diff mbox

[3/4] x86: use optimal NOPs to fill the SMAP/SMEP placeholders

Message ID 56D97F6702000078000D9565@prv-mh.provo.novell.com (mailing list archive)
State New, archived
Headers show

Commit Message

Jan Beulich March 4, 2016, 11:28 a.m. UTC 
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.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
x86: use optimal NOPs to fill the SMAP/SMEP 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.

Signed-off-by: Jan Beulich <jbeulich@suse.com>

--- 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_LM, 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_LM, 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_smep_smap_restore;                               \
         .section .altinstructions, "a";                                \
+        altinstruction_entry 667b, 667b, X86_FEATURE_LM, 5, 0;         \
         altinstruction_entry 667b, 668b, X86_FEATURE_SMEP, 5, 5;       \
         altinstruction_entry 667b, 668b, X86_FEATURE_SMAP, 5, 5;       \
         .popsection

Comments

Andrew Cooper March 7, 2016, 5:43 p.m. UTC | #1 
On 04/03/16 11:28, 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.

I would be tempted to introduce X86_FEATURE_ALWAYS as an alias of
X86_FEATURE_LM, or even a new synthetic feature.  The choice of LM is
explained in the commit message, but will be non-obvious to people
reading the code.

~Andrew
Jan Beulich March 8, 2016, 8:02 a.m. UTC | #2 
>>> On 07.03.16 at 18:43, <andrew.cooper3@citrix.com> wrote:
> On 04/03/16 11:28, 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.
> 
> I would be tempted to introduce X86_FEATURE_ALWAYS as an alias of
> X86_FEATURE_LM, or even a new synthetic feature.  The choice of LM is
> explained in the commit message, but will be non-obvious to people
> reading the code.

Okay, as an alias.

Jan
diff mbox

Patch

--- 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_LM, 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_LM, 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_smep_smap_restore;                               \
         .section .altinstructions, "a";                                \
+        altinstruction_entry 667b, 667b, X86_FEATURE_LM, 5, 0;         \
         altinstruction_entry 667b, 668b, X86_FEATURE_SMEP, 5, 5;       \
         altinstruction_entry 667b, 668b, X86_FEATURE_SMAP, 5, 5;       \
         .popsection