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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 13/05/16 17:06, Jan Beulich wrote:
>>>> 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?

Yes.

~Andrew

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