riscv/kernel/cpufeature.c
> @@ -199,6 +199,7 @@ void __init riscv_fill_hwcap(void)
> } else {
> SET_ISA_EXT_MAP("sscofpmf", RISCV_ISA_EXT_SSCOFPMF);
> SET_ISA_EXT_MAP("svpbmt", RISCV_ISA_EXT_SVPBMT);
> + SET_ISA_EXT_MAP("zicbom", RISCV_ISA_EXT_ZICBOM);
> }
> #undef SET_ISA_EXT_MAP
> }
> @@ -259,6 +260,20 @@ static bool __init_or_module cpufeature_probe_svpbmt(unsigned int stage)
> return false;
> }
>
> +static bool __init_or_module cpufeature_probe_cmo(unsigned int stage)
> +{
> +#ifdef CONFIG_RISCV_ISA_ZICBOM
> + switch (stage) {
> + case RISCV_ALTERNATIVES_EARLY_BOOT:
> + return false;
> + default:
> + return riscv_isa_extension_available(NULL, ZICBOM);
> + }
> +#endif
> +
> + return false;
> +}
> +
> /*
> * Probe presence of individual extensions.
> *
> @@ -273,6 +288,9 @@ static u32 __init_or_module cpufeature_probe(unsigned int stage)
> if (cpufeature_probe_svpbmt(stage))
> cpu_req_feature |= (1U << CPUFEATURE_SVPBMT);
>
> + if (cpufeature_probe_cmo(stage))
> + cpu_req_feature |= (1U << CPUFEATURE_CMO);
> +
> return cpu_req_feature;
> }
>
> diff --git a/arch/riscv/kernel/setup.c b/arch/riscv/kernel/setup.c
> index f0f36a4a0e9b..95ef6e2bf45c 100644
> --- a/arch/riscv/kernel/setup.c
> +++ b/arch/riscv/kernel/setup.c
> @@ -22,6 +22,7 @@
> #include <linux/crash_dump.h>
>
> #include <asm/alternative.h>
> +#include <asm/cacheflush.h>
> #include <asm/cpu_ops.h>
> #include <asm/early_ioremap.h>
> #include <asm/pgtable.h>
> @@ -296,6 +297,7 @@ void __init setup_arch(char **cmdline_p)
> #endif
>
> riscv_fill_hwcap();
> + riscv_init_cbom_blocksize();
> apply_boot_alternatives();
> }
>
> diff --git a/arch/riscv/mm/Makefile b/arch/riscv/mm/Makefile
> index ac7a25298a04..548f2f3c00e9 100644
> --- a/arch/riscv/mm/Makefile
> +++ b/arch/riscv/mm/Makefile
> @@ -30,3 +30,4 @@ endif
> endif
>
> obj-$(CONFIG_DEBUG_VIRTUAL) += physaddr.o
> +obj-$(CONFIG_RISCV_ISA_ZICBOM) += dma-noncoherent.o
> diff --git a/arch/riscv/mm/dma-noncoherent.c b/arch/riscv/mm/dma-noncoherent.c
> new file mode 100644
> index 000000000000..f77f4c529835
> --- /dev/null
> +++ b/arch/riscv/mm/dma-noncoherent.c
> @@ -0,0 +1,93 @@
> +// SPDX-License-Identifier: GPL-2.0-only
> +/*
> + * RISC-V specific functions to support DMA for non-coherent devices
> + *
> + * Copyright (c) 2021 Western Digital Corporation or its affiliates.
> + */
> +
> +#include <linux/dma-direct.h>
> +#include <linux/dma-map-ops.h>
> +#include <linux/init.h>
> +#include <linux/io.h>
> +#include <linux/libfdt.h>
> +#include <linux/mm.h>
> +#include <linux/of.h>
> +#include <linux/of_device.h>
> +#include <asm/cacheflush.h>
> +
> +static unsigned int riscv_cbom_block_size = L1_CACHE_BYTES;
> +
> +void arch_sync_dma_for_device(phys_addr_t paddr, size_t size, enum dma_data_direction dir)
> +{
> + switch (dir) {
> + case DMA_TO_DEVICE:
> + ALT_CMO_OP(CLEAN, (unsigned long)phys_to_virt(paddr), size, riscv_cbom_block_size);
> + break;
> + case DMA_FROM_DEVICE:
> + ALT_CMO_OP(INVAL, (unsigned long)phys_to_virt(paddr), size, riscv_cbom_block_size);
> + break;

arch_sync_dma_for_device(DMA_FROM_DEVICE) is a no-op from the CPU's perspective.
Invalidating the CPU's cache goes in arch_sync_dma_for_cpu(DMA_FROM_DEVICE).

> + case DMA_BIDIRECTIONAL:
> + ALT_CMO_OP(FLUSH, (unsigned long)phys_to_virt(paddr), size, riscv_cbom_block_size);
> + break;
> + default:
> + break;
> + }
> +}
> +
> +void arch_sync_dma_for_cpu(phys_addr_t paddr, size_t size, enum dma_data_direction dir)
> +{
> + switch (dir) {
> + case DMA_TO_DEVICE:
> + break;
> + case DMA_FROM_DEVICE:
> + case DMA_BIDIRECTIONAL:
> + ALT_CMO_OP(INVAL, (unsigned long)phys_to_virt(paddr), size, riscv_cbom_block_size);

For arch_sync_dma_for_cpu(DMA_BIDIRECTIONAL), we expect the CPU to have written
to the buffer, so this should flush, not invalidate.

Regards,
Samuel

> + break;
> + default:
> + break;
> + }
> +}
> +
> +void arch_dma_prep_coherent(struct page *page, size_t size)
> +{
> + void *flush_addr = page_address(page);
> +
> + memset(flush_addr, 0, size);
> + ALT_CMO_OP(FLUSH, (unsigned long)flush_addr, size, riscv_cbom_block_size);
> +}
> +
> +void arch_setup_dma_ops(struct device *dev, u64 dma_base, u64 size,
> + const struct iommu_ops *iommu, bool coherent)
> +{
> + /* If a specific device is dma-coherent, set it here */
> + dev->dma_coherent = coherent;
> +}
> +
> +void riscv_init_cbom_blocksize(void)
> +{
> + struct device_node *node;
> + int ret;
> + u32 val;
> +
> + for_each_of_cpu_node(node) {
> + int hartid = riscv_of_processor_hartid(node);
> + int cbom_hartid;
> +
> + if (hartid < 0)
> + continue;
> +
> + /* set block-size for cbom extension if available */
> + ret = of_property_read_u32(node, "riscv,cbom-block-size", &val);
> + if (ret)
> + continue;
> +
> + if (!riscv_cbom_block_size) {
> + riscv_cbom_block_size = val;
> + cbom_hartid = hartid;
> + } else {
> + if (riscv_cbom_block_size != val)
> + pr_warn("cbom-block-size mismatched between harts %d and %d\n",
> + cbom_hartid, hartid);
> + }
> + }
> +}
>