Re: [PATCH v12 4/8] arm64: kvm: Introduce MTE VM feature

[Catalin Marinas <catalin.marinas@arm.com>]

On Mon, May 17, 2021 at 01:32:35PM +0100, Steven Price wrote:
> diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
> index c5d1f3c87dbd..8660f6a03f51 100644
> --- a/arch/arm64/kvm/mmu.c
> +++ b/arch/arm64/kvm/mmu.c
> @@ -822,6 +822,31 @@ transparent_hugepage_adjust(struct kvm_memory_slot *memslot,
>  	return PAGE_SIZE;
>  }
>  
> +static int sanitise_mte_tags(struct kvm *kvm, unsigned long size,
> +			     kvm_pfn_t pfn)
> +{
> +	if (kvm_has_mte(kvm)) {
> +		/*
> +		 * The page will be mapped in stage 2 as Normal Cacheable, so
> +		 * the VM will be able to see the page's tags and therefore
> +		 * they must be initialised first. If PG_mte_tagged is set,
> +		 * tags have already been initialised.
> +		 */
> +		unsigned long i, nr_pages = size >> PAGE_SHIFT;
> +		struct page *page = pfn_to_online_page(pfn);
> +
> +		if (!page)
> +			return -EFAULT;

IIRC we ended up with pfn_to_online_page() to reject ZONE_DEVICE pages
that may be mapped into a guest and we have no idea whether they support
MTE. It may be worth adding a comment, otherwise, as Marc said, the page
wouldn't disappear.

> +
> +		for (i = 0; i < nr_pages; i++, page++) {
> +			if (!test_and_set_bit(PG_mte_tagged, &page->flags))
> +				mte_clear_page_tags(page_address(page));

We started the page->flags thread and ended up fixing it for the host
set_pte_at() as per the first patch:

https://lore.kernel.org/r/c3293d47-a5f2-ea4a-6730-f5cae26d8a7e@arm.com

Now, can we have a race between the stage 2 kvm_set_spte_gfn() and a
stage 1 set_pte_at()? Only the latter takes a lock. Or between two
kvm_set_spte_gfn() in different VMs? I think in the above thread we
concluded that there's only a problem if the page is shared between
multiple VMMs (MAP_SHARED). How realistic is this and what's the
workaround?

Either way, I think it's worth adding a comment here on the race on
page->flags as it looks strange that here it's just a test_and_set_bit()
while set_pte_at() uses a spinlock.

-- 
Catalin
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