[PATCH v5 1/6] lib: sbi: select expected trap handler per hart
Anup Patel
anup at brainfault.org
Thu Oct 22 21:08:29 PDT 2026
On Fri, Oct 23, 2026 at 9:15 AM Troy Mitchell
<troy.mitchell at linux.spacemit.com> wrote:
>
> The expected trap handler is selected once by the cold boot hart. This
> breaks heterogeneous systems where the cold boot hart implements H but
> another hart does not, because the H-aware handler accesses mtval2 and
> mtinst.
>
> Select the handler from each hart's MISA at entry to sbi_init(), before
> platform hooks can probe CSRs, and store its address in that hart's
> scratch space. Load the address directly at each use. This avoids an
> accessor call and its stack-protector instrumentation in the CSR probing
> and unprivileged memory access paths.
>
> Align scratch allocations after the enlarged fixed scratch area so that
> the new field preserves the allocator's cache-line alignment guarantee.
>
> Fixes: 1de66d170e71 ("lib: Optimize unpriv load/store implementation")
> Reported-by: Bo Gan <ganboing at gmail.com>
> Link: https://lore.kernel.org/r/e702f291-dde8-4b99-a65e-182d2b847720@gmail.com
> Suggested-by: Bo Gan <ganboing at gmail.com>
> Cc: Alvin Chang <alvinga at andestech.com>
> Link: https://lore.kernel.org/r/20260831022156.2060561-1-alvinga@andestech.com
> Tested-by: E Shattow <e at freeshell.de>
> Signed-off-by: Troy Mitchell <troy.mitchell at linux.spacemit.com>
> ---
> include/sbi/sbi_csr_detect.h | 4 ++--
> include/sbi/sbi_hart.h | 5 ++++-
> include/sbi/sbi_scratch.h | 7 ++++++-
> lib/sbi/sbi_hart.c | 8 --------
> lib/sbi/sbi_illegal_atomic.c | 4 ++--
> lib/sbi/sbi_init.c | 7 +++++++
> lib/sbi/sbi_scratch.c | 4 ++++
> lib/sbi/sbi_unpriv.c | 6 +++---
> 8 files changed, 28 insertions(+), 17 deletions(-)
>
> diff --git a/include/sbi/sbi_csr_detect.h b/include/sbi/sbi_csr_detect.h
> index 097c31c8..31e50db9 100644
> --- a/include/sbi/sbi_csr_detect.h
> +++ b/include/sbi/sbi_csr_detect.h
> @@ -16,9 +16,9 @@
>
> #define csr_read_allowed(csr_num, trap) \
> ({ \
> + register ulong mtvec = sbi_hart_expected_trap_addr(); \
> register ulong tinfo asm("a3") = (ulong)trap; \
> register ulong ttmp asm("a4"); \
> - register ulong mtvec = (ulong)sbi_hart_expected_trap; \
> register ulong ret = 0; \
> ((struct sbi_trap_info *)(trap))->cause = 0; \
> asm volatile( \
> @@ -35,9 +35,9 @@
>
> #define csr_write_allowed(csr_num, trap, value) \
> ({ \
> + register ulong mtvec = sbi_hart_expected_trap_addr(); \
> register ulong tinfo asm("a3") = (ulong)trap; \
> register ulong ttmp asm("a4"); \
> - register ulong mtvec = (ulong)sbi_hart_expected_trap; \
> ((struct sbi_trap_info *)(trap))->cause = 0; \
> asm volatile( \
> "add %[ttmp], %[tinfo], zero\n" \
> diff --git a/include/sbi/sbi_hart.h b/include/sbi/sbi_hart.h
> index 543393bb..f0a4be61 100644
> --- a/include/sbi/sbi_hart.h
> +++ b/include/sbi/sbi_hart.h
> @@ -12,6 +12,7 @@
>
> #include <sbi/sbi_types.h>
> #include <sbi/sbi_bitops.h>
> +#include <sbi/sbi_scratch.h>
>
> /** Possible privileged specification versions of a hart */
> enum sbi_hart_priv_versions {
> @@ -135,7 +136,9 @@ struct sbi_scratch;
> int sbi_hart_reinit(struct sbi_scratch *scratch);
> int sbi_hart_init(struct sbi_scratch *scratch, bool cold_boot);
>
> -extern void (*sbi_hart_expected_trap)(void);
> +/* Keep the load free of function calls, including stack-protector code. */
> +#define sbi_hart_expected_trap_addr() \
> + (sbi_scratch_thishart_ptr()->expected_trap)
>
> unsigned int sbi_hart_mhpm_mask(struct sbi_scratch *scratch);
> void sbi_hart_delegation_dump(struct sbi_scratch *scratch,
> diff --git a/include/sbi/sbi_scratch.h b/include/sbi/sbi_scratch.h
> index 06097c8a..20e7dcd0 100644
> --- a/include/sbi/sbi_scratch.h
> +++ b/include/sbi/sbi_scratch.h
> @@ -46,8 +46,10 @@
> #define SBI_SCRATCH_OPTIONS_OFFSET (14 * __SIZEOF_POINTER__)
> /** Offset of hartindex member in sbi_scratch */
> #define SBI_SCRATCH_HARTINDEX_OFFSET (15 * __SIZEOF_POINTER__)
> +/** Offset of expected_trap member in sbi_scratch */
> +#define SBI_SCRATCH_EXPECTED_TRAP_OFFSET (16 * __SIZEOF_POINTER__)
> /** Offset of extra space in sbi_scratch */
> -#define SBI_SCRATCH_EXTRA_SPACE_OFFSET (16 * __SIZEOF_POINTER__)
> +#define SBI_SCRATCH_EXTRA_SPACE_OFFSET (17 * __SIZEOF_POINTER__)
> /** Maximum size of sbi_scratch (4KB) */
> #define SBI_SCRATCH_SIZE (0x1000)
>
> @@ -91,6 +93,8 @@ struct sbi_scratch {
> unsigned long options;
> /** Index of the hart */
> unsigned long hartindex;
> + /** Address of the expected trap handler for this HART */
> + unsigned long expected_trap;
Instead of adding in struct sbi_scratch please add this to
struct sbi_hart_features.
Typically, we add stuff to struct sbi_scratch only when we
expect it to be accessed by low-level assembly entry/exit
routines.
> };
>
> /**
> @@ -113,6 +117,7 @@ assert_member_offset(struct sbi_scratch, tmp0, SBI_SCRATCH_TMP0_OFFSET);
> assert_member_offset(struct sbi_scratch, tmp1, SBI_SCRATCH_TMP1_OFFSET);
> assert_member_offset(struct sbi_scratch, options, SBI_SCRATCH_OPTIONS_OFFSET);
> assert_member_offset(struct sbi_scratch, hartindex, SBI_SCRATCH_HARTINDEX_OFFSET);
> +assert_member_offset(struct sbi_scratch, expected_trap, SBI_SCRATCH_EXPECTED_TRAP_OFFSET);
>
> /** Possible options for OpenSBI library */
> enum sbi_scratch_options {
> diff --git a/lib/sbi/sbi_hart.c b/lib/sbi/sbi_hart.c
> index bee88557..61c5f945 100644
> --- a/lib/sbi/sbi_hart.c
> +++ b/lib/sbi/sbi_hart.c
> @@ -22,11 +22,6 @@
> #include <sbi/sbi_string.h>
> #include <sbi/sbi_trap.h>
>
> -extern void __sbi_expected_trap(void);
> -extern void __sbi_expected_trap_hext(void);
> -
> -void (*sbi_hart_expected_trap)(void) = &__sbi_expected_trap;
> -
> unsigned long hart_features_offset;
>
> static void mstatus_init(struct sbi_scratch *scratch)
> @@ -712,9 +707,6 @@ int sbi_hart_init(struct sbi_scratch *scratch, bool cold_boot)
> csr_write(CSR_MIP, 0);
>
> if (cold_boot) {
> - if (misa_extension('H'))
> - sbi_hart_expected_trap = &__sbi_expected_trap_hext;
> -
> hart_features_offset = sbi_scratch_alloc_offset(
> sizeof(struct sbi_hart_features));
> if (!hart_features_offset)
> diff --git a/lib/sbi/sbi_illegal_atomic.c b/lib/sbi/sbi_illegal_atomic.c
> index 977a9ad0..30f5118e 100644
> --- a/lib/sbi/sbi_illegal_atomic.c
> +++ b/lib/sbi/sbi_illegal_atomic.c
> @@ -30,7 +30,7 @@ int sbi_illegal_atomic(ulong insn, struct sbi_trap_regs *regs)
> { \
> register ulong tinfo asm("a3"); \
> register ulong mstatus = 0; \
> - register ulong mtvec = (ulong)sbi_hart_expected_trap; \
> + register ulong mtvec = sbi_hart_expected_trap_addr(); \
> type ret = 0; \
> trap->cause = 0; \
> asm volatile( \
> @@ -57,7 +57,7 @@ int sbi_illegal_atomic(ulong insn, struct sbi_trap_regs *regs)
> { \
> register ulong tinfo asm("a3"); \
> register ulong mstatus = 0; \
> - register ulong mtvec = (ulong)sbi_hart_expected_trap; \
> + register ulong mtvec = sbi_hart_expected_trap_addr(); \
> type ret = 0; \
> trap->cause = 0; \
> asm volatile( \
> diff --git a/lib/sbi/sbi_init.c b/lib/sbi/sbi_init.c
> index 9bb1a37e..76b4fd91 100644
> --- a/lib/sbi/sbi_init.c
> +++ b/lib/sbi/sbi_init.c
> @@ -36,6 +36,9 @@
> #include <sbi/sbi_version.h>
> #include <sbi/sbi_unit_test.h>
>
> +extern void __sbi_expected_trap(void);
> +extern void __sbi_expected_trap_hext(void);
> +
> #define BANNER \
> " ____ _____ ____ _____\n" \
> " / __ \\ / ____| _ \\_ _|\n" \
> @@ -585,6 +588,10 @@ void __noreturn sbi_init(struct sbi_scratch *scratch)
> u32 hartid = current_hartid();
> const struct sbi_platform *plat = sbi_platform_ptr(scratch);
>
> + /* Platform hooks may probe CSRs before sbi_hart_init(). */
> + scratch->expected_trap = misa_extension('H') ?
> + (ulong)&__sbi_expected_trap_hext : (ulong)&__sbi_expected_trap;
> +
> switch (scratch->next_mode) {
> case PRV_M:
> next_mode_supported = true;
> diff --git a/lib/sbi/sbi_scratch.c b/lib/sbi/sbi_scratch.c
> index bb14a1a2..88ef1547 100644
> --- a/lib/sbi/sbi_scratch.c
> +++ b/lib/sbi/sbi_scratch.c
> @@ -97,6 +97,10 @@ unsigned long sbi_scratch_alloc_offset(unsigned long size)
>
> spin_lock(&extra_lock);
>
> + /* The fixed scratch fields need not end on a cache-line boundary. */
> + extra_offset = (extra_offset + scratch_alloc_align - 1) &
> + ~(scratch_alloc_align - 1);
> +
> if (SBI_SCRATCH_SIZE < (extra_offset + size))
> goto done;
>
> diff --git a/lib/sbi/sbi_unpriv.c b/lib/sbi/sbi_unpriv.c
> index 60becedc..1550d111 100644
> --- a/lib/sbi/sbi_unpriv.c
> +++ b/lib/sbi/sbi_unpriv.c
> @@ -33,9 +33,9 @@ union sbi_unpriv_data {
> type sbi_load_##type(const type *addr, \
> struct sbi_trap_info *trap) \
> { \
> + register ulong mtvec = sbi_hart_expected_trap_addr(); \
> register ulong tinfo asm("a3") = (ulong)trap; \
> register ulong mstatus = 0; \
> - register ulong mtvec = (ulong)sbi_hart_expected_trap; \
> type ret = 0; \
> trap->cause = 0; \
> asm volatile( \
> @@ -58,9 +58,9 @@ union sbi_unpriv_data {
> void sbi_store_##type(type *addr, type val, \
> struct sbi_trap_info *trap) \
> { \
> + register ulong mtvec = sbi_hart_expected_trap_addr(); \
> register ulong tinfo asm("a3") = (ulong)trap; \
> register ulong mstatus = 0; \
> - register ulong mtvec = (ulong)sbi_hart_expected_trap; \
> trap->cause = 0; \
> asm volatile( \
> "csrrw %[mtvec], " STR(CSR_MTVEC) ", %[mtvec]\n" \
> @@ -207,7 +207,7 @@ ulong sbi_get_insn(ulong mepc, struct sbi_trap_info *trap)
> register ulong tinfo asm("a3");
> register ulong ttmp asm("a4");
> register ulong mstatus = 0;
> - register ulong mtvec = (ulong)sbi_hart_expected_trap;
> + register ulong mtvec = sbi_hart_expected_trap_addr();
> ulong insn = 0;
>
> trap->cause = 0;
>
> --
> 2.55.0
>
Regards,
Anup
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