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[PATCH] x86/PV: use 64-bit subtract to adjust guest RIP upon missing SYSCALL callbacks

When discussing the shrunk down version of the commit in question it
was said (in reply to my conditional choosing of the width):

"However, the 32bit case isn't actually interesting here.  A
 guest can't execute a SYSCALL instruction on/across the 4G->0 boundary
 because the M2P is mapped NX up to the 4G boundary, so we can never
 reach this point with %eip < 2.

 Therefore, the 64bit-only form is the appropriate one to use, which
 solves any question of cleverness, or potential decode stalls it

Fixes: ca6fcf4321b3 ("x86/pv: Inject #UD for missing SYSCALL callbacks")
Signed-off-by: Jan Beulich <JBeulich@xxxxxxxx>

--- a/xen/arch/x86/x86_64/entry.S
+++ b/xen/arch/x86/x86_64/entry.S
@@ -42,7 +42,7 @@ ENTRY(switch_to_kernel)
 UNLIKELY_START(z, syscall_no_callback) /* TB_eip == 0 => #UD */
         mov   VCPU_trap_ctxt(%rbx), %rdi
         movl  $X86_EXC_UD, UREGS_entry_vector(%rsp)
-        subl  $2, UREGS_rip(%rsp)
+        subq  $2, UREGS_rip(%rsp)
         mov   X86_EXC_UD * TRAPINFO_sizeof + TRAPINFO_eip(%rdi), %rax
         testb $4, X86_EXC_UD * TRAPINFO_sizeof + TRAPINFO_flags(%rdi)
         setnz %cl



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