Hi Alex,
On Wed, Nov 17, 2021 at 7:37 AM Alexandru Elisei
<alexandru.elisei@xxxxxxx> wrote:
>
> KVM relies on doing dcache maintenance on stage 2 faults to present to a
> guest running with the MMU off the same view of memory as userspace. For
> locked memslots, KVM so far has done the dcache maintenance when a memslot
> is locked, but that leaves KVM in a rather awkward position: what userspace
> writes to guest memory after the memslot is locked, but before a VCPU is
> run, might not be visible to the guest.
>
> Fix this by deferring the dcache maintenance until the first VCPU is run.
>
> Signed-off-by: Alexandru Elisei <alexandru.elisei@xxxxxxx>
> ---
> arch/arm64/include/asm/kvm_host.h | 7 ++++
> arch/arm64/include/asm/kvm_mmu.h | 5 +++
> arch/arm64/kvm/arm.c | 3 ++
> arch/arm64/kvm/mmu.c | 55 ++++++++++++++++++++++++++++---
> 4 files changed, 66 insertions(+), 4 deletions(-)
>
> diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> index 7fd70ad90c16..3b4839b447c4 100644
> --- a/arch/arm64/include/asm/kvm_host.h
> +++ b/arch/arm64/include/asm/kvm_host.h
> @@ -113,6 +113,10 @@ struct kvm_arch_memory_slot {
> u32 flags;
> };
>
> +/* kvm->arch.mmu_pending_ops flags */
> +#define KVM_LOCKED_MEMSLOT_FLUSH_DCACHE 0
> +#define KVM_MAX_MMU_PENDING_OPS 1
> +
> struct kvm_arch {
> struct kvm_s2_mmu mmu;
>
> @@ -136,6 +140,9 @@ struct kvm_arch {
> */
> bool return_nisv_io_abort_to_user;
>
> + /* Defer MMU operations until a VCPU is run. */
> + unsigned long mmu_pending_ops;
> +
> /*
> * VM-wide PMU filter, implemented as a bitmap and big enough for
> * up to 2^10 events (ARMv8.0) or 2^16 events (ARMv8.1+).
> diff --git a/arch/arm64/include/asm/kvm_mmu.h b/arch/arm64/include/asm/kvm_mmu.h
> index 2c50734f048d..cbf57c474fea 100644
> --- a/arch/arm64/include/asm/kvm_mmu.h
> +++ b/arch/arm64/include/asm/kvm_mmu.h
> @@ -219,6 +219,11 @@ void kvm_toggle_cache(struct kvm_vcpu *vcpu, bool was_enabled);
> int kvm_mmu_lock_memslot(struct kvm *kvm, u64 slot, u64 flags);
> int kvm_mmu_unlock_memslot(struct kvm *kvm, u64 slot, u64 flags);
>
> +#define kvm_mmu_has_pending_ops(kvm) \
> + (!bitmap_empty(&(kvm)->arch.mmu_pending_ops, KVM_MAX_MMU_PENDING_OPS))
> +
> +void kvm_mmu_perform_pending_ops(struct kvm *kvm);
> +
> static inline unsigned int kvm_get_vmid_bits(void)
> {
> int reg = read_sanitised_ftr_reg(SYS_ID_AA64MMFR1_EL1);
> diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
> index b9b8b43835e3..96ed48455cdd 100644
> --- a/arch/arm64/kvm/arm.c
> +++ b/arch/arm64/kvm/arm.c
> @@ -870,6 +870,9 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu)
> if (unlikely(!kvm_vcpu_initialized(vcpu)))
> return -ENOEXEC;
>
> + if (unlikely(kvm_mmu_has_pending_ops(vcpu->kvm)))
> + kvm_mmu_perform_pending_ops(vcpu->kvm);
> +
> ret = kvm_vcpu_first_run_init(vcpu);
> if (ret)
> return ret;
> diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
> index b0a8e61315e4..8e4787019840 100644
> --- a/arch/arm64/kvm/mmu.c
> +++ b/arch/arm64/kvm/mmu.c
> @@ -1305,6 +1305,40 @@ int kvm_handle_guest_abort(struct kvm_vcpu *vcpu)
> return ret;
> }
>
> +/*
> + * It's safe to do the CMOs when the first VCPU is run because:
> + * - VCPUs cannot run until mmu_cmo_needed is cleared.
What does 'mmu_cmo_needed' mean ? Do you mean 'mmu_pending_ops' instead ?
> + * - Memslots cannot be modified because we hold the kvm->slots_lock.
> + *
> + * It's safe to periodically release the mmu_lock because:
> + * - VCPUs cannot run.
> + * - Any changes to the stage 2 tables triggered by the MMU notifiers also take
> + * the mmu_lock, which means accesses will be serialized.
> + * - Stage 2 tables cannot be freed from under us as long as at least one VCPU
> + * is live, which means that the VM will be live.
> + */
> +void kvm_mmu_perform_pending_ops(struct kvm *kvm)
> +{
> + struct kvm_memory_slot *memslot;
> +
> + mutex_lock(&kvm->slots_lock);
> + if (!kvm_mmu_has_pending_ops(kvm))
> + goto out_unlock;
> +
> + if (test_bit(KVM_LOCKED_MEMSLOT_FLUSH_DCACHE, &kvm->arch.mmu_pending_ops)) {
> + kvm_for_each_memslot(memslot, kvm_memslots(kvm)) {
> + if (!memslot_is_locked(memslot))
> + continue;
Shouldn't the code hold the mmu_lock to call stage2_flush_memslot() ?
> + stage2_flush_memslot(kvm, memslot);
Since stage2_flush_memslot() won't do anything when stage2_has_fwb()
returns true, I wonder if it can be checked even before iterating
memslots (so those iterations can be skipped when not needed).
Thanks,
Reiji
> + }
> + clear_bit(KVM_LOCKED_MEMSLOT_FLUSH_DCACHE, &kvm->arch.mmu_pending_ops);
> + }
> +
> +out_unlock:
> + mutex_unlock(&kvm->slots_lock);
> + return;
> +}
> +
> static int try_rlimit_memlock(unsigned long npages)
> {
> unsigned long lock_limit;
> @@ -1345,7 +1379,8 @@ static int lock_memslot(struct kvm *kvm, struct kvm_memory_slot *memslot,
> struct kvm_memory_slot_page *page_entry;
> bool writable = flags & KVM_ARM_LOCK_MEM_WRITE;
> enum kvm_pgtable_prot prot = KVM_PGTABLE_PROT_R;
> - struct kvm_pgtable *pgt = kvm->arch.mmu.pgt;
> + struct kvm_pgtable pgt;
> + struct kvm_pgtable_mm_ops mm_ops;
> struct vm_area_struct *vma;
> unsigned long npages = memslot->npages;
> unsigned int pin_flags = FOLL_LONGTERM;
> @@ -1363,6 +1398,16 @@ static int lock_memslot(struct kvm *kvm, struct kvm_memory_slot *memslot,
> pin_flags |= FOLL_WRITE;
> }
>
> + /*
> + * Make a copy of the stage 2 translation table struct to remove the
> + * dcache callback so we can postpone the cache maintenance operations
> + * until the first VCPU is run.
> + */
> + mm_ops = *kvm->arch.mmu.pgt->mm_ops;
> + mm_ops.dcache_clean_inval_poc = NULL;
> + pgt = *kvm->arch.mmu.pgt;
> + pgt.mm_ops = &mm_ops;
> +
> hva = memslot->userspace_addr;
> ipa = memslot->base_gfn << PAGE_SHIFT;
>
> @@ -1414,13 +1459,13 @@ static int lock_memslot(struct kvm *kvm, struct kvm_memory_slot *memslot,
> goto out_err;
> }
>
> - ret = kvm_pgtable_stage2_map(pgt, ipa, PAGE_SIZE,
> + ret = kvm_pgtable_stage2_map(&pgt, ipa, PAGE_SIZE,
> page_to_phys(page_entry->page),
> prot, &cache);
> spin_unlock(&kvm->mmu_lock);
>
> if (ret) {
> - kvm_pgtable_stage2_unmap(pgt, memslot->base_gfn << PAGE_SHIFT,
> + kvm_pgtable_stage2_unmap(&pgt, memslot->base_gfn << PAGE_SHIFT,
> i << PAGE_SHIFT);
> unpin_memslot_pages(memslot, writable);
> goto out_err;
> @@ -1439,7 +1484,7 @@ static int lock_memslot(struct kvm *kvm, struct kvm_memory_slot *memslot,
> */
> ret = account_locked_vm(current->mm, npages, true);
> if (ret) {
> - kvm_pgtable_stage2_unmap(pgt, memslot->base_gfn << PAGE_SHIFT,
> + kvm_pgtable_stage2_unmap(&pgt, memslot->base_gfn << PAGE_SHIFT,
> npages << PAGE_SHIFT);
> unpin_memslot_pages(memslot, writable);
> goto out_err;
> @@ -1449,6 +1494,8 @@ static int lock_memslot(struct kvm *kvm, struct kvm_memory_slot *memslot,
> if (writable)
> memslot->arch.flags |= KVM_MEMSLOT_LOCK_WRITE;
>
> + set_bit(KVM_LOCKED_MEMSLOT_FLUSH_DCACHE, &kvm->arch.mmu_pending_ops);
> +
> kvm_mmu_free_memory_cache(&cache);
>
> return 0;
> --
> 2.33.1
>
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