[PATCH v5 1/4] lib: sbi: Add RISC-V vector context save/restore support
Dave Patel
dave.patel at riscstar.com
Sat May 16 03:23:09 PDT 2026
On 5/15/26 08:41, Anup Patel wrote:
> On Fri, May 15, 2026 at 11:12 AM <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.
>>
>> Signed-off-by: Dave Patel <dave.patel at riscstar.com>
>> ---
>> include/sbi/sbi_vector.h | 28 ++++++++++
>> lib/sbi/objects.mk | 1 +
>> lib/sbi/sbi_vector.c | 109 +++++++++++++++++++++++++++++++++++++++
>> 3 files changed, 138 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..bbd857c3
>> --- /dev/null
>> +++ b/include/sbi/sbi_vector.h
>> @@ -0,0 +1,28 @@
>> +/*
>> + * SPDX-License-Identifier: BSD-2-Clause
>> + *
>> + * 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>
>> +
>> +struct sbi_vector_context {
>> + unsigned long vcsr;
>> + unsigned long vstart;
>> +
>> + /* size depends on VLEN */
>> + uint8_t vregs[];
>> +};
>> +
>> +void sbi_vector_save(struct sbi_vector_context *dst);
>> +void sbi_vector_restore(const struct sbi_vector_context *src);
>> +unsigned long vector_vlenb(void);
>> +
>> +#endif //__SBI_VECTOR_H__
>> +
>> diff --git a/lib/sbi/objects.mk b/lib/sbi/objects.mk
>> index 97cc4521..ddb2e7ac 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..1d2ac944
>> --- /dev/null
>> +++ b/lib/sbi/sbi_vector.c
>> @@ -0,0 +1,109 @@
>> +/*
>> + * SPDX-License-Identifier: BSD-2-Clause
>> + *
>> + * 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
>> +
>> +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;
>> +
>> + /* Step 1: Save CSRs */
>> + dst->vcsr = csr_read(vcsr);
>> + dst->vstart = csr_read(vstart);
>> +
>> + 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);
>
> Same issue as FP save/restore, this will trap and crash
> if Vector is disabled in mstatus.VS.
>
> For correct implementation, refer __riscv_v_vstate_save()/restore()
> from <linux_source>/arch/riscv/include/asm/vector.h
>
>> +
>> +#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 */
>> + /* Restore CSRs first */
>> + csr_write(vcsr, src->vcsr);
>> + csr_write(vstart, src->vstart);
>> +}
>> +
>> +#else
>> +
>> +void sbi_vector_save(struct sbi_vector_context *dst)
>> +{
>> +}
>> +
>> +void sbi_vector_restore(const struct sbi_vector_context *src)
>> +{
>> +}
>> +
>> +#endif /* OPENSBI_CC_SUPPORT_VECTOR */
>> --
>> 2.43.0
>>
>>
>> --
>> opensbi mailing list
>> opensbi at lists.infradead.org
>> http://lists.infradead.org/mailman/listinfo/opensbi
>
> Regards,
> Anup
Hi Anup,
Firstly thanks for all your review comments, I have endeavor to
cover all of them, so please confirm.
Rgarding this comment of mstatus.VS and FS (I think which includes 3
comments)
I have already have code inplace in patch 3, please see below line from
patch 3
+ /* Make sure FS and VS is on before context switch */
+ csr_set(CSR_MSTATUS, MSTATUS_FS | MSTATUS_VS);
Since the context of Floating and Vector is more granular in Opensbi I
thought of handle in context switch.
So there is no issue of trap and crash, however there is still a
question regarding if you are fine with this or you want it more fine
grained inside its individual functions?
Please let me know.
Thanks
Dave
More information about the opensbi
mailing list