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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

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.

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

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