Re: [Xen-devel] [PATCH v2 2/3] x86: use optimal NOPs to fill the SMEP/SMAP placeholders

[Andrew Cooper]

On 10/03/16 09:54, Jan Beulich wrote:
> Alternatives patching code picks the most suitable NOPs for the
> running system, so simply use it to replace the pre-populated ones.
>
> Use an arbitrary, always available feature to key off from, but
> hide this behind the new X86_FEATURE_ALWAYS.
>
> Signed-off-by: Jan Beulich 

Reviewed-by: Andrew Cooper 

___
Xen-devel mailing list
Xen-devel@lists.xen.org
http://lists.xen.org/xen-devel

[Xen-devel] [PATCH v2 2/3] x86: use optimal NOPs to fill the SMEP/SMAP placeholders

[Jan Beulich]

Alternatives patching code picks the most suitable NOPs for the
running system, so simply use it to replace the pre-populated ones.

Use an arbitrary, always available feature to key off from, but
hide this behind the new X86_FEATURE_ALWAYS.

Signed-off-by: Jan Beulich 
---
v2: Introduce and use X86_FEATURE_ALWAYS.

--- a/xen/arch/x86/x86_64/compat/entry.S
+++ b/xen/arch/x86/x86_64/compat/entry.S
@@ -185,6 +185,7 @@ ENTRY(compat_restore_all_guest)
 mov   %rax, %cr4
 .Lcr4_alt_end:
 .section .altinstructions, "a"
+altinstruction_entry .Lcr4_orig, .Lcr4_orig, X86_FEATURE_ALWAYS, 12, 0
 altinstruction_entry .Lcr4_orig, .Lcr4_alt, X86_FEATURE_SMEP, 12, \
  (.Lcr4_alt_end - .Lcr4_alt)
 altinstruction_entry .Lcr4_orig, .Lcr4_alt, X86_FEATURE_SMAP, 12, \
--- a/xen/include/asm-x86/asm_defns.h
+++ b/xen/include/asm-x86/asm_defns.h
@@ -204,6 +204,7 @@ void ret_from_intr(void);
 662: __ASM_##op;   \
 .popsection;   \
 .pushsection .altinstructions, "a";\
+altinstruction_entry 661b, 661b, X86_FEATURE_ALWAYS, 3, 0; \
 altinstruction_entry 661b, 662b, X86_FEATURE_SMAP, 3, 3;   \
 .popsection
 
@@ -215,6 +216,7 @@ void ret_from_intr(void);
 .pushsection .altinstr_replacement, "ax";  \
 668: call cr4_pv32_restore;\
 .section .altinstructions, "a";\
+altinstruction_entry 667b, 667b, X86_FEATURE_ALWAYS, 5, 0; \
 altinstruction_entry 667b, 668b, X86_FEATURE_SMEP, 5, 5;   \
 altinstruction_entry 667b, 668b, X86_FEATURE_SMAP, 5, 5;   \
 .popsection
--- a/xen/include/asm-x86/cpufeature.h
+++ b/xen/include/asm-x86/cpufeature.h
@@ -162,6 +162,9 @@
 #define cpufeat_bit(idx)   ((idx) % 32)
 #define cpufeat_mask(idx)  (_AC(1, U) << cpufeat_bit(idx))
 
+/* An alias of a feature we know is always going to be present. */
+#define X86_FEATURE_ALWAYS  X86_FEATURE_LM
+
 #if !defined(__ASSEMBLY__) && !defined(X86_FEATURES_ONLY)
 #include 
 



x86: use optimal NOPs to fill the SMEP/SMAP placeholders

Alternatives patching code picks the most suitable NOPs for the
running system, so simply use it to replace the pre-populated ones.

Use an arbitrary, always available feature to key off from, but
hide this behind the new X86_FEATURE_ALWAYS.

Signed-off-by: Jan Beulich 
---
v2: Introduce and use X86_FEATURE_ALWAYS.

--- a/xen/arch/x86/x86_64/compat/entry.S
+++ b/xen/arch/x86/x86_64/compat/entry.S
@@ -185,6 +185,7 @@ ENTRY(compat_restore_all_guest)
 mov   %rax, %cr4
 .Lcr4_alt_end:
 .section .altinstructions, "a"
+altinstruction_entry .Lcr4_orig, .Lcr4_orig, X86_FEATURE_ALWAYS, 12, 0
 altinstruction_entry .Lcr4_orig, .Lcr4_alt, X86_FEATURE_SMEP, 12, \
  (.Lcr4_alt_end - .Lcr4_alt)
 altinstruction_entry .Lcr4_orig, .Lcr4_alt, X86_FEATURE_SMAP, 12, \
--- a/xen/include/asm-x86/asm_defns.h
+++ b/xen/include/asm-x86/asm_defns.h
@@ -204,6 +204,7 @@ void ret_from_intr(void);
 662: __ASM_##op;   \
 .popsection;   \
 .pushsection .altinstructions, "a";\
+altinstruction_entry 661b, 661b, X86_FEATURE_ALWAYS, 3, 0; \
 altinstruction_entry 661b, 662b, X86_FEATURE_SMAP, 3, 3;   \
 .popsection
 
@@ -215,6 +216,7 @@ void ret_from_intr(void);
 .pushsection .altinstr_replacement, "ax";  \
 668: call cr4_pv32_restore;\
 .section .altinstructions, "a";\
+altinstruction_entry 667b, 667b, X86_FEATURE_ALWAYS, 5, 0; \
 altinstruction_entry 667b, 668b, X86_FEATURE_SMEP, 5, 5;   \
 altinstruction_entry 667b, 668b, X86_FEATURE_SMAP, 5, 5;   \
 .popsection
--- a/xen/include/asm-x86/cpufeature.h
+++ b/xen/include/asm-x86/cpufeature.h
@@ -162,6 +162,9 @@
 #define cpufeat_bit(idx)   ((idx) % 32)
 #define cpufeat_mask(idx)  (_AC(1, U) << cpufeat_bit(idx))
 
+/* An alias of a feature we know is always going to be present. */
+#define X86_FEATURE_ALWAYS  X86_FEATURE_LM
+
 #if !defined(__ASSEMBLY__) && !defined(X86_FEATURES_ONLY)
 #include 
 
___
Xen-devel mailing list
Xen-devel@lists.xen.org
http://lists.xen.org/xen-devel