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SubjectRe: [PATCH v1 09/16] kvm: arm/arm64: Delay stage2 page table allocation
On Tue, Jan 09, 2018 at 07:04:04PM +0000, Suzuki K Poulose wrote:
> We allocate the entry level page tables for stage2 when the
> VM is created. This doesn't give us the flexibility of configuring
> the physical address space size for a VM. In order to allow
> the VM to choose the required size, we delay the allocation of
> stage2 entry level tables until we really try to map something.
>
> This could be either when the VM creates a memory range or when
> it tries to map a device memory. So we add in a hook to these
> two places to make sure the tables are allocated. We use
> kvm->slots_lock to serialize the allocation entry point, since
> we add hooks to the arch specific call back with the mutex held.
>
> Cc: Marc Zyngier <marc.zyngier@arm.com>
> Cc: Christoffer Dall <cdall@linaro.org>
> Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
> ---
> virt/kvm/arm/arm.c | 18 ++++++----------
> virt/kvm/arm/mmu.c | 61 +++++++++++++++++++++++++++++++++++++++++++++---------
> 2 files changed, 57 insertions(+), 22 deletions(-)
>
> diff --git a/virt/kvm/arm/arm.c b/virt/kvm/arm/arm.c
> index 19b720ddedce..d06f00566664 100644
> --- a/virt/kvm/arm/arm.c
> +++ b/virt/kvm/arm/arm.c
> @@ -127,13 +127,13 @@ int kvm_arch_init_vm(struct kvm *kvm, unsigned long type)
> for_each_possible_cpu(cpu)
> *per_cpu_ptr(kvm->arch.last_vcpu_ran, cpu) = -1;
>
> - ret = kvm_alloc_stage2_pgd(kvm);
> - if (ret)
> - goto out_fail_alloc;
> -
> ret = create_hyp_mappings(kvm, kvm + 1, PAGE_HYP);
> - if (ret)
> - goto out_free_stage2_pgd;
> + if (ret) {
> + free_percpu(kvm->arch.last_vcpu_ran);
> + kvm->arch.last_vcpu_ran = NULL;
> + return ret;
> + }
> +
>
> kvm_vgic_early_init(kvm);
>
> @@ -145,12 +145,6 @@ int kvm_arch_init_vm(struct kvm *kvm, unsigned long type)
> kvm_vgic_get_max_vcpus() : KVM_MAX_VCPUS;
>
> return ret;
> -out_free_stage2_pgd:
> - kvm_free_stage2_pgd(kvm);
> -out_fail_alloc:
> - free_percpu(kvm->arch.last_vcpu_ran);
> - kvm->arch.last_vcpu_ran = NULL;
> - return ret;
> }
>
> bool kvm_arch_has_vcpu_debugfs(void)
> diff --git a/virt/kvm/arm/mmu.c b/virt/kvm/arm/mmu.c
> index c94c61ac38b9..257f2a8ccfc7 100644
> --- a/virt/kvm/arm/mmu.c
> +++ b/virt/kvm/arm/mmu.c
> @@ -1011,15 +1011,39 @@ static int stage2_pmdp_test_and_clear_young(pmd_t *pmd)
> return stage2_ptep_test_and_clear_young((pte_t *)pmd);
> }
>
> -/**
> - * kvm_phys_addr_ioremap - map a device range to guest IPA
> - *
> - * @kvm: The KVM pointer
> - * @guest_ipa: The IPA at which to insert the mapping
> - * @pa: The physical address of the device
> - * @size: The size of the mapping
> +/*
> + * Finalise the stage2 page table layout. Must be called with kvm->slots_lock
> + * held.
> */
> -int kvm_phys_addr_ioremap(struct kvm *kvm, phys_addr_t guest_ipa,
> +static int __kvm_init_stage2_table(struct kvm *kvm)
> +{
> + /* Double check if somebody has already allocated it */

dubious comment: Either leave it out or explain that we need to check
again with the mutex held.

> + if (likely(kvm->arch.pgd))
> + return 0;
> + return kvm_alloc_stage2_pgd(kvm);
> +}
> +
> +static int kvm_init_stage2_table(struct kvm *kvm)
> +{
> + int rc;
> +
> + /*
> + * Once allocated, the stage2 entry level tables are only
> + * freed when the KVM instance is destroyed. So, if we see
> + * something valid here, that guarantees that we have
> + * done the one time allocation and it is something valid
> + * and won't go away until the last reference to the KVM
> + * is gone.
> + */

Really not sure if this comment adds something beyond what's described
by the code already?

Thanks,
-Christoffer

> + if (likely(kvm->arch.pgd))
> + return 0;
> + mutex_lock(&kvm->slots_lock);
> + rc = __kvm_init_stage2_table(kvm);
> + mutex_unlock(&kvm->slots_lock);
> + return rc;
> +}
> +
> +static int __kvm_phys_addr_ioremap(struct kvm *kvm, phys_addr_t guest_ipa,
> phys_addr_t pa, unsigned long size, bool writable)
> {
> phys_addr_t addr, end;
> @@ -1055,6 +1079,23 @@ int kvm_phys_addr_ioremap(struct kvm *kvm, phys_addr_t guest_ipa,
> return ret;
> }
>
> +/**
> + * kvm_phys_addr_ioremap - map a device range to guest IPA.
> + * Acquires kvm->slots_lock for making sure that the stage2 is initialized.
> + *
> + * @kvm: The KVM pointer
> + * @guest_ipa: The IPA at which to insert the mapping
> + * @pa: The physical address of the device
> + * @size: The size of the mapping
> + */
> +int kvm_phys_addr_ioremap(struct kvm *kvm, phys_addr_t guest_ipa,
> + phys_addr_t pa, unsigned long size, bool writable)
> +{
> + if (unlikely(kvm_init_stage2_table(kvm)))
> + return -ENOMEM;
> + return __kvm_phys_addr_ioremap(kvm, guest_ipa, pa, size, writable);
> +}
> +
> static bool transparent_hugepage_adjust(kvm_pfn_t *pfnp, phys_addr_t *ipap)
> {
> kvm_pfn_t pfn = *pfnp;
> @@ -1912,7 +1953,7 @@ int kvm_arch_prepare_memory_region(struct kvm *kvm,
> goto out;
> }
>
> - ret = kvm_phys_addr_ioremap(kvm, gpa, pa,
> + ret = __kvm_phys_addr_ioremap(kvm, gpa, pa,
> vm_end - vm_start,
> writable);
> if (ret)
> @@ -1943,7 +1984,7 @@ void kvm_arch_free_memslot(struct kvm *kvm, struct kvm_memory_slot *free,
> int kvm_arch_create_memslot(struct kvm *kvm, struct kvm_memory_slot *slot,
> unsigned long npages)
> {
> - return 0;
> + return __kvm_init_stage2_table(kvm);
> }
>
> void kvm_arch_memslots_updated(struct kvm *kvm, struct kvm_memslots *slots)
> --
> 2.13.6
>

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