Re: [PATCH v4 30/34] KVM: arm64: Add kvm_pgtable_stage2_find_range()

[Quentin Perret <qperret@google.com>]

On Thursday 11 Mar 2021 at 19:04:07 (+0000), Will Deacon wrote:
> On Wed, Mar 10, 2021 at 05:57:47PM +0000, Quentin Perret wrote:
> > Since the host stage 2 will be identity mapped, and since it will own
> > most of memory, it would preferable for performance to try and use large
> > block mappings whenever that is possible. To ease this, introduce a new
> > helper in the KVM page-table code which allows to search for large
> > ranges of available IPA space. This will be used in the host memory
> > abort path to greedily idmap large portion of the PA space.
> > 
> > Signed-off-by: Quentin Perret <qperret@google.com>
> > ---
> >  arch/arm64/include/asm/kvm_pgtable.h | 30 ++++++++++
> >  arch/arm64/kvm/hyp/pgtable.c         | 90 +++++++++++++++++++++++++++-
> >  2 files changed, 117 insertions(+), 3 deletions(-)
> > 
> > diff --git a/arch/arm64/include/asm/kvm_pgtable.h b/arch/arm64/include/asm/kvm_pgtable.h
> > index b09af4612656..477bf10c48a9 100644
> > --- a/arch/arm64/include/asm/kvm_pgtable.h
> > +++ b/arch/arm64/include/asm/kvm_pgtable.h
> > @@ -94,6 +94,16 @@ enum kvm_pgtable_prot {
> >  #define PAGE_HYP_RO		(KVM_PGTABLE_PROT_R)
> >  #define PAGE_HYP_DEVICE		(PAGE_HYP | KVM_PGTABLE_PROT_DEVICE)
> >  
> > +/**
> > + * struct kvm_mem_range - Range of Intermediate Physical Addresses
> > + * @start:	Start of the range.
> > + * @end:	End of the range.
> > + */
> > +struct kvm_mem_range {
> > +	u64 start;
> > +	u64 end;
> > +};
> > +
> >  /**
> >   * enum kvm_pgtable_walk_flags - Flags to control a depth-first page-table walk.
> >   * @KVM_PGTABLE_WALK_LEAF:		Visit leaf entries, including invalid
> > @@ -398,4 +408,24 @@ int kvm_pgtable_stage2_flush(struct kvm_pgtable *pgt, u64 addr, u64 size);
> >  int kvm_pgtable_walk(struct kvm_pgtable *pgt, u64 addr, u64 size,
> >  		     struct kvm_pgtable_walker *walker);
> >  
> > +/**
> > + * kvm_pgtable_stage2_find_range() - Find a range of Intermediate Physical
> > + *				     Addresses with compatible permission
> > + *				     attributes.
> > + * @pgt:	Page-table structure initialised by kvm_pgtable_stage2_init().
> > + * @addr:	Address that must be covered by the range.
> > + * @prot:	Protection attributes that the range must be compatible with.
> > + * @range:	Range structure used to limit the search space at call time and
> > + *		that will hold the result.
> > + *
> > + * The offset of @addr within a page is ignored. An existing mapping is defined
> > + * as compatible with @prot if it is invalid and not owned by another entity, or
> > + * if its permission attributes are strictly similar to @prot and it has no
> > + * software bits set.
> 
> nit: I think the 'or' is ambigious here, as it makes it sound like an
> invalid entry that _is_ owned by another entity is compatible if the
> permissions attribute match. How about:
> 
>   | An IPA is compatible with prot iff its corresponding stage-2 page-table
>   | entry has default ownership and, if valid, is mapped with protection
>   | attributes identical to @prot.

Works for me.

> > + *
> > + * Return: 0 on success, negative error code on failure.
> > + */
> > +int kvm_pgtable_stage2_find_range(struct kvm_pgtable *pgt, u64 addr,
> > +				  enum kvm_pgtable_prot prot,
> > +				  struct kvm_mem_range *range);
> >  #endif	/* __ARM64_KVM_PGTABLE_H__ */
> > diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c
> > index c16e0306dd9a..f20287bb3e41 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_PTE_LEAF_ATTR_S2_RES	GENMASK(58, 55)
> 
> Please make this IGN instead of RES, to match the architecture (and because
> RES is usually used in the context of RES0 or RES1).

Ack, maybe I'll even say 'IGNORED' in full to match the Arm ARM -- IGN
sounds like an acronym otherwise.

> >  #define KVM_INVALID_PTE_OWNER_MASK	GENMASK(63, 32)
> >  
> >  struct kvm_pgtable_walk_data {
> > @@ -69,10 +71,8 @@ static u64 kvm_granule_size(u32 level)
> >  	return BIT(kvm_granule_shift(level));
> >  }
> >  
> > -static bool kvm_block_mapping_supported(u64 addr, u64 end, u64 phys, u32 level)
> > +static bool kvm_level_support_block_mappings(u32 level)
> >  {
> > -	u64 granule = kvm_granule_size(level);
> > -
> >  	/*
> >  	 * Reject invalid block mappings and don't bother with 4TB mappings for
> >  	 * 52-bit PAs.
> > @@ -80,6 +80,16 @@ static bool kvm_block_mapping_supported(u64 addr, u64 end, u64 phys, u32 level)
> >  	if (level == 0 || (PAGE_SIZE != SZ_4K && level == 1))
> >  		return false;
> >  
> > +	return true;
> 
> return !(level == 0 || (PAGE_SIZE != SZ_4K && level == 1));
> 
> > +static bool kvm_block_mapping_supported(u64 addr, u64 end, u64 phys, u32 level)
> > +{
> > +	u64 granule = kvm_granule_size(level);
> > +
> > +	if (!kvm_level_support_block_mappings(level))
> > +		return false;
> > +
> >  	if (granule > (end - addr))
> >  		return false;
> >  
> > @@ -1042,3 +1052,77 @@ void kvm_pgtable_stage2_destroy(struct kvm_pgtable *pgt)
> >  	pgt->mm_ops->free_pages_exact(pgt->pgd, pgd_sz);
> >  	pgt->pgd = NULL;
> >  }
> > +
> > +#define KVM_PTE_LEAF_S2_COMPAT_MASK	(KVM_PTE_LEAF_ATTR_S2_PERMS | \
> > +					 KVM_PTE_LEAF_ATTR_LO_S2_MEMATTR | \
> > +					 KVM_PTE_LEAF_ATTR_S2_RES)
> > +
> > +static int stage2_check_permission_walker(u64 addr, u64 end, u32 level,
> > +					  kvm_pte_t *ptep,
> > +					  enum kvm_pgtable_walk_flags flag,
> > +					  void * const arg)
> > +{
> > +	kvm_pte_t old_attr, pte = *ptep, *new_attr = arg;
> > +
> > +	/*
> > +	 * Compatible mappings are either invalid and owned by the page-table
> > +	 * owner (whose id is 0), or valid with matching permission attributes.
> > +	 */
> > +	if (kvm_pte_valid(pte)) {
> > +		old_attr = pte & KVM_PTE_LEAF_S2_COMPAT_MASK;
> > +		if (old_attr != *new_attr)
> > +			return -EEXIST;
> > +	} else if (pte) {
> > +		return -EEXIST;
> > +	}
> > +
> > +	return 0;
> > +}
> > +
> > +int kvm_pgtable_stage2_find_range(struct kvm_pgtable *pgt, u64 addr,
> > +				  enum kvm_pgtable_prot prot,
> > +				  struct kvm_mem_range *range)
> > +{
> > +	kvm_pte_t attr;
> > +	struct kvm_pgtable_walker check_perm_walker = {
> > +		.cb		= stage2_check_permission_walker,
> > +		.flags		= KVM_PGTABLE_WALK_LEAF,
> > +		.arg		= &attr,
> > +	};
> > +	u64 granule, start, end;
> > +	u32 level;
> > +	int ret;
> > +
> > +	ret = stage2_set_prot_attr(prot, &attr);
> > +	if (ret)
> > +		return ret;
> > +	attr &= KVM_PTE_LEAF_S2_COMPAT_MASK;
> > +
> > +	for (level = pgt->start_level; level < KVM_PGTABLE_MAX_LEVELS; level++) {
> > +		granule = kvm_granule_size(level);
> > +		start = ALIGN_DOWN(addr, granule);
> > +		end = start + granule;
> > +
> > +		if (!kvm_level_support_block_mappings(level))
> > +			continue;
> > +
> > +		if (start < range->start || range->end < end)
> > +			continue;
> > +
> > +		/*
> > +		 * Check the presence of existing mappings with incompatible
> > +		 * permissions within the current block range, and try one level
> > +		 * deeper if one is found.
> > +		 */
> > +		ret = kvm_pgtable_walk(pgt, start, granule, &check_perm_walker);
> > +		if (ret != -EEXIST)
> > +			break;
> > +	}
> 
> Can you write this as a:
> 
> 	do {
> 		...
> 	} while (level < KVM_PGTABLE_MAX_LEVELS && ret == -EEXIST);
> 
> loop?

I tried it but found it a little less pretty -- the pre-assignment of
level and the increment at the end make it really feel like a for loop
to me:

--- a/arch/arm64/kvm/hyp/pgtable.c
+++ b/arch/arm64/kvm/hyp/pgtable.c
@@ -1098,26 +1098,23 @@ int kvm_pgtable_stage2_find_range(struct kvm_pgtable *pgt, u64 addr,
                return ret;
        attr &= KVM_PTE_LEAF_S2_COMPAT_MASK;
 
-       for (level = pgt->start_level; level < KVM_PGTABLE_MAX_LEVELS; level++) {
+       ret = -EEXIST;
+       level = pgt->start_level;
+       do {
                granule = kvm_granule_size(level);
                start = ALIGN_DOWN(addr, granule);
                end = start + granule;
 
-               if (!kvm_level_support_block_mappings(level))
-                       continue;
-
-               if (start < range->start || range->end < end)
-                       continue;
-
                /*
                 * Check the presence of existing mappings with incompatible
                 * permissions within the current block range, and try one level
                 * deeper if one is found.
                 */
-               ret = kvm_pgtable_walk(pgt, start, granule, &check_perm_walker);
-               if (ret != -EEXIST)
-                       break;
-       }
+               if (kvm_level_support_block_mappings(level) && start >= range->start && range->end >= end)
+                       ret = kvm_pgtable_walk(pgt, start, granule, &check_perm_walker);
+
+               level++;
+       } while (level < KVM_PGTABLE_MAX_LEVELS && ret == -EEXIST);
 
        if (!ret) {


I'm guessing you're not a fan of the break case in existing loop?
Another option could be:

--- a/arch/arm64/kvm/hyp/pgtable.c
+++ b/arch/arm64/kvm/hyp/pgtable.c
@@ -1098,7 +1098,7 @@ int kvm_pgtable_stage2_find_range(struct kvm_pgtable *pgt, u64 addr,
                return ret;
        attr &= KVM_PTE_LEAF_S2_COMPAT_MASK;

-       for (level = pgt->start_level; level < KVM_PGTABLE_MAX_LEVELS; level++) {
+       for (ret = -EEXIST, level = pgt->start_level; level < KVM_PGTABLE_MAX_LEVELS && ret == -EEXIST; level++) {
                granule = kvm_granule_size(level);
                start = ALIGN_DOWN(addr, granule);
                end = start + granule;
@@ -1115,8 +1115,6 @@ int kvm_pgtable_stage2_find_range(struct kvm_pgtable *pgt, u64 addr,
                 * deeper if one is found.
                 */
                ret = kvm_pgtable_walk(pgt, start, granule, &check_perm_walker);
-               if (ret != -EEXIST)
-                       break;
        }

        if (!ret) {

No strong opinion either way. Thoughts?

Thanks,
Quentin
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