[xen staging] x86: clobber registers in switch_stack_and_jump() when !LIVEPATCH

[patchbot@xxxxxxx]

commit 033a80677087b322789d2748e83b764545f76c6b
Author:     Jan Beulich <jbeulich@xxxxxxxx>
AuthorDate: Wed Dec 16 16:41:46 2020 +0100
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Wed Dec 16 16:41:46 2020 +0100

    x86: clobber registers in switch_stack_and_jump() when !LIVEPATCH
    
    In order to have the same effect on registers as a call to
    check_for_livepatch_work() may have, clobber all call-clobbered
    registers in debug builds.
    
    Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
    Reviewed-by: Wei Liu <wl@xxxxxxx>
---
 xen/include/asm-x86/current.h | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/xen/include/asm-x86/current.h b/xen/include/asm-x86/current.h
index 4d8822f78c..231994a245 100644
--- a/xen/include/asm-x86/current.h
+++ b/xen/include/asm-x86/current.h
@@ -120,6 +120,14 @@ unsigned long get_stack_dump_bottom (unsigned long sp);
 
 #ifdef CONFIG_LIVEPATCH
 # define CHECK_FOR_LIVEPATCH_WORK "call check_for_livepatch_work;"
+#elif defined(CONFIG_DEBUG)
+/* Mimic the clobbering effect a call has on registers. */
+# define CHECK_FOR_LIVEPATCH_WORK \
+    "mov $0x1234567890abcdef, %%rax\n\t" \
+    "mov %%rax, %%rcx; mov %%rax, %%rdx\n\t" \
+    "mov %%rax, %%rsi; mov %%rax, %%rdi\n\t" \
+    "mov %%rax, %%r8; mov %%rax, %%r9\n\t" \
+    "mov %%rax, %%r10; mov %%rax, %%r11\n\t"
 #else
 # define CHECK_FOR_LIVEPATCH_WORK ""
 #endif
--
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