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ARM: NOMMU: Fix exc_ret for stack frame type

PATCH In review Jisheng Zhang 2 participants 4 messages Oct 13, 2024 – Oct 22, 2024 Arm (32-bit)

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Jisheng Zhang

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#0 lore raw

PATCH 1/1 ARM: NOMMU: Fix exc_ret for stack frame type

commit 72cd4064fcca ("ARM: 8830/1: NOMMU: Toggle only bits in EXC_RETURN we are really care of") only sets BIT[3] for Thread mode and BIT[2] for PSP, it leaves BIT[4] untouched. But there's such a
case: the pre-linux env makes use of FPU then the BIT[4] in 'lr' is cleared, this brings an umatch issue since the NOMMU kernel doesn't support FPU yet. Before the above commit, we hardcode the exc_ret as 0xfffffffd, so the flow works fine.

Fix this issue by explicitly set BIT[4] which means using standard stack frame.

Fixes 72cd4064fcca ("ARM: 8830/1: NOMMU: Toggle only bits in EXC_RETURN we are really care of")
Signed-off-by Jisheng Zhang <[email protected]>
---
 arch/arm/include/asm/v7m.h | 1 +
 arch/arm/mm/proc-v7m.S     | 1 +
 2 files changed, 2 insertions(+)
arch/arm/include/asm/v7m.h +1 -0
index 4512f7e1918f..3aea6d3c97ee 100644
@@ -51,6 +51,7 @@
  */
 #define EXC_RET_STACK_MASK			0x00000004
 #define EXC_RET_THREADMODE_PROCESSSTACK		(3 << 2)
+#define EXC_RET_FTYPE				(1 << 4)
 
 /* Cache related definitions */
arch/arm/mm/proc-v7m.S +1 -0
index ed7781c84341..fdae077d2654 100644
@@ -138,6 +138,7 @@ __v7m_setup_cont:
 1:	cpsid	i
 	/* Calculate exc_ret */
 	orr	r10, lr, #EXC_RET_THREADMODE_PROCESSSTACK
+	orr	r10, #EXC_RET_FTYPE
 	ldmia	sp, {r0-r3, r12}
 	str	r5, [r12, #11 * 4]	@ restore the original SVC vector entry
 	mov	lr, r6			@ restore LR
Signature · 1 line
2.45.2
#1 lore raw
Quoted 5 lines · Jisheng Zhang

commit 72cd4064fcca ("ARM: 8830/1: NOMMU: Toggle only bits in EXC_RETURN we are really care of") only sets BIT[3] for Thread mode and BIT[2] for PSP, it leaves BIT[4] untouched. But there's such a
case: the pre-linux env makes use of FPU then the BIT[4] in 'lr' is cleared, this brings an umatch issue since the NOMMU kernel doesn't

Can pre-linux env disable FPU before passing control to kernel (which is, as correctly pointed, doesn't know how to use FPU)?

Cheers
Vladimir

author
#2 lore raw
Quoted 9 lines · Vladimir Murzin
Quoted 5 lines · Jisheng Zhang

commit 72cd4064fcca ("ARM: 8830/1: NOMMU: Toggle only bits in EXC_RETURN we are really care of") only sets BIT[3] for Thread mode and BIT[2] for PSP, it leaves BIT[4] untouched. But there's such a
case: the pre-linux env makes use of FPU then the BIT[4] in 'lr' is cleared, this brings an umatch issue since the NOMMU kernel doesn't

Can pre-linux env disable FPU before passing control to kernel (which is, as correctly pointed, doesn't know how to use FPU)?

IIRC, I did a experiment like this by clearing the SCB CPACR related bits, but the stack frame type is still not correct. I searched in the armv8m arm or cortex-mN's TRM I didn't find the relation between the returned "lr" and the pre FPU usage.

What's more, IMHO, kernel needs to use the correct exc_ret no matter the pre-linux env does, I.E if kernel/userspace supports FPU, then clear BIT[4]; if no, set BIT[4].

PS: this is a regression: before the commit, the have-used-fpu pre-linux env + linux nommu combination works; after the commit, it fails to execute the init due to wrong stack frame type.

Thanks

Reviewed-by
#3 lore raw
Quoted 18 lines · Jisheng Zhang
Quoted 8 lines · Vladimir Murzin
Quoted 5 lines · Jisheng Zhang

commit 72cd4064fcca ("ARM: 8830/1: NOMMU: Toggle only bits in EXC_RETURN we are really care of") only sets BIT[3] for Thread mode and BIT[2] for PSP, it leaves BIT[4] untouched. But there's such a
case: the pre-linux env makes use of FPU then the BIT[4] in 'lr' is cleared, this brings an umatch issue since the NOMMU kernel doesn't

Can pre-linux env disable FPU before passing control to kernel (which is, as correctly pointed, doesn't know how to use FPU)?

IIRC, I did a experiment like this by clearing the SCB CPACR related bits, but the stack frame type is still not correct. I searched in the armv8m arm or cortex-mN's TRM I didn't find the relation between the returned "lr" and the pre FPU usage.

What's more, IMHO, kernel needs to use the correct exc_ret no matter the pre-linux env does, I.E if kernel/userspace supports FPU, then clear BIT[4]; if no, set BIT[4].

Quoted 6 lines

PS: this is a regression: before the commit, the have-used-fpu pre-linux env + linux nommu combination works; after the commit, it fails to execute the init due to wrong stack frame type.

Thanks

Fair enough.

Reviewed-by Vladimir Murzin <[email protected]>

Please, upload the patch in RMK's patch system [1]

[1] https://www.arm.linux.org.uk/developer/patches/

Vladimir