[xen master] xen/wait: Minor asm improvements

[patchbot@xxxxxxx]

commit 660d69cd5a3714777252d7b804f82e8b7a8a8313
Author:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
AuthorDate: Fri Jul 15 12:27:08 2022 +0100
Commit:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
CommitDate: Mon Jul 18 15:55:53 2022 +0100

    xen/wait: Minor asm improvements
    
    There is no point preserving all registers.  Instead, preserve an arbitrary 
6
    registers, and list the rest as clobbered.  This does not alter the register
    scheduling at all, but does reduce the amount of state needing saving.
    
    Use a named parameter for page size, instead of needing to parse which is
    parameter 3.
    
    Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
    Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
---
 xen/common/wait.c | 33 +++++++++++++++++----------------
 1 file changed, 17 insertions(+), 16 deletions(-)

diff --git a/xen/common/wait.c b/xen/common/wait.c
index 1f0285ac30..e45345ede7 100644
--- a/xen/common/wait.c
+++ b/xen/common/wait.c
@@ -151,13 +151,12 @@ static void __prepare_to_wait(struct waitqueue_vcpu *wqv)
      * copies in from wqv->stack over the active stack.
      */
     asm volatile (
-        "push %%rax; push %%rbx; push %%rdx; push %%rbp;"
-        "push %%r8;  push %%r9;  push %%r10; push %%r11;"
-        "push %%r12; push %%r13; push %%r14; push %%r15;"
+        "push %%rbx; push %%rbp; push %%r12;"
+        "push %%r13; push %%r14; push %%r15;"
 
         "sub %%esp,%%ecx;"
-        "cmp %3,%%ecx;"
-        "ja .L_skip;"
+        "cmp %[sz], %%ecx;"
+        "ja .L_skip;"       /* Bail if >4k */
         "mov %%rsp,%%rsi;"
 
         /* check_wakeup_from_wait() longjmp()'s to this point. */
@@ -165,12 +164,12 @@ static void __prepare_to_wait(struct waitqueue_vcpu *wqv)
         "mov %%rsp,%%rsi;"
 
         ".L_skip:"
-        "pop %%r15; pop %%r14; pop %%r13; pop %%r12;"
-        "pop %%r11; pop %%r10; pop %%r9;  pop %%r8;"
-        "pop %%rbp; pop %%rdx; pop %%rbx; pop %%rax"
+        "pop %%r15; pop %%r14; pop %%r13;"
+        "pop %%r12; pop %%rbp; pop %%rbx"
         : "=&S" (wqv->esp), "=&c" (dummy), "=&D" (dummy)
-        : "i" (PAGE_SIZE), "0" (0), "1" (cpu_info), "2" (wqv->stack)
-        : "memory" );
+        : "0" (0), "1" (cpu_info), "2" (wqv->stack),
+          [sz] "i" (PAGE_SIZE)
+        : "memory", "rax", "rdx", "r8", "r9", "r10", "r11" );
 
     if ( unlikely(wqv->esp == 0) )
     {
@@ -221,13 +220,15 @@ void check_wakeup_from_wait(void)
      * rep movs in __prepare_to_wait(), it copies from wqv->stack over the
      * active stack.
      *
-     * All other GPRs are available for use; they're restored from the stack.
+     * All other GPRs are available for use; They're restored from the stack,
+     * or explicitly clobbered.
      */
-    asm volatile (
-        "mov %1,%%"__OP"sp; jmp .L_wq_resume;"
-        : : "S" (wqv->stack), "D" (wqv->esp),
-          "c" ((char *)get_cpu_info() - (char *)wqv->esp)
-        : "memory" );
+    asm volatile ( "mov %%rdi, %%rsp;"
+                   "jmp .L_wq_resume"
+                   :
+                   : "S" (wqv->stack), "D" (wqv->esp),
+                     "c" ((char *)get_cpu_info() - (char *)wqv->esp)
+                   : "memory" );
     unreachable();
 }
 
--
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