Re: [PATCH 2/2] arm64: errata: Workaround possible Cortex-A715 [ESR|FAR]_ELx corruption

[Anshuman Khandual <anshuman.khandual@arm.com>]

On 11/10/22 00:48, Catalin Marinas wrote:
> On Thu, Oct 27, 2022 at 08:09:15AM +0530, Anshuman Khandual wrote:
>> +#define __HAVE_ARCH_PTEP_MODIFY_PROT_TRANSACTION
>> +static inline pte_t ptep_modify_prot_start(struct vm_area_struct *vma,
>> +					   unsigned long addr,
>> +					   pte_t *ptep)
>> +{
>> +	pte_t pte = ptep_get_and_clear(vma->vm_mm, addr, ptep);
>>  
>> +	if (IS_ENABLED(CONFIG_ARM64_WORKAROUND_2645198)) {
>> +		/*
>> +		 * Break-before-make (BBM) is required for all user space mappings
>> +		 * when the permission changes from executable to non-executable
>> +		 * in cases where cpu is affected with errata #2645198.
>> +		 */
>> +		if (pte_user_exec(pte) && cpus_have_const_cap(ARM64_WORKAROUND_2645198))
>> +			__flush_tlb_range(vma, addr, addr + PAGE_SIZE, PAGE_SIZE, false, 3);
> 
> Why not flush_tlb_page() here?
> 
> But more importantly, can we not use ptep_clear_flush() instead (and

Something like ...

ptep_modify_prot_start -

if (IS_ENABLED(CONFIG_ARM64_WORKAROUND_2645198)) {
	if (pte_user_exec(READ_ONCE(*ptep)) && cpus_have_const_cap(ARM64_WORKAROUND_2645198))
		return ptep_clear_flush(vma, addr, ptep);
} else {
	return ptep_get_and_clear(vma->vm_mm, addr, ptep);
}

> huge_ptep_clear_flush())? They return the pte and do the TLBI.

huge_ptep_modify_prot_start -

if (IS_ENABLED(CONFIG_ARM64_WORKAROUND_2645198)) {
	if (pte_user_exec(READ_ONCE(*ptep)) && cpus_have_const_cap(ARM64_WORKAROUND_2645198))
		return huge_ptep_clear_flush(vma, addr, ptep);
} else {
	return huge_ptep_get_and_clear(vma->vm_mm, addr, ptep);
}

pte_user_exec(READ_ONCE(*ptep) should identify an user exec mapping even though
ptep represents a cont PTE/PMD huge page ? OR should huge_ptep_get() helper be
used instead ? Regardless, using [huge_]ptep_clear_flush() here seems better.