Re: [PATCH v4 28/34] KVM: arm64: Use page-table to track page ownership

[Will Deacon <will@kernel.org>]

On Wed, Mar 10, 2021 at 05:57:45PM +0000, Quentin Perret wrote:
> As the host stage 2 will be identity mapped, all the .hyp memory regions
> and/or memory pages donated to protected guestis will have to marked
> invalid in the host stage 2 page-table. At the same time, the hypervisor
> will need a way to track the ownership of each physical page to ensure
> memory sharing or donation between entities (host, guests, hypervisor) is
> legal.
> 
> In order to enable this tracking at EL2, let's use the host stage 2
> page-table itself. The idea is to use the top bits of invalid mappings
> to store the unique identifier of the page owner. The page-table owner
> (the host) gets identifier 0 such that, at boot time, it owns the entire
> IPA space as the pgd starts zeroed.
> 
> Provide kvm_pgtable_stage2_set_owner() which allows to modify the
> ownership of pages in the host stage 2. It re-uses most of the map()
> logic, but ends up creating invalid mappings instead. This impacts
> how we do refcount as we now need to count invalid mappings when they
> are used for ownership tracking.
> 
> Signed-off-by: Quentin Perret <qperret@google.com>
> ---
>  arch/arm64/include/asm/kvm_pgtable.h | 21 +++++++
>  arch/arm64/kvm/hyp/pgtable.c         | 92 ++++++++++++++++++++++++----
>  2 files changed, 101 insertions(+), 12 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/kvm_pgtable.h b/arch/arm64/include/asm/kvm_pgtable.h
> index 4ae19247837b..b09af4612656 100644
> --- a/arch/arm64/include/asm/kvm_pgtable.h
> +++ b/arch/arm64/include/asm/kvm_pgtable.h
> @@ -238,6 +238,27 @@ int kvm_pgtable_stage2_map(struct kvm_pgtable *pgt, u64 addr, u64 size,
>  			   u64 phys, enum kvm_pgtable_prot prot,
>  			   void *mc);
>  
> +/**
> + * kvm_pgtable_stage2_set_owner() - Annotate invalid mappings with metadata
> + *				    encoding the ownership of a page in the
> + *				    IPA space.
> + * @pgt:	Page-table structure initialised by kvm_pgtable_stage2_init().
> + * @addr:	Intermediate physical address at which to place the annotation.

This confused me a bit, as the annotation is stored in the page-table, not
at the memory identified by @addr. How about:

  "Base intermediate physical address to annotate"

> + * @size:	Size of the IPA range to annotate.

  "Size of the annotated range"

> + * @mc:		Cache of pre-allocated and zeroed memory from which to allocate
> + *		page-table pages.
> + * @owner_id:	Unique identifier for the owner of the page.
> + *
> + * The page-table owner has identifier 0.

Perhaps, "By default, all page-tables are owned by identifier 0"

> + *
> + * Return: 0 on success, negative error code on failure.
> + */
> +int kvm_pgtable_stage2_set_owner(struct kvm_pgtable *pgt, u64 addr, u64 size,
> +				 void *mc, u32 owner_id);

Is there a need for the owner_id to be 32-bit rather than e.g. 16-bit? Just
strikes me that it might be difficult to recover these bits in future if we
give them out freely now.

> +
>  /**
>   * kvm_pgtable_stage2_unmap() - Remove a mapping from a guest stage-2 page-table.
>   * @pgt:	Page-table structure initialised by kvm_pgtable_stage2_init().
> diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c
> index f37b4179b880..e4670b639726 100644
> --- a/arch/arm64/kvm/hyp/pgtable.c
> +++ b/arch/arm64/kvm/hyp/pgtable.c
> @@ -48,6 +48,8 @@
>  					 KVM_PTE_LEAF_ATTR_LO_S2_S2AP_W | \
>  					 KVM_PTE_LEAF_ATTR_HI_S2_XN)
>  
> +#define KVM_INVALID_PTE_OWNER_MASK	GENMASK(63, 32)

Ah, so that '02' earlier was a typo for '32'.

> +
>  struct kvm_pgtable_walk_data {
>  	struct kvm_pgtable		*pgt;
>  	struct kvm_pgtable_walker	*walker;
> @@ -186,6 +188,11 @@ static kvm_pte_t kvm_init_valid_leaf_pte(u64 pa, kvm_pte_t attr, u32 level)
>  	return pte;
>  }
>  
> +static kvm_pte_t kvm_init_invalid_leaf_owner(u32 owner_id)
> +{
> +	return FIELD_PREP(KVM_INVALID_PTE_OWNER_MASK, owner_id);
> +}
> +
>  static int kvm_pgtable_visitor_cb(struct kvm_pgtable_walk_data *data, u64 addr,
>  				  u32 level, kvm_pte_t *ptep,
>  				  enum kvm_pgtable_walk_flags flag)
> @@ -440,6 +447,7 @@ void kvm_pgtable_hyp_destroy(struct kvm_pgtable *pgt)
>  struct stage2_map_data {
>  	u64				phys;
>  	kvm_pte_t			attr;
> +	u32				owner_id;
>  
>  	kvm_pte_t			*anchor;
>  	kvm_pte_t			*childp;
> @@ -506,6 +514,24 @@ static int stage2_map_set_prot_attr(enum kvm_pgtable_prot prot,
>  	return 0;
>  }
>  
> +static bool stage2_is_permission_change(kvm_pte_t old, kvm_pte_t new)
> +{
> +	if (!kvm_pte_valid(old) || !kvm_pte_valid(new))
> +		return false;
> +
> +	return !((old ^ new) & (~KVM_PTE_LEAF_ATTR_S2_PERMS));
> +}

This new name throws me, because it will return true if old == new.

> +static bool stage2_pte_is_counted(kvm_pte_t pte)
> +{
> +	/*
> +	 * The refcount tracks valid entries as well as invalid entries if they
> +	 * encode ownership of a page to another entity than the page-table
> +	 * owner, whose id is 0.
> +	 */
> +	return !!pte;
> +}
> +
>  static int stage2_map_walker_try_leaf(u64 addr, u64 end, u32 level,
>  				      kvm_pte_t *ptep,
>  				      struct stage2_map_data *data)
> @@ -517,28 +543,36 @@ static int stage2_map_walker_try_leaf(u64 addr, u64 end, u32 level,
>  	if (!kvm_block_mapping_supported(addr, end, phys, level))
>  		return -E2BIG;
>  
> -	new = kvm_init_valid_leaf_pte(phys, data->attr, level);
> -	if (kvm_pte_valid(old)) {
> +	if (kvm_pte_valid(data->attr))

This feels like a bit of a hack to me: the 'attr' field in stage2_map_data
is intended to correspond directly to the lower/upper attributes of the
descriptor as per the architecture, so tagging the valid bit in there is
pretty grotty. However, I can see the significant advantage in being able
to re-use the stage2_map_walker functionality, so about instead of nobbling
attr, you set phys to something invalid instead, e.g.:

	#define KVM_PHYS_SET_OWNER	(-1ULL)

> +		new = kvm_init_valid_leaf_pte(phys, data->attr, level);
> +	else
> +		new = kvm_init_invalid_leaf_owner(data->owner_id);
> +
> +	if (stage2_pte_is_counted(old)) {
>  		/*
>  		 * Skip updating the PTE if we are trying to recreate the exact
>  		 * same mapping or only change the access permissions. Instead,
>  		 * the vCPU will exit one more time from guest if still needed
>  		 * and then go through the path of relaxing permissions.
>  		 */
> -		if (!((old ^ new) & (~KVM_PTE_LEAF_ATTR_S2_PERMS)))
> +		if (stage2_is_permission_change(old, new))
>  			return -EAGAIN;
>  
>  		/*
> -		 * There's an existing different valid leaf entry, so perform
> -		 * break-before-make.
> +		 * Clear the existing PTE, and perform break-before-make with
> +		 * TLB maintenance if it was valid.
>  		 */
>  		kvm_clear_pte(ptep);
> -		kvm_call_hyp(__kvm_tlb_flush_vmid_ipa, data->mmu, addr, level);
> +		if (kvm_pte_valid(old)) {
> +			kvm_call_hyp(__kvm_tlb_flush_vmid_ipa, data->mmu, addr,
> +				     level);
> +		}

Why do you clear the pte unconditionally here? I think you can move that
into the 'if' as well.

>  		mm_ops->put_page(ptep);
>  	}
>  
>  	smp_store_release(ptep, new);
> -	mm_ops->get_page(ptep);
> +	if (stage2_pte_is_counted(new))
> +		mm_ops->get_page(ptep);
>  	data->phys += granule;
>  	return 0;
>  }
> @@ -574,7 +608,7 @@ static int stage2_map_walk_leaf(u64 addr, u64 end, u32 level, kvm_pte_t *ptep,
>  	int ret;
>  
>  	if (data->anchor) {
> -		if (kvm_pte_valid(pte))
> +		if (stage2_pte_is_counted(pte))
>  			mm_ops->put_page(ptep);
>  
>  		return 0;
> @@ -599,9 +633,10 @@ static int stage2_map_walk_leaf(u64 addr, u64 end, u32 level, kvm_pte_t *ptep,
>  	 * a table. Accesses beyond 'end' that fall within the new table
>  	 * will be mapped lazily.
>  	 */
> -	if (kvm_pte_valid(pte)) {
> +	if (stage2_pte_is_counted(pte)) {
>  		kvm_clear_pte(ptep);
> -		kvm_call_hyp(__kvm_tlb_flush_vmid_ipa, data->mmu, addr, level);
> +		if (kvm_pte_valid(pte))
> +			kvm_call_hyp(__kvm_tlb_flush_vmid_ipa, data->mmu, addr, level);

Same here.

>  		mm_ops->put_page(ptep);
>  	}
>  
> @@ -683,6 +718,7 @@ int kvm_pgtable_stage2_map(struct kvm_pgtable *pgt, u64 addr, u64 size,
>  		.mmu		= pgt->mmu,
>  		.memcache	= mc,
>  		.mm_ops		= pgt->mm_ops,
> +		.owner_id	= 0,

Not needed.

>  	};
>  	struct kvm_pgtable_walker walker = {
>  		.cb		= stage2_map_walker,
> @@ -696,6 +732,33 @@ int kvm_pgtable_stage2_map(struct kvm_pgtable *pgt, u64 addr, u64 size,
>  	if (ret)
>  		return ret;
>  
> +	/* Set the valid flag to distinguish with the set_owner() path. */
> +	map_data.attr |= KVM_PTE_VALID;

(see earlier comments)

> +	ret = kvm_pgtable_walk(pgt, addr, size, &walker);
> +	dsb(ishst);
> +	return ret;
> +}
> +
> +int kvm_pgtable_stage2_set_owner(struct kvm_pgtable *pgt, u64 addr, u64 size,
> +				 void *mc, u32 owner_id)
> +{
> +	int ret;
> +	struct stage2_map_data map_data = {
> +		.mmu		= pgt->mmu,
> +		.memcache	= mc,
> +		.mm_ops		= pgt->mm_ops,
> +		.owner_id	= owner_id,
> +		.attr		= 0,

Not needed.

> +	};
> +	struct kvm_pgtable_walker walker = {
> +		.cb		= stage2_map_walker,
> +		.flags		= KVM_PGTABLE_WALK_TABLE_PRE |
> +				  KVM_PGTABLE_WALK_LEAF |
> +				  KVM_PGTABLE_WALK_TABLE_POST,

Oh man, now I see why phys is zero -- so that the table callbacks will
put down the largest possible block. That needs a comment because it's
pretty horrible, and also means my idea of using -1 won't help you. Hmm.

Is there ever a reason to use kvm_pgtable_stage2_set_owner() to set an
owner of 0, or should you just use the map/unmap APIs for that? If so,
then maybe the key is simply if owner_id is non-zero, then an invalid
entry is installed?

Will
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