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Message-ID: <2ab9323a-40a1-d223-f692-0a19207e16a9@huawei.com>
Date: Mon, 14 Dec 2020 15:20:56 +0800
From: "wangyanan (Y)" <wangyanan55@...wei.com>
To: Will Deacon <will@...nel.org>, Marc Zyngier <maz@...nel.org>
CC: <linux-kernel@...r.kernel.org>,
<linux-arm-kernel@...ts.infradead.org>,
Catalin Marinas <catalin.marinas@....com>,
James Morse <james.morse@....com>,
Julien Thierry <julien.thierry.kdev@...il.com>,
Suzuki K Poulose <suzuki.poulose@....com>,
Gavin Shan <gshan@...hat.com>,
Quentin Perret <qperret@...gle.com>,
<wanghaibin.wang@...wei.com>, <yezengruan@...wei.com>,
<zhukeqian1@...wei.com>, <yuzenghui@...wei.com>,
<jiangkunkun@...wei.com>, <wangjingyi11@...wei.com>,
<lushenming@...wei.com>
Subject: Re: [RFC PATCH] KVM: arm64: Add prejudgement for relaxing permissions
only case in stage2 translation fault handler
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
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