Re: [PATCH v6 03/12] KVM: arm64: Add helper for creating unlinked stage2 subtrees

[Ricardo Koller <ricarkol@google.com>]

On Sun, Mar 12, 2023 at 11:06:31AM +0000, Marc Zyngier wrote:
> On Tue, 07 Mar 2023 03:45:46 +0000,
> Ricardo Koller <ricarkol@google.com> 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 PMD and PUD blocks into subtrees of PAGE_SIZE
> 
> Please drop the PMD/PUD verbiage. That's specially confusing when
> everything is described in terms of 'level'
> 
> > PTEs.  For example, a PUD can be split into PAGE_SIZE PTEs by first
> 
> for example: s/a PUD/a level 1 mapping/
> 
> > creating a fully populated tree, and then use it to replace the PUD in
> > a single step.  This will be used in a subsequent commit for eager
> > huge-page splitting (a dirty-logging optimization).
> > 
> > No functional change intended. This new function will be used in a
> > subsequent commit.
> 
> Drop this last sentence, it doesn't say anything that you haven't
> already said.
> 
> > 
> > Signed-off-by: Ricardo Koller <ricarkol@google.com>
> > Reviewed-by: Shaoqin Huang <shahuang@redhat.com>
> > ---
> >  arch/arm64/include/asm/kvm_pgtable.h | 28 +++++++++++++++++
> >  arch/arm64/kvm/hyp/pgtable.c         | 46 ++++++++++++++++++++++++++++
> >  2 files changed, 74 insertions(+)
> > 
> > diff --git a/arch/arm64/include/asm/kvm_pgtable.h b/arch/arm64/include/asm/kvm_pgtable.h
> > index c7a269cad053..b7b3fc0fa7a5 100644
> > --- a/arch/arm64/include/asm/kvm_pgtable.h
> > +++ b/arch/arm64/include/asm/kvm_pgtable.h
> > @@ -468,6 +468,34 @@ 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
> > + *		page-table pages.
> > + * @force_pte:  Force mappings to PAGE_SIZE granularity.
> > + *
> > + * Returns an unlinked page-table tree. If @force_pte is true or
> > + * @level is 2 (the PMD level), then the tree is mapped up to the
> > + * PAGE_SIZE leaf PTE; the tree is mapped up one level otherwise.
> 
> I wouldn't make this "one level" assumption, as this really depends on
> the size of what gets mapped (and future evolution of this code).
> 
> > + * 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.
> 
> What guarantees that this new unlinked structure is kept in sync with
> the original one? AFAICT, nothing does.
> 

That should be the job of the caller: kvm_pgtable_stage2_split() in this
series.

> > + */
> > +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 4f703cc4cb03..6bdfcb671b32 100644
> > --- a/arch/arm64/kvm/hyp/pgtable.c
> > +++ b/arch/arm64/kvm/hyp/pgtable.c
> > @@ -1212,6 +1212,52 @@ 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 |
> > +				  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,
> 
> Is that always true? What if the caller expect a non-block-aligned
> mapping? You should at least check that phys is aligned to the granule
> size of 'level', or bad stuff may happen.
> 

Good point, it should be true, but I will add a check to fail with
EINVAL on that case. Will also double check the caller's code to be sure
mappings are always block-aligned.

> > +		.end	= kvm_granule_size(level),
> > +	};
> > +	struct kvm_pgtable_mm_ops *mm_ops = pgt->mm_ops;
> > +	kvm_pte_t *pgtable;
> > +	int ret;
> > +
> > +	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,
> 
> 	M.
> 
> -- 

ACK on all the other comments.

Thanks,
Ricardo

> Without deviation from the norm, progress is not possible.