[xen staging-4.16] x86/spec-ctrl: Rework SPEC_CTRL_ENTRY_FROM_INTR_IST

[patchbot@xxxxxxx]

commit 2a9e690a0ad5d54dca4166e089089a07bbe7fc85
Author:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
AuthorDate: Fri Jul 1 15:59:40 2022 +0100
Commit:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
CommitDate: Tue Jul 12 16:25:33 2022 +0100

    x86/spec-ctrl: Rework SPEC_CTRL_ENTRY_FROM_INTR_IST
    
    We are shortly going to add a conditional IBPB in this path.
    
    Therefore, we cannot hold spec_ctrl_flags in %eax, and rely on only 
clobbering
    it after we're done with its contents.  %rbx is available for use, and the
    more normal register to hold preserved information in.
    
    With %rax freed up, use it instead of %rdx for the RSB tmp register, and for
    the adjustment to spec_ctrl_flags.
    
    This leaves no use of %rdx, except as 0 for the upper half of WRMSR.  In
    practice, %rdx is 0 from SAVE_ALL on all paths and isn't likely to change in
    the foreseeable future, so update the macro entry requirements to state this
    dependency.  This marginal optimisation can be revisited if circumstances
    change.
    
    No practical change.
    
    This is part of XSA-407.
    
    Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
    Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
    (cherry picked from commit e9b8d31981f184c6539f91ec54bd9cae29cdae36)
---
 xen/arch/x86/x86_64/entry.S         |  4 ++--
 xen/include/asm-x86/spec_ctrl_asm.h | 21 ++++++++++-----------
 2 files changed, 12 insertions(+), 13 deletions(-)

diff --git a/xen/arch/x86/x86_64/entry.S b/xen/arch/x86/x86_64/entry.S
index 2a86938f1f..a1810bf4d3 100644
--- a/xen/arch/x86/x86_64/entry.S
+++ b/xen/arch/x86/x86_64/entry.S
@@ -932,7 +932,7 @@ ENTRY(double_fault)
 
         GET_STACK_END(14)
 
-        SPEC_CTRL_ENTRY_FROM_INTR_IST /* Req: %rsp=regs, %r14=end, Clob: acd */
+        SPEC_CTRL_ENTRY_FROM_INTR_IST /* Req: %rsp=regs, %r14=end, %rdx=0, 
Clob: abcd */
         /* WARNING! `ret`, `call *`, `jmp *` not safe before this point. */
 
         mov   STACK_CPUINFO_FIELD(xen_cr3)(%r14), %rbx
@@ -968,7 +968,7 @@ handle_ist_exception:
 
         GET_STACK_END(14)
 
-        SPEC_CTRL_ENTRY_FROM_INTR_IST /* Req: %rsp=regs, %r14=end, Clob: acd */
+        SPEC_CTRL_ENTRY_FROM_INTR_IST /* Req: %rsp=regs, %r14=end, %rdx=0, 
Clob: abcd */
         /* WARNING! `ret`, `call *`, `jmp *` not safe before this point. */
 
         mov   STACK_CPUINFO_FIELD(xen_cr3)(%r14), %rcx
diff --git a/xen/include/asm-x86/spec_ctrl_asm.h 
b/xen/include/asm-x86/spec_ctrl_asm.h
index 0ff1b118f8..15e24cde00 100644
--- a/xen/include/asm-x86/spec_ctrl_asm.h
+++ b/xen/include/asm-x86/spec_ctrl_asm.h
@@ -251,34 +251,33 @@
  */
 .macro SPEC_CTRL_ENTRY_FROM_INTR_IST
 /*
- * Requires %rsp=regs, %r14=stack_end
- * Clobbers %rax, %rcx, %rdx
+ * Requires %rsp=regs, %r14=stack_end, %rdx=0
+ * Clobbers %rax, %rbx, %rcx, %rdx
  *
  * This is logical merge of DO_OVERWRITE_RSB and DO_SPEC_CTRL_ENTRY
  * maybexen=1, but with conditionals rather than alternatives.
  */
-    movzbl STACK_CPUINFO_FIELD(spec_ctrl_flags)(%r14), %eax
+    movzbl STACK_CPUINFO_FIELD(spec_ctrl_flags)(%r14), %ebx
 
-    test $SCF_ist_rsb, %al
+    test $SCF_ist_rsb, %bl
     jz .L\@_skip_rsb
 
-    DO_OVERWRITE_RSB tmp=rdx /* Clobbers %rcx/%rdx */
+    DO_OVERWRITE_RSB         /* Clobbers %rax/%rcx */
 
 .L\@_skip_rsb:
 
-    test $SCF_ist_sc_msr, %al
+    test $SCF_ist_sc_msr, %bl
     jz .L\@_skip_msr_spec_ctrl
 
-    xor %edx, %edx
+    xor %eax, %eax
     testb $3, UREGS_cs(%rsp)
-    setnz %dl
-    not %edx
-    and %dl, STACK_CPUINFO_FIELD(spec_ctrl_flags)(%r14)
+    setnz %al
+    not %eax
+    and %al, STACK_CPUINFO_FIELD(spec_ctrl_flags)(%r14)
 
     /* Load Xen's intended value. */
     mov $MSR_SPEC_CTRL, %ecx
     movzbl STACK_CPUINFO_FIELD(xen_spec_ctrl)(%r14), %eax
-    xor %edx, %edx
     wrmsr
 
     /* Opencoded UNLIKELY_START() with no condition. */
--
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