[PATCH 3/8] ARM: vfp: Reimplement VFP exception entry in C code
Ard Biesheuvel
ardb at kernel.org
Tue May 9 05:29:33 PDT 2023
En/disabling softirqs from asm code turned out to be trickier than
expected, so vfp_support_entry now returns by tail calling
__local_enable_bh_ip() and passing the same arguments that a C call to
local_bh_enable() would pass. However, this is slightly hacky, as we
don't want to carry our own implementation of local_bh_enable().
So let's bite the bullet, and get rid of the asm logic in
vfp_support_entry that reasons about whether or not to save and/or
reload the VFP state, and about whether or not an FP exception is
pending, and only keep the VFP loading logic as a function that is
callable from C.
Replicate the removed logic in vfp_entry(), and use the exact same
reasoning as in the asm code. To emphasize the correspondance, retain
some of the asm comments in the C version as well.
Signed-off-by: Ard Biesheuvel <ardb at kernel.org>
Acked-by: Linus Walleij <linus.walleij at linaro.org>
---
arch/arm/vfp/entry.S | 12 +-
arch/arm/vfp/vfp.h | 1 +
arch/arm/vfp/vfphw.S | 204 ++------------------
arch/arm/vfp/vfpmodule.c | 123 ++++++++++--
4 files changed, 124 insertions(+), 216 deletions(-)
diff --git a/arch/arm/vfp/entry.S b/arch/arm/vfp/entry.S
index 62206ef250371cd3..547c94c62cd3a66a 100644
--- a/arch/arm/vfp/entry.S
+++ b/arch/arm/vfp/entry.S
@@ -22,10 +22,10 @@
@ IRQs enabled.
@
ENTRY(do_vfp)
- mov r1, r10
- str lr, [sp, #-8]!
- add r3, sp, #4
- str r9, [r3]
- bl vfp_entry
- ldr pc, [sp], #8
+ mov r1, r0 @ pass trigger opcode via R1
+ mov r0, sp @ pass struct pt_regs via R0
+ bl vfp_support_entry @ dispatch the VFP exception
+ cmp r0, #0 @ handled successfully?
+ reteq r9 @ then use R9 as return address
+ ret lr @ pass to undef handler
ENDPROC(do_vfp)
diff --git a/arch/arm/vfp/vfp.h b/arch/arm/vfp/vfp.h
index 5cd6d5053271760e..e43a630f8a164f9d 100644
--- a/arch/arm/vfp/vfp.h
+++ b/arch/arm/vfp/vfp.h
@@ -375,3 +375,4 @@ struct op {
};
asmlinkage void vfp_save_state(void *location, u32 fpexc);
+asmlinkage u32 vfp_load_state(const void *location);
diff --git a/arch/arm/vfp/vfphw.S b/arch/arm/vfp/vfphw.S
index 0aeb60ac3b5376a8..d5a03f3c10c500f3 100644
--- a/arch/arm/vfp/vfphw.S
+++ b/arch/arm/vfp/vfphw.S
@@ -4,12 +4,6 @@
*
* Copyright (C) 2004 ARM Limited.
* Written by Deep Blue Solutions Limited.
- *
- * This code is called from the kernel's undefined instruction trap.
- * r1 holds the thread_info pointer
- * r3 holds the return address for successful handling.
- * lr holds the return address for unrecognised instructions.
- * sp points to a struct pt_regs (as defined in include/asm/proc/ptrace.h)
*/
#include <linux/init.h>
#include <linux/linkage.h>
@@ -19,20 +13,6 @@
#include <asm/assembler.h>
#include <asm/asm-offsets.h>
- .macro DBGSTR, str
-#ifdef DEBUG
- stmfd sp!, {r0-r3, ip, lr}
- ldr r0, =1f
- bl _printk
- ldmfd sp!, {r0-r3, ip, lr}
-
- .pushsection .rodata, "a"
-1: .ascii KERN_DEBUG "VFP: \str\n"
- .byte 0
- .previous
-#endif
- .endm
-
.macro DBGSTR1, str, arg
#ifdef DEBUG
stmfd sp!, {r0-r3, ip, lr}
@@ -48,177 +28,25 @@
#endif
.endm
- .macro DBGSTR3, str, arg1, arg2, arg3
-#ifdef DEBUG
- stmfd sp!, {r0-r3, ip, lr}
- mov r3, \arg3
- mov r2, \arg2
- mov r1, \arg1
- ldr r0, =1f
- bl _printk
- ldmfd sp!, {r0-r3, ip, lr}
-
- .pushsection .rodata, "a"
-1: .ascii KERN_DEBUG "VFP: \str\n"
- .byte 0
- .previous
-#endif
- .endm
-
-
-@ VFP hardware support entry point.
-@
-@ r0 = instruction opcode (32-bit ARM or two 16-bit Thumb)
-@ r1 = thread_info pointer
-@ r2 = PC value to resume execution after successful emulation
-@ r3 = normal "successful" return address
-@ lr = unrecognised instruction return address
-@ IRQs enabled.
-ENTRY(vfp_support_entry)
- ldr r11, [r1, #TI_CPU] @ CPU number
- add r10, r1, #TI_VFPSTATE @ r10 = workspace
-
- DBGSTR3 "instr %08x pc %08x state %p", r0, r2, r10
-
- .fpu vfpv2
- VFPFMRX r1, FPEXC @ Is the VFP enabled?
- DBGSTR1 "fpexc %08x", r1
- tst r1, #FPEXC_EN
- bne look_for_VFP_exceptions @ VFP is already enabled
-
- DBGSTR1 "enable %x", r10
- ldr r9, vfp_current_hw_state_address
- orr r1, r1, #FPEXC_EN @ user FPEXC has the enable bit set
- ldr r4, [r9, r11, lsl #2] @ vfp_current_hw_state pointer
- bic r5, r1, #FPEXC_EX @ make sure exceptions are disabled
- cmp r4, r10 @ this thread owns the hw context?
-#ifndef CONFIG_SMP
- @ For UP, checking that this thread owns the hw context is
- @ sufficient to determine that the hardware state is valid.
- beq vfp_hw_state_valid
-
- @ On UP, we lazily save the VFP context. As a different
- @ thread wants ownership of the VFP hardware, save the old
- @ state if there was a previous (valid) owner.
-
- VFPFMXR FPEXC, r5 @ enable VFP, disable any pending
- @ exceptions, so we can get at the
- @ rest of it
-
- DBGSTR1 "save old state %p", r4
- cmp r4, #0 @ if the vfp_current_hw_state is NULL
- beq vfp_reload_hw @ then the hw state needs reloading
- VFPFSTMIA r4, r5 @ save the working registers
- VFPFMRX r5, FPSCR @ current status
- tst r1, #FPEXC_EX @ is there additional state to save?
- beq 1f
- VFPFMRX r6, FPINST @ FPINST (only if FPEXC.EX is set)
- tst r1, #FPEXC_FP2V @ is there an FPINST2 to read?
- beq 1f
- VFPFMRX r8, FPINST2 @ FPINST2 if needed (and present)
-1:
- stmia r4, {r1, r5, r6, r8} @ save FPEXC, FPSCR, FPINST, FPINST2
-vfp_reload_hw:
-
-#else
- @ For SMP, if this thread does not own the hw context, then we
- @ need to reload it. No need to save the old state as on SMP,
- @ we always save the state when we switch away from a thread.
- bne vfp_reload_hw
-
- @ This thread has ownership of the current hardware context.
- @ However, it may have been migrated to another CPU, in which
- @ case the saved state is newer than the hardware context.
- @ Check this by looking at the CPU number which the state was
- @ last loaded onto.
- ldr ip, [r10, #VFP_CPU]
- teq ip, r11
- beq vfp_hw_state_valid
-
-vfp_reload_hw:
- @ We're loading this threads state into the VFP hardware. Update
- @ the CPU number which contains the most up to date VFP context.
- str r11, [r10, #VFP_CPU]
-
- VFPFMXR FPEXC, r5 @ enable VFP, disable any pending
- @ exceptions, so we can get at the
- @ rest of it
-#endif
-
- DBGSTR1 "load state %p", r10
- str r10, [r9, r11, lsl #2] @ update the vfp_current_hw_state pointer
+ENTRY(vfp_load_state)
+ @ Load the current VFP state
+ @ r0 - load location
+ @ returns FPEXC
+ DBGSTR1 "load VFP state %p", r0
@ Load the saved state back into the VFP
- VFPFLDMIA r10, r5 @ reload the working registers while
+ VFPFLDMIA r0, r1 @ reload the working registers while
@ FPEXC is in a safe state
- ldmia r10, {r1, r5, r6, r8} @ load FPEXC, FPSCR, FPINST, FPINST2
- tst r1, #FPEXC_EX @ is there additional state to restore?
+ ldmia r0, {r0-r3} @ load FPEXC, FPSCR, FPINST, FPINST2
+ tst r0, #FPEXC_EX @ is there additional state to restore?
beq 1f
- VFPFMXR FPINST, r6 @ restore FPINST (only if FPEXC.EX is set)
- tst r1, #FPEXC_FP2V @ is there an FPINST2 to write?
+ VFPFMXR FPINST, r2 @ restore FPINST (only if FPEXC.EX is set)
+ tst r0, #FPEXC_FP2V @ is there an FPINST2 to write?
beq 1f
- VFPFMXR FPINST2, r8 @ FPINST2 if needed (and present)
+ VFPFMXR FPINST2, r3 @ FPINST2 if needed (and present)
1:
- VFPFMXR FPSCR, r5 @ restore status
-
-@ The context stored in the VFP hardware is up to date with this thread
-vfp_hw_state_valid:
- tst r1, #FPEXC_EX
- bne process_exception @ might as well handle the pending
- @ exception before retrying branch
- @ out before setting an FPEXC that
- @ stops us reading stuff
- VFPFMXR FPEXC, r1 @ Restore FPEXC last
- mov sp, r3 @ we think we have handled things
- pop {lr}
- sub r2, r2, #4 @ Retry current instruction - if Thumb
- str r2, [sp, #S_PC] @ mode it's two 16-bit instructions,
- @ else it's one 32-bit instruction, so
- @ always subtract 4 from the following
- @ instruction address.
-
-local_bh_enable_and_ret:
- adr r0, .
- mov r1, #SOFTIRQ_DISABLE_OFFSET
- b __local_bh_enable_ip @ tail call
-
-look_for_VFP_exceptions:
- @ Check for synchronous or asynchronous exception
- tst r1, #FPEXC_EX | FPEXC_DEX
- bne process_exception
- @ On some implementations of the VFP subarch 1, setting FPSCR.IXE
- @ causes all the CDP instructions to be bounced synchronously without
- @ setting the FPEXC.EX bit
- VFPFMRX r5, FPSCR
- tst r5, #FPSCR_IXE
- bne process_exception
-
- tst r5, #FPSCR_LENGTH_MASK
- beq skip
- orr r1, r1, #FPEXC_DEX
- b process_exception
-skip:
-
- @ Fall into hand on to next handler - appropriate coproc instr
- @ not recognised by VFP
-
- DBGSTR "not VFP"
- b local_bh_enable_and_ret
-
-process_exception:
- DBGSTR "bounce"
- mov sp, r3 @ setup for a return to the user code.
- pop {lr}
- mov r2, sp @ nothing stacked - regdump is at TOS
-
- @ Now call the C code to package up the bounce to the support code
- @ r0 holds the trigger instruction
- @ r1 holds the FPEXC value
- @ r2 pointer to register dump
- b VFP_bounce @ we have handled this - the support
- @ code will raise an exception if
- @ required. If not, the user code will
- @ retry the faulted instruction
-ENDPROC(vfp_support_entry)
+ VFPFMXR FPSCR, r1 @ restore status
+ ret lr
+ENDPROC(vfp_load_state)
ENTRY(vfp_save_state)
@ Save the current VFP state
@@ -238,10 +66,6 @@ ENTRY(vfp_save_state)
ret lr
ENDPROC(vfp_save_state)
- .align
-vfp_current_hw_state_address:
- .word vfp_current_hw_state
-
.macro tbl_branch, base, tmp, shift
#ifdef CONFIG_THUMB2_KERNEL
adr \tmp, 1f
diff --git a/arch/arm/vfp/vfpmodule.c b/arch/arm/vfp/vfpmodule.c
index 95628e57807b1e79..7572cb5b28a2eb08 100644
--- a/arch/arm/vfp/vfpmodule.c
+++ b/arch/arm/vfp/vfpmodule.c
@@ -30,11 +30,6 @@
#include "vfpinstr.h"
#include "vfp.h"
-/*
- * Our undef handlers (in entry.S)
- */
-asmlinkage void vfp_support_entry(u32, void *, u32, u32);
-
static bool have_vfp __ro_after_init;
/*
@@ -325,7 +320,7 @@ static u32 vfp_emulate_instruction(u32 inst, u32 fpscr, struct pt_regs *regs)
/*
* Package up a bounce condition.
*/
-void VFP_bounce(u32 trigger, u32 fpexc, struct pt_regs *regs)
+static void VFP_bounce(u32 trigger, u32 fpexc, struct pt_regs *regs)
{
u32 fpscr, orig_fpscr, fpsid, exceptions;
@@ -374,7 +369,7 @@ void VFP_bounce(u32 trigger, u32 fpexc, struct pt_regs *regs)
* on VFP subarch 1.
*/
vfp_raise_exceptions(VFP_EXCEPTION_ERROR, trigger, fpscr, regs);
- goto exit;
+ return;
}
/*
@@ -405,7 +400,7 @@ void VFP_bounce(u32 trigger, u32 fpexc, struct pt_regs *regs)
* the FPEXC.FP2V bit is valid only if FPEXC.EX is 1.
*/
if ((fpexc & (FPEXC_EX | FPEXC_FP2V)) != (FPEXC_EX | FPEXC_FP2V))
- goto exit;
+ return;
/*
* The barrier() here prevents fpinst2 being read
@@ -418,8 +413,6 @@ void VFP_bounce(u32 trigger, u32 fpexc, struct pt_regs *regs)
exceptions = vfp_emulate_instruction(trigger, orig_fpscr, regs);
if (exceptions)
vfp_raise_exceptions(exceptions, trigger, orig_fpscr, regs);
- exit:
- local_bh_enable();
}
static void vfp_enable(void *unused)
@@ -649,22 +642,112 @@ static int vfp_starting_cpu(unsigned int unused)
}
/*
- * Entered with:
+ * vfp_support_entry - Handle VFP exception from user mode
*
- * r0 = instruction opcode (32-bit ARM or two 16-bit Thumb)
- * r1 = thread_info pointer
- * r2 = PC value to resume execution after successful emulation
- * r3 = normal "successful" return address
- * lr = unrecognised instruction return address
+ * @regs: pt_regs structure holding the register state at exception entry
+ * @trigger: The opcode of the instruction that triggered the exception
+ *
+ * Returns 0 if the exception was handled, or an error code otherwise.
*/
-asmlinkage void vfp_entry(u32 trigger, struct thread_info *ti, u32 resume_pc,
- u32 resume_return_address)
+asmlinkage int vfp_support_entry(struct pt_regs *regs, u32 trigger)
{
+ struct thread_info *ti = current_thread_info();
+ u32 fpexc;
+
if (unlikely(!have_vfp))
- return;
+ return -ENODEV;
local_bh_disable();
- vfp_support_entry(trigger, ti, resume_pc, resume_return_address);
+ fpexc = fmrx(FPEXC);
+
+ /*
+ * If the VFP unit was not enabled yet, we have to check whether the
+ * VFP state in the CPU's registers is the most recent VFP state
+ * associated with the process. On UP systems, we don't save the VFP
+ * state eagerly on a context switch, so we may need to save the
+ * VFP state to memory first, as it may belong to another process.
+ */
+ if (!(fpexc & FPEXC_EN)) {
+ /*
+ * Enable the VFP unit but mask the FP exception flag for the
+ * time being, so we can access all the registers.
+ */
+ fpexc |= FPEXC_EN;
+ fmxr(FPEXC, fpexc & ~FPEXC_EX);
+
+ /*
+ * Check whether or not the VFP state in the CPU's registers is
+ * the most recent VFP state associated with this task. On SMP,
+ * migration may result in multiple CPUs holding VFP states
+ * that belong to the same task, but only the most recent one
+ * is valid.
+ */
+ if (!vfp_state_in_hw(ti->cpu, ti)) {
+ if (!IS_ENABLED(CONFIG_SMP) &&
+ vfp_current_hw_state[ti->cpu] != NULL) {
+ /*
+ * This CPU is currently holding the most
+ * recent VFP state associated with another
+ * task, and we must save that to memory first.
+ */
+ vfp_save_state(vfp_current_hw_state[ti->cpu],
+ fpexc);
+ }
+
+ /*
+ * We can now proceed with loading the task's VFP state
+ * from memory into the CPU registers.
+ */
+ fpexc = vfp_load_state(&ti->vfpstate);
+ vfp_current_hw_state[ti->cpu] = &ti->vfpstate;
+#ifdef CONFIG_SMP
+ /*
+ * Record that this CPU is now the one holding the most
+ * recent VFP state of the task.
+ */
+ ti->vfpstate.hard.cpu = ti->cpu;
+#endif
+ }
+
+ if (fpexc & FPEXC_EX)
+ /*
+ * Might as well handle the pending exception before
+ * retrying branch out before setting an FPEXC that
+ * stops us reading stuff.
+ */
+ goto bounce;
+
+ /*
+ * No FP exception is pending: just enable the VFP and
+ * replay the instruction that trapped.
+ */
+ fmxr(FPEXC, fpexc);
+ regs->ARM_pc -= 4;
+ } else {
+ /* Check for synchronous or asynchronous exceptions */
+ if (!(fpexc & (FPEXC_EX | FPEXC_DEX))) {
+ u32 fpscr = fmrx(FPSCR);
+
+ /*
+ * On some implementations of the VFP subarch 1,
+ * setting FPSCR.IXE causes all the CDP instructions to
+ * be bounced synchronously without setting the
+ * FPEXC.EX bit
+ */
+ if (!(fpscr & FPSCR_IXE)) {
+ if (!(fpscr & FPSCR_LENGTH_MASK)) {
+ pr_debug("not VFP\n");
+ local_bh_enable();
+ return -ENOEXEC;
+ }
+ fpexc |= FPEXC_DEX;
+ }
+ }
+bounce: VFP_bounce(trigger, fpexc, regs);
+ }
+
+ local_bh_enable();
+ return 0;
}
#ifdef CONFIG_KERNEL_MODE_NEON
--
2.39.2
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