Re: [PATCH v3 03/21] KVM: arm64: Add support for creating kernel-agnostic stage-1 page tables

Alexandru Elisei
2020-08-28T15:36:08Z

Hi Will,

On 8/25/20 10:39 AM, Will Deacon wrote:
> The generic page-table walker is pretty useless as it stands, because it
> doesn't understand enough to allocate anything. Teach it about stage-1
> page-tables, and hook up an API for allocating these for the hypervisor
> at EL2.
>
> Cc: Marc Zyngier <maz@kernel.org>
> Cc: Quentin Perret <qperret@google.com>
> Signed-off-by: Will Deacon <will@kernel.org>
> ---
> arch/arm64/include/asm/kvm_pgtable.h | 34 +++++++
> arch/arm64/kvm/hyp/pgtable.c | 131 +++++++++++++++++++++++++++
> 2 files changed, 165 insertions(+)
>
> diff --git a/arch/arm64/include/asm/kvm_pgtable.h b/arch/arm64/include/asm/kvm_pgtable.h
> index 51ccbbb0efae..ec9f98527dcc 100644
> --- a/arch/arm64/include/asm/kvm_pgtable.h
> +++ b/arch/arm64/include/asm/kvm_pgtable.h
> @@ -77,6 +77,40 @@ struct kvm_pgtable_walker {
> const enum kvm_pgtable_walk_flags flags;
> };
>
> +/**
> + * kvm_pgtable_hyp_init() - Initialise a hypervisor stage-1 page-table.
> + * @pgt: Uninitialised page-table structure to initialise.
> + * @va_bits: Maximum virtual address bits.
> + *
> + * Return: 0 on success, negative error code on failure.
> + */
> +int kvm_pgtable_hyp_init(struct kvm_pgtable *pgt, u32 va_bits);
> +
> +/**
> + * kvm_pgtable_hyp_destroy() - Destroy an unused hypervisor stage-1 page-table.
> + * @pgt: Page-table structure initialised by kvm_pgtable_hyp_init().
> + *
> + * The page-table is assumed to be unreachable by any hardware walkers prior
> + * to freeing and therefore no TLB invalidation is performed.
> + */
> +void kvm_pgtable_hyp_destroy(struct kvm_pgtable *pgt);
> +
> +/**
> + * kvm_pgtable_hyp_map() - Install a mapping in a hypervisor stage-1 page-table.
> + * @pgt: Page-table structure initialised by kvm_pgtable_hyp_init().
> + * @addr: Virtual address at which to place the mapping.
> + * @size: Size of the mapping.
> + * @phys: Physical address of the memory to map.
> + * @prot: Permissions and attributes for the mapping.
> + *
> + * If device attributes are not explicitly requested in @prot, then the
> + * mapping will be normal, cacheable.
> + *
> + * Return: 0 on success, negative error code on failure.

From my understanding of the code, when the caller replaces an existing leaf entry
or a table with a different one, KVM will print a warning instead of using
break-before-make (if necessary). It might be worth pointing out that it is
expected from the callers not to do that, because it's not immediately obvious.

> + */
> +int kvm_pgtable_hyp_map(struct kvm_pgtable *pgt, u64 addr, u64 size, u64 phys,
> + enum kvm_pgtable_prot prot);
> +
> /**
> * kvm_pgtable_walk() - Walk a page-table.
> * @pgt: Page-table structure initialised by kvm_pgtable_*_init().
> diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c
> index 462001bbe028..d75166823ad9 100644
> --- a/arch/arm64/kvm/hyp/pgtable.c
> +++ b/arch/arm64/kvm/hyp/pgtable.c
> @@ -24,8 +24,18 @@
>
> #define KVM_PTE_LEAF_ATTR_LO GENMASK(11, 2)
>
> +#define KVM_PTE_LEAF_ATTR_LO_S1_ATTRIDX GENMASK(4, 2)
> +#define KVM_PTE_LEAF_ATTR_LO_S1_AP GENMASK(7, 6)
> +#define KVM_PTE_LEAF_ATTR_LO_S1_AP_RO 3
> +#define KVM_PTE_LEAF_ATTR_LO_S1_AP_RW 1
> +#define KVM_PTE_LEAF_ATTR_LO_S1_SH GENMASK(9, 8)
> +#define KVM_PTE_LEAF_ATTR_LO_S1_SH_IS 3
> +#define KVM_PTE_LEAF_ATTR_LO_S1_AF BIT(10)
> +
> #define KVM_PTE_LEAF_ATTR_HI GENMASK(63, 51)
>
> +#define KVM_PTE_LEAF_ATTR_HI_S1_XN BIT(54)

I compared the macros to the Arm ARM attribute fields in stage 1 VMSAv8-64 block
and page descriptors, and they match.

I looked at the algorithm below, and for what it's worth it looks alright to me.

Thanks,

Alex

> +
> struct kvm_pgtable_walk_data {
> struct kvm_pgtable *pgt;
> struct kvm_pgtable_walker *walker;
> @@ -288,3 +298,124 @@ int kvm_pgtable_walk(struct kvm_pgtable *pgt, u64 addr, u64 size,
>
> return _kvm_pgtable_walk(&walk_data);
> }
> +
> +struct hyp_map_data {
> + u64 phys;
> + kvm_pte_t attr;
> +};
> +
> +static int hyp_map_set_prot_attr(enum kvm_pgtable_prot prot,
> + struct hyp_map_data *data)
> +{
> + bool device = prot & KVM_PGTABLE_PROT_DEVICE;
> + u32 mtype = device ? MT_DEVICE_nGnRE : MT_NORMAL;
> + kvm_pte_t attr = FIELD_PREP(KVM_PTE_LEAF_ATTR_LO_S1_ATTRIDX, mtype);
> + u32 sh = KVM_PTE_LEAF_ATTR_LO_S1_SH_IS;
> + u32 ap = (prot & KVM_PGTABLE_PROT_W) ? KVM_PTE_LEAF_ATTR_LO_S1_AP_RW :
> + KVM_PTE_LEAF_ATTR_LO_S1_AP_RO;
> +
> + if (!(prot & KVM_PGTABLE_PROT_R))
> + return -EINVAL;
> +
> + if (prot & KVM_PGTABLE_PROT_X) {
> + if (prot & KVM_PGTABLE_PROT_W)
> + return -EINVAL;
> +
> + if (device)
> + return -EINVAL;
> + } else {
> + attr |= KVM_PTE_LEAF_ATTR_HI_S1_XN;
> + }
> +
> + attr |= FIELD_PREP(KVM_PTE_LEAF_ATTR_LO_S1_AP, ap);
> + attr |= FIELD_PREP(KVM_PTE_LEAF_ATTR_LO_S1_SH, sh);
> + attr |= KVM_PTE_LEAF_ATTR_LO_S1_AF;
> + data->attr = attr;
> + return 0;
> +}
> +
> +static bool hyp_map_walker_try_leaf(u64 addr, u64 end, u32 level,
> + kvm_pte_t *ptep, struct hyp_map_data *data)
> +{
> + 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));
> + data->phys += granule;
> + return true;
> +}
> +
> +static int hyp_map_walker(u64 addr, u64 end, u32 level, kvm_pte_t *ptep,
> + enum kvm_pgtable_walk_flags flag, void * const arg)
> +{
> + kvm_pte_t *childp;
> +
> + if (hyp_map_walker_try_leaf(addr, end, level, ptep, arg))
> + return 0;
> +
> + if (WARN_ON(level == KVM_PGTABLE_MAX_LEVELS - 1))
> + return -EINVAL;
> +
> + childp = (kvm_pte_t *)get_zeroed_page(GFP_KERNEL);
> + if (!childp)
> + return -ENOMEM;
> +
> + kvm_set_table_pte(ptep, childp);
> + return 0;
> +}
> +
> +int kvm_pgtable_hyp_map(struct kvm_pgtable *pgt, u64 addr, u64 size, u64 phys,
> + enum kvm_pgtable_prot prot)
> +{
> + int ret;
> + struct hyp_map_data map_data = {
> + .phys = ALIGN_DOWN(phys, PAGE_SIZE),
> + };
> + struct kvm_pgtable_walker walker = {
> + .cb = hyp_map_walker,
> + .flags = KVM_PGTABLE_WALK_LEAF,
> + .arg = &map_data,
> + };
> +
> + ret = hyp_map_set_prot_attr(prot, &map_data);
> + if (ret)
> + return ret;
> +
> + ret = kvm_pgtable_walk(pgt, addr, size, &walker);
> + dsb(ishst);
> + isb();
> + return ret;
> +}
> +
> +int kvm_pgtable_hyp_init(struct kvm_pgtable *pgt, u32 va_bits)
> +{
> + pgt->pgd = (kvm_pte_t *)get_zeroed_page(GFP_KERNEL);
> + if (!pgt->pgd)
> + return -ENOMEM;
> +
> + pgt->ia_bits = va_bits;
> + pgt->start_level = kvm_start_level(va_bits);
> + pgt->mmu = NULL;
> + return 0;
> +}
> +
> +static int hyp_free_walker(u64 addr, u64 end, u32 level, kvm_pte_t *ptep,
> + enum kvm_pgtable_walk_flags flag, void * const arg)
> +{
> + free_page((unsigned long)kvm_pte_follow(*ptep));
> + return 0;
> +}
> +
> +void kvm_pgtable_hyp_destroy(struct kvm_pgtable *pgt)
> +{
> + struct kvm_pgtable_walker walker = {
> + .cb = hyp_free_walker,
> + .flags = KVM_PGTABLE_WALK_TABLE_POST,
> + };
> +
> + WARN_ON(kvm_pgtable_walk(pgt, 0, BIT(pgt->ia_bits), &walker));
> + free_page((unsigned long)pgt->pgd);
> + pgt->pgd = NULL;
> +}