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

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Message-ID: <73c40107-0d7a-d988-c817-7bba6d72c371@arm.com>
Date:   Thu, 10 Nov 2022 08:45:07 +0530
From:   Anshuman Khandual <anshuman.khandual@....com>
To:     Catalin Marinas <catalin.marinas@....com>
Cc:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        will@...nel.org, Suzuki K Poulose <suzuki.poulose@....com>,
        James Morse <james.morse@....com>,
        Jonathan Corbet <corbet@....net>,
        Mark Rutland <mark.rutland@....com>, linux-doc@...r.kernel.org
Subject: Re: [PATCH 2/2] arm64: errata: Workaround possible Cortex-A715
 [ESR|FAR]_ELx corruption



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.