[PATCH v4 1/3] lib: sbi: Add RISC-V vector context save/restore support
Samuel Holland
samuel.holland at sifive.com
Mon Apr 6 15:39:58 PDT 2026
Hi Dave,
On 2026-04-06 2:31 PM, dave.patel at riscstar.com wrote:
> From: Dave Patel <dave.patel at riscstar.com>
>
> Eager context switch: Add support for saving and restoring RISC-V vector
> extension state in OpenSBI. This introduces a per-hart vector context
> structure and helper routines to perform full context save and restore.
>
> The vector context includes vcsr CSRs along with storage for all 32 vector
> registers. The register state is saved and restored using byte-wise vector
> load/store instructions (vs8r/vl8r).
>
> The implementation follows an eager context switching model where the entire
> vector state is saved and restored on every context switch. This provides a
> simple and deterministic mechanism without requiring lazy trap-based
> management.
>
> Notes:
> - The SBI_MAX_VLENB is configured using CONFIG_SBI_MAX_VLENB.
>
> Signed-off-by: Dave Patel <dave.patel at riscstar.com>
> ---
> include/sbi/sbi_vector.h | 29 ++++++++
> lib/sbi/Kconfig | 4 +
> lib/sbi/objects.mk | 1 +
> lib/sbi/sbi_vector.c | 154 +++++++++++++++++++++++++++++++++++++++
> 4 files changed, 188 insertions(+)
> create mode 100644 include/sbi/sbi_vector.h
> create mode 100644 lib/sbi/sbi_vector.c
>
> diff --git a/include/sbi/sbi_vector.h b/include/sbi/sbi_vector.h
> new file mode 100644
> index 00000000..13048716
> --- /dev/null
> +++ b/include/sbi/sbi_vector.h
> @@ -0,0 +1,29 @@
> +/* SPDX-License-Identifier: GPL-2.0
This license is not compatible with the OpenSBI project. You need to release
these changes under a compatible license (assuming you are able to do so, i.e.
they are not derived from GPL code, etc. I am not a lawyer).
> + *
> + * Copyright (c) 2026 RISCstar Solutions.
> + *
> + * Authors:
> + * Dave Patel <dave.patel at riscstar.com>
> + */
> +
> +#ifndef __SBI_VECTOR_H__
> +#define __SBI_VECTOR_H__
> +
> +#include <sbi/sbi_types.h>
> +
> +#define SBI_MAX_VLENB CONFIG_SBI_MAX_VLENB
> +
> +struct sbi_vector_context {
> + unsigned long vcsr;
> + unsigned long vstart;
> +
> + /* size depends on VLEN */
> + uint8_t vregs[32 * SBI_MAX_VLENB];
> +};
> +
> +void sbi_vector_save(struct sbi_vector_context *dst);
> +void sbi_vector_restore(const struct sbi_vector_context *src);
> +int sbi_vector_domain_init(void);
> +
> +#endif //__SBI_VECTOR_H__
> +
> diff --git a/lib/sbi/Kconfig b/lib/sbi/Kconfig
> index 8479f861..b2432150 100644
> --- a/lib/sbi/Kconfig
> +++ b/lib/sbi/Kconfig
> @@ -74,4 +74,8 @@ config SBI_ECALL_VIRQ
> bool "VIRQ extension"
> default y
>
> +config SBI_MAX_VLENB
> + int "Vector VLENB size"
> + default 256
> +
> endmenu
> diff --git a/lib/sbi/objects.mk b/lib/sbi/objects.mk
> index 68fc2036..ecb2b54e 100644
> --- a/lib/sbi/objects.mk
> +++ b/lib/sbi/objects.mk
> @@ -109,3 +109,4 @@ libsbi-objs-y += sbi_trap_v_ldst.o
> libsbi-objs-y += sbi_unpriv.o
> libsbi-objs-y += sbi_expected_trap.o
> libsbi-objs-y += sbi_cppc.o
> +libsbi-objs-y += sbi_vector.o
> diff --git a/lib/sbi/sbi_vector.c b/lib/sbi/sbi_vector.c
> new file mode 100644
> index 00000000..85134f8b
> --- /dev/null
> +++ b/lib/sbi/sbi_vector.c
> @@ -0,0 +1,154 @@
> +/* SPDX-License-Identifier: GPL-2.0
> + *
> + * Copyright (c) 2026 RISCstar Solutions.
> + *
> + * Authors:
> + * Dave Patel <dave.patel at riscstar.com>
> + */
> +
> +#include <sbi/sbi_domain.h>
> +#include <sbi/riscv_encoding.h>
> +#include <sbi/riscv_asm.h>
> +#include <sbi/sbi_vector.h>
> +#include <sbi/sbi_types.h>
> +#include <sbi/sbi_hart.h>
> +#include <sbi/sbi_error.h>
> +#include <sbi/sbi_console.h>
> +
> +#ifdef OPENSBI_CC_SUPPORT_VECTOR
> +
> +static inline unsigned long vector_vlenb(void)
> +{
> + unsigned long vlenb = 0;
> +
> + asm volatile (
> + ".option push\n\t"
> + ".option arch, +v\n\t"
> + "csrr %0, vlenb\n\t"
> + ".option pop\n\t"
> + : "=r"(vlenb)
> + :
> + : "memory");
> +
> + return vlenb;
> +}
> +
> +void sbi_vector_save(struct sbi_vector_context *dst)
> +{
> + if (!dst)
> + return;
> +
> +#define READ_CSR(dst, csr) \
> + ({ \
> + asm volatile ( \
> + " .option push\n\t" \
> + " .option arch, +v\n\t" \
> + " csrr %0, " #csr "\n\t" \
> + " .option pop\n\t" \
> + : "=r"(dst) \
> + : \
> + : "memory"); \
> + }) \
> +
> + /* Step 1: Save CSRs */
> + READ_CSR(dst->vcsr, vcsr);
> + READ_CSR(dst->vstart, vstart);
> +
> +#undef READ_CSR
> +
> + ulong vlenb = vector_vlenb();
> + uint8_t *base = dst->vregs;
> +
> + /* Step 3: Save vector registers */
> +#define SAVE_VREG(i) \
> + ({ \
> + asm volatile( \
> + " .option push\n\t" \
> + " .option arch, +v\n\t" \
> + " vs8r.v v" #i ", (%0)\n\t" \
> + " .option pop\n\t" \
> + :: "r"(base + (i) * vlenb) : "memory"); \
> + }) \
> +
> + SAVE_VREG(0);
> + SAVE_VREG(8);
> + SAVE_VREG(16);
> + SAVE_VREG(24);
> +
> +#undef SAVE_VREG
> +}
> +
> +void sbi_vector_restore(const struct sbi_vector_context *src)
> +{
> + if (!src)
> + return;
> +
> + const uint8_t *base = src->vregs;
> + ulong vlenb = vector_vlenb();
> +
> + /* Step 2: Restore vector registers */
> +#define RESTORE_VREG(i) \
> + ({ \
> + asm volatile( \
> + " .option push\n\t" \
> + " .option arch, +v\n\t" \
> + " vl8r.v v" #i ", (%0)\n\t" \
> + " .option pop\n\t" \
> + :: "r"(base + (i) * vlenb) : "memory"); \
> + }) \
> +
> + RESTORE_VREG(0);
> + RESTORE_VREG(8);
> + RESTORE_VREG(16);
> + RESTORE_VREG(24);
> +#undef RESTORE_VREG
> +
> + /* Step 3: Restore CSR's last */
> +#define WRITE_CSR(csr, val) \
> + ({ \
> + asm volatile( \
> + " .option push\n\t" \
> + " .option arch, +v\n\t" \
> + " csrw " #csr ", %0\n\t" \
> + " .option pop\n\t" \
> + : \
> + : "r"(val) \
> + : "memory"); \
> + }) \
> +
> + /* Restore CSRs first */
> + WRITE_CSR(vcsr, src->vcsr);
> + WRITE_CSR(vstart, src->vstart);
> +#undef WRITE_CSR
> +}
> +
> +int sbi_vector_domain_init(void)
> +{
> + csr_set(CSR_MSTATUS, MSTATUS_VS);
> + ulong vlenb = vector_vlenb();
> +
> + if (vlenb > SBI_MAX_VLENB) {
> + sbi_printf("[Vector ERR:] vlenb range error\n");
> + return SBI_ERR_BAD_RANGE;
> + }
> +
> + csr_clear(CSR_MSTATUS, MSTATUS_VS);
What is the purpose of this CSR write? If you want to set the initial state of
the vector registers, this doesn't accomplish that; you have to write the vector
registers.
Regards,
Samuel
> + return SBI_OK;
> +}
> +
> +#else
> +
> +void sbi_vector_save(struct sbi_vector_context *dst)
> +{
> +}
> +
> +void sbi_vector_restore(const struct sbi_vector_context *src)
> +{
> +}
> +
> +int sbi_vector_domain_init(void)
> +{
> + return SBI_OK;
> +}
> +
> +#endif /* OPENSBI_CC_SUPPORT_VECTOR */
> --
> 2.43.0
>
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