Re: [RFC PATCH 01/12] KVM: arm64: Relax WARN check in stage2_make_pte()

[Oliver Upton <oliver.upton@linux.dev>]

Hi Ricardo,

On Sat, Nov 12, 2022 at 08:17:03AM +0000, Ricardo Koller wrote:
> stage2_make_pte() throws a warning when used in a non-shared walk, as PTEs
> are not "locked" when walking non-shared. Add a check so it can be used
> non-shared.
> 
> Signed-off-by: Ricardo Koller <ricarkol@google.com>

I would very much prefer to leave this WARN as-is. Correct me if I am
wrong, but I do not believe this warning is firing with the existing
code.

While the locking portion doesn't make a whole lot of sense for a
non-shared walk, it is also a magic value that indicates we've already
done the break side of break-before-make. If the warning fires then that
would suggest our break-before-make implementation isn't working as
expected.

--
Thanks,
Oliver

> ---
>  arch/arm64/kvm/hyp/pgtable.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c
> index c12462439e70..b16107bf917c 100644
> --- a/arch/arm64/kvm/hyp/pgtable.c
> +++ b/arch/arm64/kvm/hyp/pgtable.c
> @@ -733,7 +733,8 @@ static void stage2_make_pte(const struct kvm_pgtable_visit_ctx *ctx, kvm_pte_t n
>  {
>  	struct kvm_pgtable_mm_ops *mm_ops = ctx->mm_ops;
>  
> -	WARN_ON(!stage2_pte_is_locked(*ctx->ptep));
> +	if (kvm_pgtable_walk_shared(ctx))
> +		WARN_ON(!stage2_pte_is_locked(*ctx->ptep));
>  
>  	if (stage2_pte_is_counted(new))
>  		mm_ops->get_page(ctx->ptep);
> -- 
> 2.38.1.431.g37b22c650d-goog
> 
>