[PATCH v2] riscv: misaligned: Make enabling delegation depend on NONPORTABLE

[PATCH v2] riscv: misaligned: Make enabling delegation depend on NONPORTABLE

[Vivian Wang]

The unaligned access emulation code in Linux has various deficiencies.
For example, it doesn't emulate vector instructions [1] [2], and doesn't
emulate KVM guest accesses. Therefore, requesting misaligned exception
delegation with SBI FWFT actually regresses vector instructions' and KVM
guests' behavior.

Until Linux can handle it properly, guard these sbi_fwft_set() calls
behind RISCV_SBI_FWFT_DELEGATE_MISALIGNED, which in turn depends on
NONPORTABLE. Those who are sure that this wouldn't be a problem can
enable this option, perhaps getting better performance.

The rest of the existing code proceeds as before, except as if
SBI_FWFT_MISALIGNED_EXC_DELEG is not available, to handle any remaining
address misaligned exceptions on a best-effort basis. The KVM SBI FWFT
implementation is also not touched, but it is disabled if the firmware
emulates unaligned accesses.

Cc: stable@vger.kernel.org
Fixes: cf5a8abc6560 ("riscv: misaligned: request misaligned exception from SBI")
Reported-by: Songsong Zhang <U2FsdGVkX1@gmail.com> # KVM
Link: https://lore.kernel.org/linux-riscv/38ce44c1-08cf-4e3f-8ade-20da224f529c@iscas.ac.cn/ [1]
Link: https://lore.kernel.org/linux-riscv/b3cfcdac-0337-4db0-a611-258f2868855f@iscas.ac.cn/ [2]
Signed-off-by: Vivian Wang <wangruikang@iscas.ac.cn>
---
Changes in v2:
- BROKEN -> NONPORTABLE (Conor)
- Elaborated the config help text (Conor)
- Add one more breakage report link
- Link to v1: https://patch.msgid.link/20260330-riscv-misaligned-dont-delegate-v1-1-68b089b306c3@iscas.ac.cn
---
 arch/riscv/Kconfig                   | 22 ++++++++++++++++++++++
 arch/riscv/kernel/traps_misaligned.c |  2 +-
 2 files changed, 23 insertions(+), 1 deletion(-)

diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
index 90c531e6abf5..56a4fad9b7c2 100644
--- a/arch/riscv/Kconfig
+++ b/arch/riscv/Kconfig
@@ -941,6 +941,28 @@ config RISCV_VECTOR_MISALIGNED
 	help
 	  Enable detecting support for vector misaligned loads and stores.
 
+config RISCV_SBI_FWFT_DELEGATE_MISALIGNED
+	bool "Request firmware delegation of unaligned access exceptions"
+	depends on RISCV_SBI
+	depends on NONPORTABLE
+	help
+	  Use SBI FWFT to request delegation of load address misaligned and
+	  store address misaligned exceptions, if possible, and prefer Linux
+	  kernel emulation of these accesses to firmware emulation.
+
+	  Unfortunately, Linux's emulation is still incomplete. Namely, it
+	  currently does not handle vector instructions and KVM guest accesses.
+	  On platforms where these accesses would have been handled by firmware,
+	  enabling this causes unexpected kernel oopses, userspaces crashes and
+	  KVM guest crashes. If you are sure that these are not a problem for
+	  your platform, you can say Y here, which may improve performance.
+
+	  Saying N here will not worsen emulation support for unaligned accesses
+	  even in the case where the firmware also has incomplete support. It
+	  simply keeps the firmware's emulation enabled.
+
+	  If you don't know what to do here, say N.
+
 choice
 	prompt "Unaligned Accesses Support"
 	default RISCV_PROBE_UNALIGNED_ACCESS
diff --git a/arch/riscv/kernel/traps_misaligned.c b/arch/riscv/kernel/traps_misaligned.c
index 2a27d3ff4ac6..81b7682e6c6d 100644
--- a/arch/riscv/kernel/traps_misaligned.c
+++ b/arch/riscv/kernel/traps_misaligned.c
@@ -584,7 +584,7 @@ static int cpu_online_check_unaligned_access_emulated(unsigned int cpu)
 
 static bool misaligned_traps_delegated;
 
-#ifdef CONFIG_RISCV_SBI
+#if defined(CONFIG_RISCV_SBI_FWFT_DELEGATE_MISALIGNED)
 
 static int cpu_online_sbi_unaligned_setup(unsigned int cpu)
 {

---
base-commit: f338e77383789c0cae23ca3d48adcc5e9e137e3c
change-id: 20260330-riscv-misaligned-dont-delegate-3cf98c76ee08

Best regards,
--  
Vivian "dramforever" Wang

Re: [PATCH v2] riscv: misaligned: Make enabling delegation depend on NONPORTABLE

[Conor Dooley]

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On Wed, Apr 01, 2026 at 09:53:17AM +0800, Vivian Wang wrote:
> The unaligned access emulation code in Linux has various deficiencies.
> For example, it doesn't emulate vector instructions [1] [2], and doesn't
> emulate KVM guest accesses. Therefore, requesting misaligned exception
> delegation with SBI FWFT actually regresses vector instructions' and KVM
> guests' behavior.
> 
> Until Linux can handle it properly, guard these sbi_fwft_set() calls
> behind RISCV_SBI_FWFT_DELEGATE_MISALIGNED, which in turn depends on
> NONPORTABLE. Those who are sure that this wouldn't be a problem can
> enable this option, perhaps getting better performance.
> 
> The rest of the existing code proceeds as before, except as if
> SBI_FWFT_MISALIGNED_EXC_DELEG is not available, to handle any remaining
> address misaligned exceptions on a best-effort basis. The KVM SBI FWFT
> implementation is also not touched, but it is disabled if the firmware
> emulates unaligned accesses.
> 
> Cc: stable@vger.kernel.org
> Fixes: cf5a8abc6560 ("riscv: misaligned: request misaligned exception from SBI")
> Reported-by: Songsong Zhang <U2FsdGVkX1@gmail.com> # KVM
> Link: https://lore.kernel.org/linux-riscv/38ce44c1-08cf-4e3f-8ade-20da224f529c@iscas.ac.cn/ [1]
> Link: https://lore.kernel.org/linux-riscv/b3cfcdac-0337-4db0-a611-258f2868855f@iscas.ac.cn/ [2]
> Signed-off-by: Vivian Wang <wangruikang@iscas.ac.cn>

Acked-by: Conor Dooley <conor.dooley@microchip.com>

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Re: [PATCH v2] riscv: misaligned: Make enabling delegation depend on NONPORTABLE

[Paul Walmsley]

On Wed, 1 Apr 2026, Vivian Wang wrote:

> The unaligned access emulation code in Linux has various deficiencies.
> For example, it doesn't emulate vector instructions [1] [2], and doesn't
> emulate KVM guest accesses. Therefore, requesting misaligned exception
> delegation with SBI FWFT actually regresses vector instructions' and KVM
> guests' behavior.
> 
> Until Linux can handle it properly, guard these sbi_fwft_set() calls
> behind RISCV_SBI_FWFT_DELEGATE_MISALIGNED, which in turn depends on
> NONPORTABLE. Those who are sure that this wouldn't be a problem can
> enable this option, perhaps getting better performance.
> 
> The rest of the existing code proceeds as before, except as if
> SBI_FWFT_MISALIGNED_EXC_DELEG is not available, to handle any remaining
> address misaligned exceptions on a best-effort basis. The KVM SBI FWFT
> implementation is also not touched, but it is disabled if the firmware
> emulates unaligned accesses.
> 
> Cc: stable@vger.kernel.org
> Fixes: cf5a8abc6560 ("riscv: misaligned: request misaligned exception from SBI")
> Reported-by: Songsong Zhang <U2FsdGVkX1@gmail.com> # KVM
> Link: https://lore.kernel.org/linux-riscv/38ce44c1-08cf-4e3f-8ade-20da224f529c@iscas.ac.cn/ [1]
> Link: https://lore.kernel.org/linux-riscv/b3cfcdac-0337-4db0-a611-258f2868855f@iscas.ac.cn/ [2]
> Signed-off-by: Vivian Wang <wangruikang@iscas.ac.cn>

Thanks, queued for v7.1-rc.


- Paul