[PATCH v5 00/13] riscv: add SBI FWFT misaligned exception delegation support

Clément Léger
2025-04-17T12:36:11Z

The SBI Firmware Feature extension allows the S-mode to request some
specific features (either hardware or software) to be enabled. This
series uses this extension to request misaligned access exception
delegation to S-mode in order to let the kernel handle it. It also adds
support for the KVM FWFT SBI extension based on the misaligned access
handling infrastructure.

FWFT SBI extension is part of the SBI V3.0 specifications [1]. It can be
tested using the qemu provided at [2] which contains the series from
[3]. Upstream kvm-unit-tests can be used inside kvm to tests the correct
delegation of misaligned exceptions. Upstream OpenSBI can be used.

Note: Since SBI V3.0 is not yet ratified, FWFT extension API is split
between interface only and implementation, allowing to pick only the
interface which do not have hard dependencies on SBI.

The tests can be run using the kselftest from series [4].

$ qemu-system-riscv64 \
-cpu rv64,trap-misaligned-access=true,v=true \
-M virt \
-m 1024M \
-bios fw_dynamic.bin \
-kernel Image
...

# ./misaligned
TAP version 13
1..23
# Starting 23 tests from 1 test cases.
# RUN global.gp_load_lh ...
# OK global.gp_load_lh
ok 1 global.gp_load_lh
# RUN global.gp_load_lhu ...
# OK global.gp_load_lhu
ok 2 global.gp_load_lhu
# RUN global.gp_load_lw ...
# OK global.gp_load_lw
ok 3 global.gp_load_lw
# RUN global.gp_load_lwu ...
# OK global.gp_load_lwu
ok 4 global.gp_load_lwu
# RUN global.gp_load_ld ...
# OK global.gp_load_ld
ok 5 global.gp_load_ld
# RUN global.gp_load_c_lw ...
# OK global.gp_load_c_lw
ok 6 global.gp_load_c_lw
# RUN global.gp_load_c_ld ...
# OK global.gp_load_c_ld
ok 7 global.gp_load_c_ld
# RUN global.gp_load_c_ldsp ...
# OK global.gp_load_c_ldsp
ok 8 global.gp_load_c_ldsp
# RUN global.gp_load_sh ...
# OK global.gp_load_sh
ok 9 global.gp_load_sh
# RUN global.gp_load_sw ...
# OK global.gp_load_sw
ok 10 global.gp_load_sw
# RUN global.gp_load_sd ...
# OK global.gp_load_sd
ok 11 global.gp_load_sd
# RUN global.gp_load_c_sw ...
# OK global.gp_load_c_sw
ok 12 global.gp_load_c_sw
# RUN global.gp_load_c_sd ...
# OK global.gp_load_c_sd
ok 13 global.gp_load_c_sd
# RUN global.gp_load_c_sdsp ...
# OK global.gp_load_c_sdsp
ok 14 global.gp_load_c_sdsp
# RUN global.fpu_load_flw ...
# OK global.fpu_load_flw
ok 15 global.fpu_load_flw
# RUN global.fpu_load_fld ...
# OK global.fpu_load_fld
ok 16 global.fpu_load_fld
# RUN global.fpu_load_c_fld ...
# OK global.fpu_load_c_fld
ok 17 global.fpu_load_c_fld
# RUN global.fpu_load_c_fldsp ...
# OK global.fpu_load_c_fldsp
ok 18 global.fpu_load_c_fldsp
# RUN global.fpu_store_fsw ...
# OK global.fpu_store_fsw
ok 19 global.fpu_store_fsw
# RUN global.fpu_store_fsd ...
# OK global.fpu_store_fsd
ok 20 global.fpu_store_fsd
# RUN global.fpu_store_c_fsd ...
# OK global.fpu_store_c_fsd
ok 21 global.fpu_store_c_fsd
# RUN global.fpu_store_c_fsdsp ...
# OK global.fpu_store_c_fsdsp
ok 22 global.fpu_store_c_fsdsp
# RUN global.gen_sigbus ...
[12797.988647] misaligned[618]: unhandled signal 7 code 0x1 at 0x0000000000014dc0 in misaligned[4dc0,10000+76000]
[12797.988990] CPU: 0 UID: 0 PID: 618 Comm: misaligned Not tainted 6.13.0-rc6-00008-g4ec4468967c9-dirty #51
[12797.989169] Hardware name: riscv-virtio,qemu (DT)
[12797.989264] epc : 0000000000014dc0 ra : 0000000000014d00 sp : 00007fffe165d100
[12797.989407] gp : 000000000008f6e8 tp : 0000000000095760 t0 : 0000000000000008
[12797.989544] t1 : 00000000000965d8 t2 : 000000000008e830 s0 : 00007fffe165d160
[12797.989692] s1 : 000000000000001a a0 : 0000000000000000 a1 : 0000000000000002
[12797.989831] a2 : 0000000000000000 a3 : 0000000000000000 a4 : ffffffffdeadbeef
[12797.989964] a5 : 000000000008ef61 a6 : 626769735f6e0000 a7 : fffffffffffff000
[12797.990094] s2 : 0000000000000001 s3 : 00007fffe165d838 s4 : 00007fffe165d848
[12797.990238] s5 : 000000000000001a s6 : 0000000000010442 s7 : 0000000000010200
[12797.990391] s8 : 000000000000003a s9 : 0000000000094508 s10: 0000000000000000
[12797.990526] s11: 0000555567460668 t3 : 00007fffe165d070 t4 : 00000000000965d0
[12797.990656] t5 : fefefefefefefeff t6 : 0000000000000073
[12797.990756] status: 0000000200004020 badaddr: 000000000008ef61 cause: 0000000000000006
[12797.990911] Code: 8793 8791 3423 fcf4 3783 fc84 c737 dead 0713 eef7 (c398) 0001
# OK global.gen_sigbus
ok 23 global.gen_sigbus
# PASSED: 23 / 23 tests passed.
# Totals: pass:23 fail:0 xfail:0 xpass:0 skip:0 error:0

With kvm-tools:

# lkvm run -k sbi.flat -m 128
Info: # lkvm run -k sbi.flat -m 128 -c 1 --name guest-97
Info: Removed ghost socket file "/root/.lkvm//guest-97.sock".

##########################################################################
# kvm-unit-tests
##########################################################################

... [test messages elided]
PASS: sbi: fwft: FWFT extension probing no error
PASS: sbi: fwft: get/set reserved feature 0x6 error == SBI_ERR_DENIED
PASS: sbi: fwft: get/set reserved feature 0x3fffffff error == SBI_ERR_DENIED
PASS: sbi: fwft: get/set reserved feature 0x80000000 error == SBI_ERR_DENIED
PASS: sbi: fwft: get/set reserved feature 0xbfffffff error == SBI_ERR_DENIED
PASS: sbi: fwft: misaligned_deleg: Get misaligned deleg feature no error
PASS: sbi: fwft: misaligned_deleg: Set misaligned deleg feature invalid value error
PASS: sbi: fwft: misaligned_deleg: Set misaligned deleg feature invalid value error
PASS: sbi: fwft: misaligned_deleg: Set misaligned deleg feature value no error
PASS: sbi: fwft: misaligned_deleg: Set misaligned deleg feature value 0
PASS: sbi: fwft: misaligned_deleg: Set misaligned deleg feature value no error
PASS: sbi: fwft: misaligned_deleg: Set misaligned deleg feature value 1
PASS: sbi: fwft: misaligned_deleg: Verify misaligned load exception trap in supervisor
SUMMARY: 50 tests, 2 unexpected failures, 12 skipped

This series is available at [5].

Link: https://github.com/riscv-non-isa/riscv-sbi-doc/releases/download/vv3.0-rc2/riscv-sbi.pdf [1]
Link: https://github.com/rivosinc/qemu/tree/dev/cleger/misaligned [2]
Link: https://lore.kernel.org/all/20241211211933.198792-3-fkonrad@amd.com/T/ [3]
Link: https://lore.kernel.org/linux-riscv/20250414123543.1615478-1-cleger@rivosinc.com [4]
Link: https://github.com/rivosinc/linux/tree/dev/cleger/fwft [5]
---

V5:
- Return ERANGE as mapping for SBI_ERR_BAD_RANGE
- Removed unused sbi_fwft_get()
- Fix kernel for sbi_fwft_local_set_cpumask()
- Fix indentation for sbi_fwft_local_set()
- Remove spurious space in kvm_sbi_fwft_ops.
- Rebased on origin/master
- Remove fixes commits and sent them as a separate series [4]

V4:
- Check SBI version 3.0 instead of 2.0 for FWFT presence
- Use long for kvm_sbi_fwft operation return value
- Init KVM sbi extension even if default_disabled
- Remove revert_on_fail parameter for sbi_fwft_feature_set().
- Fix comments for sbi_fwft_set/get()
- Only handle local features (there are no globals yet in the spec)
- Add new SBI errors to sbi_err_map_linux_errno()

V3:
- Added comment about kvm sbi fwft supported/set/get callback
requirements
- Move struct kvm_sbi_fwft_feature in kvm_sbi_fwft.c
- Add a FWFT interface

V2:
- Added Kselftest for misaligned testing
- Added get_user() usage instead of __get_user()
- Reenable interrupt when possible in misaligned access handling
- Document that riscv supports unaligned-traps
- Fix KVM extension state when an init function is present
- Rework SBI misaligned accesses trap delegation code
- Added support for CPU hotplugging
- Added KVM SBI reset callback
- Added reset for KVM SBI FWFT lock
- Return SBI_ERR_DENIED_LOCKED when LOCK flag is set

Clément Léger (13):
riscv: sbi: add Firmware Feature (FWFT) SBI extensions definitions
riscv: sbi: add new SBI error mappings
riscv: sbi: add FWFT extension interface
riscv: sbi: add SBI FWFT extension calls
riscv: misaligned: request misaligned exception from SBI
riscv: misaligned: use on_each_cpu() for scalar misaligned access
probing
riscv: misaligned: use correct CONFIG_ ifdef for
misaligned_access_speed
riscv: misaligned: move emulated access uniformity check in a function
riscv: misaligned: add a function to check misalign trap delegability
RISC-V: KVM: add SBI extension init()/deinit() functions
RISC-V: KVM: add SBI extension reset callback
RISC-V: KVM: add support for FWFT SBI extension
RISC-V: KVM: add support for SBI_FWFT_MISALIGNED_DELEG

arch/riscv/include/asm/cpufeature.h | 8 +-
arch/riscv/include/asm/kvm_host.h | 5 +-
arch/riscv/include/asm/kvm_vcpu_sbi.h | 12 +
arch/riscv/include/asm/kvm_vcpu_sbi_fwft.h | 29 +++
arch/riscv/include/asm/sbi.h | 60 +++++
arch/riscv/include/uapi/asm/kvm.h | 1 +
arch/riscv/kernel/sbi.c | 75 ++++++
arch/riscv/kernel/traps_misaligned.c | 110 ++++++++-
arch/riscv/kernel/unaligned_access_speed.c | 8 +-
arch/riscv/kvm/Makefile | 1 +
arch/riscv/kvm/vcpu.c | 7 +-
arch/riscv/kvm/vcpu_sbi.c | 54 +++++
arch/riscv/kvm/vcpu_sbi_fwft.c | 252 +++++++++++++++++++++
arch/riscv/kvm/vcpu_sbi_sta.c | 3 +-
14 files changed, 610 insertions(+), 15 deletions(-)
create mode 100644 arch/riscv/include/asm/kvm_vcpu_sbi_fwft.h
create mode 100644 arch/riscv/kvm/vcpu_sbi_fwft.c