Re: [Xen-devel] [PATCH v3 3/4] x86: use optimal NOPs to fill the SMEP/SMAP placeholders

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 13.05.16 at 17:57, <andrew.cooper3@xxxxxxxxxx> wrote:
> On 17/03/16 08:03, 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 <jbeulich@xxxxxxxx>
>> ---
>> v3: Re-base.
>> 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
>> @@ -175,12 +175,7 @@ compat_bad_hypercall:
>>  ENTRY(compat_restore_all_guest)
>>          ASSERT_INTERRUPTS_DISABLED
>>  .Lcr4_orig:
>> -        ASM_NOP8 /* testb $3,UREGS_cs(%rsp) */
>> -        ASM_NOP2 /* jpe   .Lcr4_alt_end */
>> -        ASM_NOP8 /* mov   CPUINFO_cr4...(%rsp), %rax */
>> -        ASM_NOP6 /* and   $..., %rax */
>> -        ASM_NOP8 /* mov   %rax, CPUINFO_cr4...(%rsp) */
>> -        ASM_NOP3 /* mov   %rax, %cr4 */
>> +        .skip (.Lcr4_alt_end - .Lcr4_alt) - (. - .Lcr4_orig), 0x90
>>  .Lcr4_orig_end:
>>          .pushsection .altinstr_replacement, "ax"
>>  .Lcr4_alt:
> 
> This hunk should live in patch 2.

No. In patch 2 we want to leverage multi-byte NOPs. Here, knowing
they're going to be replaced anyway, we are fine with using the
simpler .fill (producing many single byte ones).

> Reviewed-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>

Does this stand nevertheless?

Jan


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