Re: [PATCH v3 03/15] KVM: arm64: Refcount hyp stage-1 pgtable pages
From: Quentin Perret <hidden>
Date: 2021-12-10 14:34:24
Also in:
kvmarm, lkml
Subsystem:
arm64 port (aarch64 architecture), kernel virtual machine for arm64 (kvm/arm64), the rest · Maintainers:
Catalin Marinas, Will Deacon, Marc Zyngier, Oliver Upton, Linus Torvalds
On Thursday 09 Dec 2021 at 10:29:24 (+0000), Will Deacon wrote:
On Wed, Dec 01, 2021 at 05:03:57PM +0000, Quentin Perret wrote:quoted
To prepare the ground for allowing hyp stage-1 mappings to be removed at run-time, update the KVM page-table code to maintain a correct refcount using the ->{get,put}_page() function callbacks. Signed-off-by: Quentin Perret <redacted> --- arch/arm64/kvm/hyp/pgtable.c | 17 ++++++++++++++--- 1 file changed, 14 insertions(+), 3 deletions(-)diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c index f8ceebe4982e..768a58835153 100644 --- a/arch/arm64/kvm/hyp/pgtable.c +++ b/arch/arm64/kvm/hyp/pgtable.c@@ -408,8 +408,10 @@ static bool hyp_map_walker_try_leaf(u64 addr, u64 end, u32 level, return false; new = kvm_init_valid_leaf_pte(phys, data->attr, level); - if (hyp_pte_needs_update(old, new)) + if (hyp_pte_needs_update(old, new)) { smp_store_release(ptep, new); + data->mm_ops->get_page(ptep);In the case where we're just updating software bits for a valid pte, doesn't this result in us taking a spurious reference to the page?
Ahem, yes, that is the case. I ended up with the below diff to fix it, which I intend to fold in the next version:
diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c
index 6ad4cb2d6947..e2047d3f05a2 100644
--- a/arch/arm64/kvm/hyp/pgtable.c
+++ b/arch/arm64/kvm/hyp/pgtable.c@@ -383,21 +383,6 @@ enum kvm_pgtable_prot kvm_pgtable_hyp_pte_prot(kvm_pte_t pte) return prot; } -static bool hyp_pte_needs_update(kvm_pte_t old, kvm_pte_t new) -{ - /* - * Tolerate KVM recreating the exact same mapping, or changing software - * bits if the existing mapping was valid. - */ - if (old == new) - return false; - - if (!kvm_pte_valid(old)) - return true; - - return !WARN_ON((old ^ new) & ~KVM_PTE_LEAF_ATTR_HI_SW); -} - static bool hyp_map_walker_try_leaf(u64 addr, u64 end, u32 level, kvm_pte_t *ptep, struct hyp_map_data *data) {
@@ -407,13 +392,16 @@ static bool hyp_map_walker_try_leaf(u64 addr, u64 end, u32 level, if (!kvm_block_mapping_supported(addr, end, phys, level)) return false; + data->phys += granule; new = kvm_init_valid_leaf_pte(phys, data->attr, level); - if (hyp_pte_needs_update(old, new)) { - smp_store_release(ptep, new); + if (old == new) + return true; + else if (!kvm_pte_valid(old)) data->mm_ops->get_page(ptep); - } + else if (WARN_ON((old ^ new) & ~KVM_PTE_LEAF_ATTR_HI_SW)) + return false; - data->phys += granule; + smp_store_release(ptep, new); return true; }
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