[Bug 262691] [patch] Save only callee-saved registers in pcb

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=262691

            Bug ID: 262691
           Summary: [patch] Save only callee-saved registers in pcb
           Product: Base System
           Version: CURRENT
          Hardware: arm64
                OS: Any
            Status: New
          Severity: Affects Some People
          Priority: ---
         Component: arm
          Assignee: [email protected]
          Reporter: [email protected]

On AArch64, registers x9-x18 are not callee-saved, yet they are preserved at
many placed in swtch.S. This patch removes code that preserves these registers.

It may also be worth it to remove the unused entries in the `pcb_x` field of
`struct pcb`, but this would be a more disruptive change with compatibility
concerns (e.g. `kgdb` would need to be changed to recognise the new layout the
struct).

--- a/sys/arm64/arm64/swtch.S
+++ b/sys/arm64/arm64/swtch.S
@@ -94,18 +94,12 @@ ENTRY(cpu_throw)
        msr     tpidr_el0, x6
        ldr     x6, [x4, #PCB_TPIDRRO]
        msr     tpidrro_el0, x6
-       ldp     x8, x9, [x4, #PCB_REGS + 8 * 8]
-       ldp     x10, x11, [x4, #PCB_REGS + 10 * 8]
-       ldp     x12, x13, [x4, #PCB_REGS + 12 * 8]
-       ldp     x14, x15, [x4, #PCB_REGS + 14 * 8]
-       ldp     x16, x17, [x4, #PCB_REGS + 16 * 8]
-       ldr          x19, [x4, #PCB_REGS + 19 * 8]
-       ldp     x20, x21, [x4, #PCB_REGS + 20 * 8]
-       ldp     x22, x23, [x4, #PCB_REGS + 22 * 8]
-       ldp     x24, x25, [x4, #PCB_REGS + 24 * 8]
-       ldp     x26, x27, [x4, #PCB_REGS + 26 * 8]
-       ldp     x28, x29, [x4, #PCB_REGS + 28 * 8]
-       ldr     lr, [x4, #PCB_LR]
+       ldp     x19, x20, [x4, #PCB_REGS + 19 * 8]
+       ldp     x21, x22, [x4, #PCB_REGS + 21 * 8]
+       ldp     x23, x24, [x4, #PCB_REGS + 23 * 8]
+       ldp     x25, x26, [x4, #PCB_REGS + 25 * 8]
+       ldp     x27, x28, [x4, #PCB_REGS + 27 * 8]
+       ldp     x29, lr, [x4, #PCB_REGS + 29 * 8]

        ret
 END(cpu_throw)
@@ -125,18 +119,12 @@ ENTRY(cpu_switch)
        ldr     x4, [x0, #TD_PCB]

        /* Store the callee-saved registers */
-       stp     x8, x9, [x4, #PCB_REGS + 8 * 8]
-       stp     x10, x11, [x4, #PCB_REGS + 10 * 8]
-       stp     x12, x13, [x4, #PCB_REGS + 12 * 8]
-       stp     x14, x15, [x4, #PCB_REGS + 14 * 8]
-       stp     x16, x17, [x4, #PCB_REGS + 16 * 8]
-       stp     x18, x19, [x4, #PCB_REGS + 18 * 8]
-       stp     x20, x21, [x4, #PCB_REGS + 20 * 8]
-       stp     x22, x23, [x4, #PCB_REGS + 22 * 8]
-       stp     x24, x25, [x4, #PCB_REGS + 24 * 8]
-       stp     x26, x27, [x4, #PCB_REGS + 26 * 8]
-       stp     x28, x29, [x4, #PCB_REGS + 28 * 8]
-       str     lr, [x4, #PCB_LR]
+       stp     x19, x20, [x4, #PCB_REGS + 19 * 8]
+       stp     x21, x22, [x4, #PCB_REGS + 21 * 8]
+       stp     x23, x24, [x4, #PCB_REGS + 23 * 8]
+       stp     x25, x26, [x4, #PCB_REGS + 25 * 8]
+       stp     x27, x28, [x4, #PCB_REGS + 27 * 8]
+       stp     x29, lr, [x4, #PCB_REGS + 29 * 8]
        /* And the old stack pointer */
        mov     x5, sp
        mrs     x6, tpidrro_el0
@@ -195,18 +183,12 @@ ENTRY(cpu_switch)
        msr     tpidr_el0, x6
        ldr     x6, [x4, #PCB_TPIDRRO]
        msr     tpidrro_el0, x6
-       ldp     x8, x9, [x4, #PCB_REGS + 8 * 8]
-       ldp     x10, x11, [x4, #PCB_REGS + 10 * 8]
-       ldp     x12, x13, [x4, #PCB_REGS + 12 * 8]
-       ldp     x14, x15, [x4, #PCB_REGS + 14 * 8]
-       ldp     x16, x17, [x4, #PCB_REGS + 16 * 8]
-       ldr          x19, [x4, #PCB_REGS + 19 * 8]
-       ldp     x20, x21, [x4, #PCB_REGS + 20 * 8]
-       ldp     x22, x23, [x4, #PCB_REGS + 22 * 8]
-       ldp     x24, x25, [x4, #PCB_REGS + 24 * 8]
-       ldp     x26, x27, [x4, #PCB_REGS + 26 * 8]
-       ldp     x28, x29, [x4, #PCB_REGS + 28 * 8]
-       ldr     lr, [x4, #PCB_LR]
+       ldp     x19, x20, [x4, #PCB_REGS + 19 * 8]
+       ldp     x21, x22, [x4, #PCB_REGS + 21 * 8]
+       ldp     x23, x24, [x4, #PCB_REGS + 23 * 8]
+       ldp     x25, x26, [x4, #PCB_REGS + 25 * 8]
+       ldp     x27, x28, [x4, #PCB_REGS + 27 * 8]
+       ldp     x29, lr, [x4, #PCB_REGS + 29 * 8]

        str     xzr, [x4, #PCB_REGS + 18 * 8]
        ret
@@ -258,23 +240,17 @@ ENTRY(fork_trampoline)
         * will be set to the desired value anyway.
         */
        ERET
-       
+
 END(fork_trampoline)

 ENTRY(savectx)
        /* Store the callee-saved registers */
-       stp     x8,  x9,  [x0, #PCB_REGS + 8 * 8]
-       stp     x10, x11, [x0, #PCB_REGS + 10 * 8]
-       stp     x12, x13, [x0, #PCB_REGS + 12 * 8]
-       stp     x14, x15, [x0, #PCB_REGS + 14 * 8]
-       stp     x16, x17, [x0, #PCB_REGS + 16 * 8]
-       stp     x18, x19, [x0, #PCB_REGS + 18 * 8]
-       stp     x20, x21, [x0, #PCB_REGS + 20 * 8]
-       stp     x22, x23, [x0, #PCB_REGS + 22 * 8]
-       stp     x24, x25, [x0, #PCB_REGS + 24 * 8]
-       stp     x26, x27, [x0, #PCB_REGS + 26 * 8]
-       stp     x28, x29, [x0, #PCB_REGS + 28 * 8]
-       str     lr, [x0, #PCB_LR]
+       stp     x19, x20, [x0, #PCB_REGS + 19 * 8]
+       stp     x21, x22, [x0, #PCB_REGS + 21 * 8]
+       stp     x23, x24, [x0, #PCB_REGS + 23 * 8]
+       stp     x25, x26, [x0, #PCB_REGS + 25 * 8]
+       stp     x27, x28, [x0, #PCB_REGS + 27 * 8]
+       stp     x29, lr, [x0, #PCB_REGS + 29 * 8]
        /* And the old stack pointer */
        mov     x5, sp
        mrs     x6, tpidrro_el0
diff --git a/sys/arm64/arm64/vm_machdep.c b/sys/arm64/arm64/vm_machdep.c
index c52a7e2fc5..feb439314f 100644
--- a/sys/arm64/arm64/vm_machdep.c
+++ b/sys/arm64/arm64/vm_machdep.c
@@ -105,8 +105,8 @@ cpu_fork(struct thread *td1, struct proc *p2, struct thread
*td2, int flags)
        td2->td_frame = tf;

        /* Set the return value registers for fork() */
-       td2->td_pcb->pcb_x[8] = (uintptr_t)fork_return;
-       td2->td_pcb->pcb_x[9] = (uintptr_t)td2;
+       td2->td_pcb->pcb_x[19] = (uintptr_t)fork_return;
+       td2->td_pcb->pcb_x[20] = (uintptr_t)td2;
        td2->td_pcb->pcb_lr = (uintptr_t)fork_trampoline;
        td2->td_pcb->pcb_sp = (uintptr_t)td2->td_frame;
        td2->td_pcb->pcb_fpusaved = &td2->td_pcb->pcb_fpustate;
@@ -183,8 +183,8 @@ cpu_copy_thread(struct thread *td, struct thread *td0)
        bcopy(td0->td_frame, td->td_frame, sizeof(struct trapframe));
        bcopy(td0->td_pcb, td->td_pcb, sizeof(struct pcb));

-       td->td_pcb->pcb_x[8] = (uintptr_t)fork_return;
-       td->td_pcb->pcb_x[9] = (uintptr_t)td;
+       td->td_pcb->pcb_x[19] = (uintptr_t)fork_return;
+       td->td_pcb->pcb_x[20] = (uintptr_t)td;
        td->td_pcb->pcb_lr = (uintptr_t)fork_trampoline;
        td->td_pcb->pcb_sp = (uintptr_t)td->td_frame;
        td->td_pcb->pcb_fpflags &= ~(PCB_FP_STARTED | PCB_FP_KERN |
PCB_FP_NOSAVE);
@@ -287,8 +287,8 @@ void
 cpu_fork_kthread_handler(struct thread *td, void (*func)(void *), void *arg)
 {

-       td->td_pcb->pcb_x[8] = (uintptr_t)func;
-       td->td_pcb->pcb_x[9] = (uintptr_t)arg;
+       td->td_pcb->pcb_x[19] = (uintptr_t)func;
+       td->td_pcb->pcb_x[20] = (uintptr_t)arg;
 }

 void

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