[Xen-devel] [PATCH v5 1/2] x86/XPTI: use %r12 to write zero into xen_cr3

["Jan Beulich" <JBeulich@xxxxxxxx>]

Now that we zero all registers early on all entry paths, use that to
avoid a couple of immediates here.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
Acked-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
Tested-by: Juergen Gross <jgross@xxxxxxxx>
Reviewed-by: Juergen Gross <jgross@xxxxxxxx>
Reviewed-by: Wei Liu <wei.liu2@xxxxxxxxxx>
---
v5: Re-base.
v4: Add comments about the %r12 being zero
---
We may want to consider eliminating a few more $0 this way. But
especially for byte ones I'm not sure it's worth it, due to the REX
prefix the use of %r12 would incur.

--- a/xen/arch/x86/x86_64/compat/entry.S
+++ b/xen/arch/x86/x86_64/compat/entry.S
@@ -220,7 +220,8 @@ ENTRY(cstar_enter)
         jz    .Lcstar_cr3_okay
         movb  $0, STACK_CPUINFO_FIELD(use_pv_cr3)(%rbx)
         mov   %rcx, %cr3
-        movq  $0, STACK_CPUINFO_FIELD(xen_cr3)(%rbx)
+        /* %r12 is still zero at this point. */
+        mov   %r12, STACK_CPUINFO_FIELD(xen_cr3)(%rbx)
 .Lcstar_cr3_okay:
         sti
 
@@ -244,7 +245,8 @@ UNLIKELY_START(z, compat_syscall_gpf)
         movq  VCPU_trap_ctxt(%rbx),%rdi
         movl  $TRAP_gp_fault,UREGS_entry_vector(%rsp)
         subl  $2,UREGS_rip(%rsp)
-        movl  $0,TRAPBOUNCE_error_code(%rdx)
+        /* %r12 is still zero at this point. */
+        mov   %r12d, TRAPBOUNCE_error_code(%rdx)
         movl  TRAP_gp_fault * TRAPINFO_sizeof + TRAPINFO_eip(%rdi),%eax
         movzwl TRAP_gp_fault * TRAPINFO_sizeof + TRAPINFO_cs(%rdi),%esi
         testb $4,TRAP_gp_fault * TRAPINFO_sizeof + TRAPINFO_flags(%rdi)
--- a/xen/arch/x86/x86_64/entry.S
+++ b/xen/arch/x86/x86_64/entry.S
@@ -266,7 +266,8 @@ ENTRY(lstar_enter)
         jz    .Llstar_cr3_okay
         movb  $0, STACK_CPUINFO_FIELD(use_pv_cr3)(%rbx)
         mov   %rcx, %cr3
-        movq  $0, STACK_CPUINFO_FIELD(xen_cr3)(%rbx)
+        /* %r12 is still zero at this point. */
+        mov   %r12, STACK_CPUINFO_FIELD(xen_cr3)(%rbx)
 .Llstar_cr3_okay:
         sti
 
@@ -302,7 +303,8 @@ GLOBAL(sysenter_eflags_saved)
         jz    .Lsyse_cr3_okay
         movb  $0, STACK_CPUINFO_FIELD(use_pv_cr3)(%rbx)
         mov   %rcx, %cr3
-        movq  $0, STACK_CPUINFO_FIELD(xen_cr3)(%rbx)
+        /* %r12 is still zero at this point. */
+        mov   %r12, STACK_CPUINFO_FIELD(xen_cr3)(%rbx)
 .Lsyse_cr3_okay:
         sti
 
@@ -351,7 +353,8 @@ ENTRY(int80_direct_trap)
         jz    .Lint80_cr3_okay
         movb  $0, STACK_CPUINFO_FIELD(use_pv_cr3)(%rbx)
         mov   %rcx, %cr3
-        movq  $0, STACK_CPUINFO_FIELD(xen_cr3)(%rbx)
+        /* %r12 is still zero at this point. */
+        mov   %r12, STACK_CPUINFO_FIELD(xen_cr3)(%rbx)
 .Lint80_cr3_okay:
         sti
 
@@ -566,11 +569,11 @@ ENTRY(common_interrupt)
         jz    .Lintr_cr3_okay
         movb  $0, STACK_CPUINFO_FIELD(use_pv_cr3)(%r14)
         mov   %rcx, %cr3
-        xor   %ecx, %ecx
-        mov   %rcx, STACK_CPUINFO_FIELD(xen_cr3)(%r14)
+        /* %r12 is still zero at this point. */
+        mov   %r12, STACK_CPUINFO_FIELD(xen_cr3)(%r14)
         testb $3, UREGS_cs(%rsp)
-        cmovnz %rcx, %r15
-        cmovnz %rcx, %rbx
+        cmovnz %r12, %r15
+        cmovnz %r12d, %ebx
 .Lintr_cr3_okay:
 
         CR4_PV32_RESTORE
@@ -598,11 +601,11 @@ GLOBAL(handle_exception)
         jz    .Lxcpt_cr3_okay
         movb  $0, STACK_CPUINFO_FIELD(use_pv_cr3)(%r14)
         mov   %rcx, %cr3
-        xor   %ecx, %ecx
-        mov   %rcx, STACK_CPUINFO_FIELD(xen_cr3)(%r14)
+        /* %r12 is still zero at this point. */
+        mov   %r12, STACK_CPUINFO_FIELD(xen_cr3)(%r14)
         testb $3, UREGS_cs(%rsp)
-        cmovnz %rcx, %r15
-        cmovnz %rcx, %r13
+        cmovnz %r12, %r15
+        cmovnz %r12d, %r13d
 .Lxcpt_cr3_okay:
 
 handle_exception_saved:
@@ -775,7 +778,8 @@ ENTRY(double_fault)
         jz    .Ldblf_cr3_okay
         movb  $0, STACK_CPUINFO_FIELD(use_pv_cr3)(%r14)
         mov   %rbx, %cr3
-        movq $0, STACK_CPUINFO_FIELD(xen_cr3)(%r14)
+        /* %r12 is still zero at this point. */
+        mov   %r12, STACK_CPUINFO_FIELD(xen_cr3)(%r14)
 .Ldblf_cr3_okay:
 
         movq  %rsp,%rdi
@@ -809,7 +813,8 @@ handle_ist_exception:
         jz    .List_cr3_okay
         movb  $0, STACK_CPUINFO_FIELD(use_pv_cr3)(%r14)
         mov   %rcx, %cr3
-        movq  $0, STACK_CPUINFO_FIELD(xen_cr3)(%r14)
+        /* %r12 is still zero at this point. */
+        mov   %r12, STACK_CPUINFO_FIELD(xen_cr3)(%r14)
 .List_cr3_okay:
 
         CR4_PV32_RESTORE




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