Re: [PATCH v7 03/12] KVM: arm64: Add helper for creating unlinked stage2 subtrees

[Ricardo Koller <ricarkol@google.com>]

On Mon, Apr 17, 2023 at 02:18:26PM +0800, Gavin Shan wrote:
> On 4/9/23 2:29 PM, Ricardo Koller wrote:
> > Add a stage2 helper, kvm_pgtable_stage2_create_unlinked(), for
> > creating unlinked tables (which is the opposite of
> > kvm_pgtable_stage2_free_unlinked()).  Creating an unlinked table is
> > useful for splitting level 1 and 2 entries into subtrees of PAGE_SIZE
> > PTEs.  For example, a level 1 entry can be split into PAGE_SIZE PTEs
> > by first creating a fully populated tree, and then use it to replace
> > the level 1 entry in a single step.  This will be used in a subsequent
> > commit for eager huge-page splitting (a dirty-logging optimization).
> > 
> > Signed-off-by: Ricardo Koller <ricarkol@google.com>
> > Reviewed-by: Shaoqin Huang <shahuang@redhat.com>
> > ---
> >   arch/arm64/include/asm/kvm_pgtable.h | 26 +++++++++++++++
> >   arch/arm64/kvm/hyp/pgtable.c         | 49 ++++++++++++++++++++++++++++
> >   2 files changed, 75 insertions(+)
> > 
> 
> With the following nits addressed:
> 
> Reviewed-by: Gavin Shan <gshan@redhat.com>
> 
> > diff --git a/arch/arm64/include/asm/kvm_pgtable.h b/arch/arm64/include/asm/kvm_pgtable.h
> > index 3f2d43ba2b628..c8e0e7d9303b2 100644
> > --- a/arch/arm64/include/asm/kvm_pgtable.h
> > +++ b/arch/arm64/include/asm/kvm_pgtable.h
> > @@ -458,6 +458,32 @@ void kvm_pgtable_stage2_destroy(struct kvm_pgtable *pgt);
> >    */
> >   void kvm_pgtable_stage2_free_unlinked(struct kvm_pgtable_mm_ops *mm_ops, void *pgtable, u32 level);
> > +/**
> > + * kvm_pgtable_stage2_create_unlinked() - Create an unlinked stage-2 paging structure.
> > + * @pgt:	Page-table structure initialised by kvm_pgtable_stage2_init*().
> > + * @phys:	Physical address of the memory to map.
> > + * @level:	Starting level of the stage-2 paging structure to be created.
> > + * @prot:	Permissions and attributes for the mapping.
> > + * @mc:		Cache of pre-allocated and zeroed memory from which to allocate
>                 ^^^^^^^^
> Alignment.

This seems to be due to the "+ ". It looks like this without it:

 * kvm_pgtable_stage2_create_unlinked() - Create an unlinked stage-2 paging structure.
 * @pgt:        Page-table structure initialised by kvm_pgtable_stage2_init*().
 * @phys:       Physical address of the memory to map.
 * @level:      Starting level of the stage-2 paging structure to be created.
 * @prot:       Permissions and attributes for the mapping.
 * @mc:         Cache of pre-allocated and zeroed memory from which to allocate
 *              page-table pages.

> 
> > + *		page-table pages.
> > + * @force_pte:  Force mappings to PAGE_SIZE granularity.
> > + *
> > + * Returns an unlinked page-table tree.  This new page-table tree is
> > + * not reachable (i.e., it is unlinked) from the root pgd and it's
> > + * therefore unreachableby the hardware page-table walker. No TLB
> > + * invalidation or CMOs are performed.
> > + *
> > + * If device attributes are not explicitly requested in @prot, then the
> > + * mapping will be normal, cacheable.
> > + *
> > + * Return: The fully populated (unlinked) stage-2 paging structure, or
> > + * an ERR_PTR(error) on failure.
> > + */
> > +kvm_pte_t *kvm_pgtable_stage2_create_unlinked(struct kvm_pgtable *pgt,
> > +					      u64 phys, u32 level,
> > +					      enum kvm_pgtable_prot prot,
> > +					      void *mc, bool force_pte);
> > +
> >   /**
> >    * kvm_pgtable_stage2_map() - Install a mapping in 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 633679ee3c49a..477d2be67d401 100644
> > --- a/arch/arm64/kvm/hyp/pgtable.c
> > +++ b/arch/arm64/kvm/hyp/pgtable.c
> > @@ -1222,6 +1222,55 @@ int kvm_pgtable_stage2_flush(struct kvm_pgtable *pgt, u64 addr, u64 size)
> >   	return kvm_pgtable_walk(pgt, addr, size, &walker);
> >   }
> > +kvm_pte_t *kvm_pgtable_stage2_create_unlinked(struct kvm_pgtable *pgt,
> > +					      u64 phys, u32 level,
> > +					      enum kvm_pgtable_prot prot,
> > +					      void *mc, bool force_pte)
> > +{
> > +	struct stage2_map_data map_data = {
> > +		.phys		= phys,
> > +		.mmu		= pgt->mmu,
> > +		.memcache	= mc,
> > +		.force_pte	= force_pte,
> > +	};
> > +	struct kvm_pgtable_walker walker = {
> > +		.cb		= stage2_map_walker,
> > +		.flags		= KVM_PGTABLE_WALK_LEAF |
> > +				  KVM_PGTABLE_WALK_SKIP_BBM_TLBI |
> > +				  KVM_PGTABLE_WALK_SKIP_CMO,
> > +		.arg		= &map_data,
> > +	};
> > +	/* .addr (the IPA) is irrelevant for an unlinked table */
> > +	struct kvm_pgtable_walk_data data = {
> > +		.walker	= &walker,
> > +		.addr	= 0,
> > +		.end	= kvm_granule_size(level),
> > +	};
> 
> The comment about '.addr' seems incorrect. The IPA address is still
> used to locate the page table entry, so I think it would be something
> like below:
> 
> 	/* The IPA address (.addr) is relative to zero */
> 

Extended it to say this:

         * The IPA address (.addr) is relative to zero. The goal is to
         * map "kvm_granule_size(level) - 0" worth of pages.

> > +	struct kvm_pgtable_mm_ops *mm_ops = pgt->mm_ops;
> > +	kvm_pte_t *pgtable;
> > +	int ret;
> > +
> > +	if (!IS_ALIGNED(phys, kvm_granule_size(level)))
> > +		return ERR_PTR(-EINVAL);
> > +
> > +	ret = stage2_set_prot_attr(pgt, prot, &map_data.attr);
> > +	if (ret)
> > +		return ERR_PTR(ret);
> > +
> > +	pgtable = mm_ops->zalloc_page(mc);
> > +	if (!pgtable)
> > +		return ERR_PTR(-ENOMEM);
> > +
> > +	ret = __kvm_pgtable_walk(&data, mm_ops, (kvm_pteref_t)pgtable,
> > +				 level + 1);
> > +	if (ret) {
> > +		kvm_pgtable_stage2_free_unlinked(mm_ops, pgtable, level);
> > +		mm_ops->put_page(pgtable);
> > +		return ERR_PTR(ret);
> > +	}
> > +
> > +	return pgtable;
> > +}
> >   int __kvm_pgtable_stage2_init(struct kvm_pgtable *pgt, struct kvm_s2_mmu *mmu,
> >   			      struct kvm_pgtable_mm_ops *mm_ops,
> > 
> 
> Thanks,
> Gavin
>