Re: [PATCH v2 12/39] x86/mm: Update ptep_set_wrprotect() and pmdp_set_wrprotect() for transition from _PAGE_DIRTY to _PAGE_COW

Dave Hansen
2022-10-03T18:51:23Z

On 10/3/22 11:11, Nadav Amit wrote:
>> +#ifdef CONFIG_X86_SHADOW_STACK
>> + /*
>> + * Avoid accidentally creating shadow stack PTEs
>> + * (Write=0,Dirty=1). Use cmpxchg() to prevent races with
>> + * the hardware setting Dirty=1.
>> + */
>> + if (cpu_feature_enabled(X86_FEATURE_SHSTK)) {
>> + pte_t old_pte, new_pte;
>> +
>> + old_pte = READ_ONCE(*ptep);
>> + do {
>> + new_pte = pte_wrprotect(old_pte);
>> + } while (!try_cmpxchg(&ptep->pte, &old_pte.pte, new_pte.pte));
>> +
>> + return;
>> + }
>> +#endif
> There is no way of using IS_ENABLED() here instead of these ifdefs?

Actually, both the existing #ifdef and an IS_ENABLED() check would be
is superfluous as-is.

Adding X86_FEATURE_SHSTK disabled-features.h gives cpu_feature_enabled()
compile-time optimizations for free. No need for *any* additional
CONFIG_* checks.

The only issue would be if the #ifdef'd code won't even compile with
X86_FEATURE_SHSTK disabled.