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]

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.