Re: [RFC PATCH 06/17] KVM: arm64: Implement break-before-make sequence for parallel walks

[Quentin Perret <qperret@google.com>]

On Friday 15 Apr 2022 at 21:58:50 (+0000), Oliver Upton wrote:
> +/*
> + * Used to indicate a pte for which a 'make-before-break' sequence is in

'break-before-make' presumably :-) ?

<snip>
> +static void stage2_make_pte(kvm_pte_t *ptep, kvm_pte_t new, struct kvm_pgtable_mm_ops *mm_ops)
> +{
> +	/* Yikes! We really shouldn't install to an entry we don't own. */
> +	WARN_ON(!stage2_pte_is_locked(*ptep));
> +
> +	if (stage2_pte_is_counted(new))
> +		mm_ops->get_page(ptep);
> +
> +	if (kvm_pte_valid(new)) {
> +		WRITE_ONCE(*ptep, new);
> +		dsb(ishst);
> +	} else {
> +		smp_store_release(ptep, new);
> +	}
> +}

I'm struggling a bit to understand this pattern. We currently use
smp_store_release() to install valid mappings, which this patch seems
to change. Is the behaviour change intentional? If so, could you please
share some details about the reasoning that applies here?

Thanks,
Quentin