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SubjectRe: [PATCH] kvm: arm: Skip stage2 huge mappings for unaligned ipa backed by THP
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On 2019/4/2 19:06, Suzuki K Poulose wrote:
> With commit a80868f398554842b14, we no longer ensure that the
> THP page is properly aligned in the guest IPA. Skip the stage2
> huge mapping for unaligned IPA backed by transparent hugepages.
>
> Fixes: a80868f398554842b14 ("KVM: arm/arm64: Enforce PTE mappings at stage2 when needed")
> Reported-by: Eric Auger <eric.auger@redhat.com>
> Cc: Marc Zyngier <marc.zyngier@arm.com>
> Cc: Chirstoffer Dall <christoffer.dall@arm.com>
> Cc: Zenghui Yu <yuzenghui@huawei.com>
> Cc: Zheng Xiang <zhengxiang9@huawei.com>
> Tested-by: Eric Auger <eric.auger@arm.com>
> Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>

Hi Suzuki,

Why not making use of fault_supports_stage2_huge_mapping()? Let it do
some checks for us.

fault_supports_stage2_huge_mapping() was intended to do a *two-step*
check to tell us that can we create stage2 huge block mappings, and this
check is both for hugetlbfs and THP. With commit a80868f398554842b14,
we pass PAGE_SIZE as "map_size" for normal size pages (which turned out
to be almost meaningless), and unfortunately the THP check no longer
works.

So we want to rework *THP* check process. Your patch fixes the first
checking-step, but the second is still missed, am I wrong?

Can you please give a look at the below diff?


thanks,

zenghui

> ---
> virt/kvm/arm/mmu.c | 4 +++-
> 1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/virt/kvm/arm/mmu.c b/virt/kvm/arm/mmu.c
> index 27c9583..4a22f5b 100644
> --- a/virt/kvm/arm/mmu.c
> +++ b/virt/kvm/arm/mmu.c
> @@ -1412,7 +1412,9 @@ static bool transparent_hugepage_adjust(kvm_pfn_t *pfnp, phys_addr_t *ipap)
> * page accordingly.
> */
> mask = PTRS_PER_PMD - 1;
> - VM_BUG_ON((gfn & mask) != (pfn & mask));
Somehow, I'd prefer keeping the VM_BUG_ON() here, let it report some
potential issues in the future (of course I hope none:) )
> + /* Skip memslots with unaligned IPA and user address */
> + if ((gfn & mask) != (pfn & mask))
> + return false;
> if (pfn & mask) {
> *ipap &= PMD_MASK;
> kvm_release_pfn_clean(pfn);
>

---8>---

Rework fault_supports_stage2_huge_mapping(), let it check THP again.

Signed-off-by: Zenghui Yu <yuzenghui@huawei.com>
---
virt/kvm/arm/mmu.c | 11 ++++++++++-
1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/virt/kvm/arm/mmu.c b/virt/kvm/arm/mmu.c
index 27c9583..5e1b258 100644
--- a/virt/kvm/arm/mmu.c
+++ b/virt/kvm/arm/mmu.c
@@ -1632,6 +1632,15 @@ static bool
fault_supports_stage2_huge_mapping(struct kvm_memory_slot *memslot,
uaddr_end = uaddr_start + size;

/*
+ * If the memslot is _not_ backed by hugetlbfs, then check if it
+ * can be backed by transparent hugepages.
+ *
+ * Currently only PMD_SIZE THPs are supported, revisit it later.
+ */
+ if (map_size == PAGE_SIZE)
+ map_size = PMD_SIZE;
+
+ /*
* Pages belonging to memslots that don't have the same alignment
* within a PMD/PUD for userspace and IPA cannot be mapped with stage-2
* PMD/PUD entries, because we'll end up mapping the wrong pages.
@@ -1643,7 +1652,7 @@ static bool
fault_supports_stage2_huge_mapping(struct kvm_memory_slot *memslot,
* |abcde|fgh Stage-1 block | Stage-1 block tv|xyz|
* +-----+--------------------+--------------------+---+
*
- * memslot->base_gfn << PAGE_SIZE:
+ * memslot->base_gfn << PAGE_SHIFT:
* +---+--------------------+--------------------+-----+
* |abc|def Stage-2 block | Stage-2 block |tvxyz|
* +---+--------------------+--------------------+-----+
--


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