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

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Date:   Fri, 11 Nov 2022 22:36:31 +0000
From:   Catalin Marinas <catalin.marinas@....com>
To:     Anshuman Khandual <anshuman.khandual@....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 Thu, Nov 10, 2022 at 08:45:07AM +0530, Anshuman Khandual wrote:
> 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);
> }

Yes, this should work but avoid the 'else' when you have a return, so
something like:

	if (IS_ENABLED(CONFIG_ARM64_WORKAROUND_2645198) &&
	    cpus_have_const_cap(ARM64_WORKAROUND_2645198) &&
	    pte_user_exec(READ_ONCE(*ptep)))
		return ptep_clear_flush(vma, addr, ptep);

	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 ?

This should work as a shortcut. The contiguous ptes should all be the
same, so it's sufficient to check one of them.

-- 
Catalin