Re: [RFC PATCH] KVM: arm64: Add prejudgement for relaxing permissions only case in stage2 translation fault handler

[wangyanan (Y)]

Hi Will, Marc,

On 2020/12/11 18:00, Will Deacon wrote:
> On Fri, Dec 11, 2020 at 09:49:28AM +0000, Marc Zyngier wrote:
> > On 2020-12-11 08:01, Yanan Wang wrote:
> > > @@ -461,25 +462,56 @@ static int stage2_map_set_prot_attr(enum
> > > kvm_pgtable_prot prot,
> > >   	return 0;
> > >   }
> > >
> > > +static bool stage2_set_valid_leaf_pte_pre(u64 addr, u32 level,
> > > +					  kvm_pte_t *ptep, kvm_pte_t new,
> > > +					  struct stage2_map_data *data)
> > > +{
> > > +	kvm_pte_t old = *ptep, old_attr, new_attr;
> > > +
> > > +	if ((old ^ new) & (~KVM_PTE_LEAF_ATTR_PERMS))
> > > +		return false;
> > > +
> > > +	/*
> > > +	 * Skip updating if we are trying to recreate exactly the same mapping
> > > +	 * or to reduce the access permissions only. And update the valid leaf
> > > +	 * PTE without break-before-make if we are trying to add more access
> > > +	 * permissions only.
> > > +	 */
> > > +	old_attr = (old & KVM_PTE_LEAF_ATTR_PERMS) ^
> > > KVM_PTE_LEAF_ATTR_HI_S2_XN;
> > > +	new_attr = (new & KVM_PTE_LEAF_ATTR_PERMS) ^
> > > KVM_PTE_LEAF_ATTR_HI_S2_XN;
> > > +	if (new_attr <= old_attr)
> > > +		return true;
> > > +
> > > +	WRITE_ONCE(*ptep, new);
> > > +	kvm_call_hyp(__kvm_tlb_flush_vmid_ipa, data->mmu, addr, level);
> > I think what bothers me the most here is that we are turning a mapping into
> > a permission update, which makes the code really hard to read, and mixes
> > two things that were so far separate.
> >
> > I wonder whether we should instead abort the update and simply take the
> > fault
> > again, if we ever need to do it.
> That's a nice idea. If we could enforce that we don't alter permissions on
> the map path, and instead just return e.g. -EAGAIN then that would be a
> very neat solution and would cement the permission vs translation fault
> division.

I agree with that we can indeed simplify the code, separate 
permission-relaxing and

mapping by the *straightly return* way, although the cost is one more 
vCPU trap on

permission fault next time possibly.

So how about the new two diffs below? I split them into two patches with 
different aims.

Thanks,

Yanan.


diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c
index 23a01dfcb27a..a74a62283012 100644
--- a/arch/arm64/kvm/hyp/pgtable.c
+++ b/arch/arm64/kvm/hyp/pgtable.c
@@ -170,10 +170,9 @@ static void kvm_set_table_pte(kvm_pte_t *ptep, 
kvm_pte_t *childp)
        smp_store_release(ptep, pte);
 }

-static bool kvm_set_valid_leaf_pte(kvm_pte_t *ptep, u64 pa, kvm_pte_t attr,
-                                  u32 level)
+static kvm_pte_t kvm_init_valid_leaf_pte(u64 pa, kvm_pte_t attr, u32 level)
 {
-       kvm_pte_t old = *ptep, pte = kvm_phys_to_pte(pa);
+       kvm_pte_t pte = kvm_phys_to_pte(pa);
        u64 type = (level == KVM_PGTABLE_MAX_LEVELS - 1) ? 
KVM_PTE_TYPE_PAGE :
KVM_PTE_TYPE_BLOCK;

@@ -181,12 +180,7 @@ static bool kvm_set_valid_leaf_pte(kvm_pte_t *ptep, 
u64 pa, kvm_pte_t attr,
        pte |= FIELD_PREP(KVM_PTE_TYPE, type);
        pte |= KVM_PTE_VALID;

-       /* Tolerate KVM recreating the exact same mapping. */
-       if (kvm_pte_valid(old))
-               return old == pte;
-
-       smp_store_release(ptep, pte);
-       return true;
+       return pte;
 }

 static int kvm_pgtable_visitor_cb(struct kvm_pgtable_walk_data *data, 
u64 addr,
@@ -341,12 +335,17 @@ static int hyp_map_set_prot_attr(enum 
kvm_pgtable_prot prot,
 static bool hyp_map_walker_try_leaf(u64 addr, u64 end, u32 level,
                                    kvm_pte_t *ptep, struct 
hyp_map_data *data)
 {
+       kvm_pte_t new, old = *ptep;
        u64 granule = kvm_granule_size(level), phys = data->phys;

        if (!kvm_block_mapping_supported(addr, end, phys, level))
                return false;

-       WARN_ON(!kvm_set_valid_leaf_pte(ptep, phys, data->attr, level));
+       /* Tolerate KVM recreating the exact same mapping. */
+       new = kvm_init_valid_leaf_pte(phys, data->attr, level);
+       if (old != new && !WARN_ON(kvm_pte_valid(old)))
+               smp_store_release(ptep, new);
+
        data->phys += granule;
        return true;
 }
@@ -465,21 +464,29 @@ static bool stage2_map_walker_try_leaf(u64 addr, 
u64 end, u32 level,
                                       kvm_pte_t *ptep,
                                       struct stage2_map_data *data)
 {
+       kvm_pte_t new, old = *ptep;
        u64 granule = kvm_granule_size(level), phys = data->phys;
+       struct page *page = virt_to_page(ptep);

        if (!kvm_block_mapping_supported(addr, end, phys, level))
                return false;

-       if (kvm_pte_valid(*ptep))
-               put_page(virt_to_page(ptep));
+       new = kvm_init_valid_leaf_pte(phys, data->attr, level);
+       if (kvm_pte_valid(old)) {
+               /* Tolerate KVM recreating the exact same mapping. */
+               if (old == new)
+                       goto out;

-       if (kvm_set_valid_leaf_pte(ptep, phys, data->attr, level))
-               goto out;
+               /* There's an existing different valid leaf entry, so 
perform
+                * break-before-make.
+                */
+               kvm_set_invalid_pte(ptep);
+               kvm_call_hyp(__kvm_tlb_flush_vmid_ipa, data->mmu, addr, 
level);
+               put_page(page);
+       }

-       /* There's an existing valid leaf entry, so perform 
break-before-make */
-       kvm_set_invalid_pte(ptep);
-       kvm_call_hyp(__kvm_tlb_flush_vmid_ipa, data->mmu, addr, level);
-       kvm_set_valid_leaf_pte(ptep, phys, data->attr, level);
+       smp_store_release(ptep, new);
+       get_page(page);
 out:
        data->phys += granule;
        return true;
@@ -521,7 +528,7 @@ static int stage2_map_walk_leaf(u64 addr, u64 end, 
u32 level, kvm_pte_t *ptep,
        }

        if (stage2_map_walker_try_leaf(addr, end, level, ptep, data))
-               goto out_get_page;
+               return 0;

        if (WARN_ON(level == KVM_PGTABLE_MAX_LEVELS - 1))
                return -EINVAL;
@@ -545,9 +552,8 @@ static int stage2_map_walk_leaf(u64 addr, u64 end, 
u32 level, kvm_pte_t *ptep,
        }

        kvm_set_table_pte(ptep, childp);
-
-out_get_page:
        get_page(page);
+
        return 0;
 }


diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c
index a74a62283012..e3c6133567c4 100644
--- a/arch/arm64/kvm/hyp/pgtable.c
+++ b/arch/arm64/kvm/hyp/pgtable.c
@@ -45,6 +45,10 @@

 #define KVM_PTE_LEAF_ATTR_HI_S2_XN     BIT(54)

+#define KVM_PTE_LEAF_ATTR_S2_PERMS (KVM_PTE_LEAF_ATTR_LO_S2_S2AP_R | \
+        KVM_PTE_LEAF_ATTR_LO_S2_S2AP_W | \
+ KVM_PTE_LEAF_ATTR_HI_S2_XN)
+
 struct kvm_pgtable_walk_data {
        struct kvm_pgtable                   *pgt;
        struct kvm_pgtable_walker       *walker;
@@ -473,8 +477,13 @@ static bool stage2_map_walker_try_leaf(u64 addr, 
u64 end, u32 level,

        new = kvm_init_valid_leaf_pte(phys, data->attr, level);
        if (kvm_pte_valid(old)) {
-               /* Tolerate KVM recreating the exact same mapping. */
-               if (old == new)
+               /*
+                * Skip updating the PTE with break-before-make if we 
are trying
+                * to recreate the exact same mapping or only change the 
access
+                * permissions. Actually, change of permissions will be 
handled
+                * through the relax_perms path next time if necessary.
+                */
+               if (!((old ^ new) & (~KVM_PTE_LEAF_ATTR_S2_PERMS)))
                        goto out;

                /* There's an existing different valid leaf entry, so 
perform