[Xen-devel] [PATCH v4 4/8] 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>
---
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
@@ -216,7 +216,8 @@ ENTRY(cstar_enter)
         mov   %rcx, STACK_CPUINFO_FIELD(xen_cr3)(%rbx)
         neg   %rcx
         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:
         ALTERNATIVE_NOP .Lcstar_cr3_start, .Lcstar_cr3_okay, 
X86_FEATURE_NO_XPTI
         sti
--- a/xen/arch/x86/x86_64/entry.S
+++ b/xen/arch/x86/x86_64/entry.S
@@ -185,7 +185,8 @@ ENTRY(lstar_enter)
         mov   %rcx, STACK_CPUINFO_FIELD(xen_cr3)(%rbx)
         neg   %rcx
         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:
         ALTERNATIVE_NOP .Llstar_cr3_start, .Llstar_cr3_okay, 
X86_FEATURE_NO_XPTI
         sti
@@ -296,7 +297,8 @@ GLOBAL(sysenter_eflags_saved)
         mov   %rcx, STACK_CPUINFO_FIELD(xen_cr3)(%rbx)
         neg   %rcx
         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:
         ALTERNATIVE_NOP .Lsyse_cr3_start, .Lsyse_cr3_okay, X86_FEATURE_NO_XPTI
         sti
@@ -348,7 +350,8 @@ ENTRY(int80_direct_trap)
         mov   %rcx, STACK_CPUINFO_FIELD(xen_cr3)(%rbx)
         neg   %rcx
         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:
         ALTERNATIVE_NOP .Lint80_cr3_start, .Lint80_cr3_okay, 
X86_FEATURE_NO_XPTI
         sti
@@ -561,10 +564,10 @@ ENTRY(common_interrupt)
         neg   %rcx
 .Lintr_cr3_load:
         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 %r12, %r15
 .Lintr_cr3_okay:
         ALTERNATIVE_NOP .Lintr_cr3_start, .Lintr_cr3_okay, X86_FEATURE_NO_XPTI
 
@@ -605,10 +608,10 @@ GLOBAL(handle_exception)
         neg   %rcx
 .Lxcpt_cr3_load:
         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 %r12, %r15
 .Lxcpt_cr3_okay:
         ALTERNATIVE_NOP .Lxcpt_cr3_start, .Lxcpt_cr3_okay, X86_FEATURE_NO_XPTI
 
@@ -824,7 +827,8 @@ handle_ist_exception:
         neg   %rcx
 .List_cr3_load:
         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:
         ALTERNATIVE_NOP .List_cr3_start, .List_cr3_okay, X86_FEATURE_NO_XPTI
 




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