Re: [PATCH v3 06/21] KVM: arm64: Add support for stage-2 map()/unmap() in generic page-table

Alexandru Elisei
2020-09-01T16:25:41Z

Hi Will,

On 8/25/20 10:39 AM, Will Deacon wrote:
> Add stage-2 map() and unmap() operations to the generic page-table code.
>
> 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 | 39 ++++
> arch/arm64/kvm/hyp/pgtable.c | 262 +++++++++++++++++++++++++++
> 2 files changed, 301 insertions(+)
>
> diff --git a/arch/arm64/include/asm/kvm_pgtable.h b/arch/arm64/include/asm/kvm_pgtable.h
> index 3389f978d573..8ab0d5f43817 100644
> --- a/arch/arm64/include/asm/kvm_pgtable.h
> +++ b/arch/arm64/include/asm/kvm_pgtable.h
> @@ -134,6 +134,45 @@ int kvm_pgtable_stage2_init(struct kvm_pgtable *pgt, struct kvm *kvm);
> */
> void kvm_pgtable_stage2_destroy(struct kvm_pgtable *pgt);
>
> +/**
> + * kvm_pgtable_stage2_map() - Install a mapping in a guest stage-2 page-table.
> + * @pgt: Page-table structure initialised by kvm_pgtable_stage2_init().
> + * @addr: Intermediate physical 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.
> + * @mc: Cache of pre-allocated GFP_PGTABLE_USER memory from which to
> + * allocate page-table pages.
> + *
> + * If device attributes are not explicitly requested in @prot, then the
> + * mapping will be normal, cacheable.
> + *
> + * Note that this function will both coalesce existing table entries and split
> + * existing block mappings, relying on page-faults to fault back areas outside
> + * of the new mapping lazily.
> + *
> + * Return: 0 on success, negative error code on failure.
> + */
> +int kvm_pgtable_stage2_map(struct kvm_pgtable *pgt, u64 addr, u64 size,
> + u64 phys, enum kvm_pgtable_prot prot,
> + struct kvm_mmu_memory_cache *mc);
> +
> +/**
> + * kvm_pgtable_stage2_unmap() - Remove a mapping from a guest stage-2 page-table.
> + * @pgt: Page-table structure initialised by kvm_pgtable_stage2_init().
> + * @addr: Intermediate physical address from which to remove the mapping.
> + * @size: Size of the mapping.
> + *
> + * TLB invalidation is performed for each page-table entry cleared during the
> + * unmapping operation and the reference count for the page-table page
> + * containing the cleared entry is decremented, with unreferenced pages being
> + * freed. Unmapping a cacheable page will ensure that it is clean to the PoC if
> + * FWB is not supported by the CPU.
> + *
> + * Return: 0 on success, negative error code on failure.
> + */
> +int kvm_pgtable_stage2_unmap(struct kvm_pgtable *pgt, u64 addr, u64 size);
> +
> /**
> * 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 b8550ccaef4d..41ee8f3c0369 100644
> --- a/arch/arm64/kvm/hyp/pgtable.c
> +++ b/arch/arm64/kvm/hyp/pgtable.c
> @@ -32,10 +32,19 @@
> #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_LO_S2_MEMATTR GENMASK(5, 2)
> +#define KVM_PTE_LEAF_ATTR_LO_S2_S2AP_R BIT(6)
> +#define KVM_PTE_LEAF_ATTR_LO_S2_S2AP_W BIT(7)
> +#define KVM_PTE_LEAF_ATTR_LO_S2_SH GENMASK(9, 8)
> +#define KVM_PTE_LEAF_ATTR_LO_S2_SH_IS 3
> +#define KVM_PTE_LEAF_ATTR_LO_S2_AF BIT(10)
> +
> #define KVM_PTE_LEAF_ATTR_HI GENMASK(63, 51)
>
> #define KVM_PTE_LEAF_ATTR_HI_S1_XN BIT(54)
>
> +#define KVM_PTE_LEAF_ATTR_HI_S2_XN BIT(54)

Checked the bitfields against ARM DDI 0487F.b, they match.

> +
> struct kvm_pgtable_walk_data {
> struct kvm_pgtable *pgt;
> struct kvm_pgtable_walker *walker;
> @@ -420,6 +429,259 @@ void kvm_pgtable_hyp_destroy(struct kvm_pgtable *pgt)
> pgt->pgd = NULL;
> }
>
> +struct stage2_map_data {
> + u64 phys;
> + kvm_pte_t attr;
> +
> + kvm_pte_t *anchor;
> +
> + struct kvm_s2_mmu *mmu;
> + struct kvm_mmu_memory_cache *memcache;
> +};
> +
> +static kvm_pte_t *stage2_memcache_alloc_page(struct stage2_map_data *data)
> +{
> + kvm_pte_t *ptep = NULL;
> + struct kvm_mmu_memory_cache *mc = data->memcache;
> +
> + /* Allocated with GFP_PGTABLE_USER, so no need to zero */
> + if (mc && mc->nobjs)
> + ptep = mc->objects[--mc->nobjs];
> +
> + return ptep;
> +}
> +
> +static int stage2_map_set_prot_attr(enum kvm_pgtable_prot prot,
> + struct stage2_map_data *data)
> +{
> + bool device = prot & KVM_PGTABLE_PROT_DEVICE;
> + kvm_pte_t attr = device ? PAGE_S2_MEMATTR(DEVICE_nGnRE) :
> + PAGE_S2_MEMATTR(NORMAL);
> + u32 sh = KVM_PTE_LEAF_ATTR_LO_S2_SH_IS;
> +
> + if (!(prot & KVM_PGTABLE_PROT_X))
> + attr |= KVM_PTE_LEAF_ATTR_HI_S2_XN;
> + else if (device)
> + return -EINVAL;
> +
> + if (prot & KVM_PGTABLE_PROT_R)
> + attr |= KVM_PTE_LEAF_ATTR_LO_S2_S2AP_R;
> +
> + if (prot & KVM_PGTABLE_PROT_W)
> + attr |= KVM_PTE_LEAF_ATTR_LO_S2_S2AP_W;
> +
> + attr |= FIELD_PREP(KVM_PTE_LEAF_ATTR_LO_S2_SH, sh);
> + attr |= KVM_PTE_LEAF_ATTR_LO_S2_AF;
> + data->attr = attr;
> + return 0;
> +}
> +
> +static bool stage2_map_walker_try_leaf(u64 addr, u64 end, u32 level,
> + kvm_pte_t *ptep,
> + struct stage2_map_data *data)
> +{
> + u64 granule = kvm_granule_size(level), phys = data->phys;
> +
> + if (!kvm_block_mapping_supported(addr, end, phys, level))
> + return false;
> +
> + if (kvm_set_valid_leaf_pte(ptep, phys, data->attr, level))
> + goto out;
> +
> + kvm_set_invalid_pte(ptep);
> + kvm_call_hyp(__kvm_tlb_flush_vmid_ipa, data->mmu, addr, level);
> + kvm_set_valid_leaf_pte(ptep, phys, data->attr, level);

One has to read the kvm_set_valid_leaf_pte code very carefully to understand why
we're doing the above (found an old, valid entry in the stage 2 code, the page
tables are in use so we're doing break-before-make to replace it with the new
one), especially since we don't this with the hyp tables. Perhaps a comment
explaining what's happening would be useful.

> +out:
> + data->phys += granule;
> + return true;
> +}
> +
> +static int stage2_map_walk_table_pre(u64 addr, u64 end, u32 level,
> + kvm_pte_t *ptep,
> + struct stage2_map_data *data)
> +{
> + if (data->anchor)
> + return 0;
> +
> + if (!kvm_block_mapping_supported(addr, end, data->phys, level))
> + return 0;
> +
> + kvm_set_invalid_pte(ptep);
> + kvm_call_hyp(__kvm_tlb_flush_vmid_ipa, data->mmu, addr, 0);
> + data->anchor = ptep;
> + return 0;
> +}
> +
> +static int stage2_map_walk_leaf(u64 addr, u64 end, u32 level, kvm_pte_t *ptep,
> + struct stage2_map_data *data)
> +{
> + kvm_pte_t *childp, pte = *ptep;
> + struct page *page = virt_to_page(ptep);
> +
> + if (data->anchor) {
> + if (kvm_pte_valid(pte))
> + put_page(page);
> +
> + return 0;
> + }
> +
> + if (stage2_map_walker_try_leaf(addr, end, level, ptep, data))
> + goto out_get_page;
> +
> + if (WARN_ON(level == KVM_PGTABLE_MAX_LEVELS - 1))
> + return -EINVAL;
> +
> + childp = stage2_memcache_alloc_page(data);
> + if (!childp)
> + return -ENOMEM;
> +
> + /*
> + * If we've run into an existing block mapping then replace it with
> + * a table. Accesses beyond 'end' that fall within the new table
> + * will be mapped lazily.
> + */
> + if (kvm_pte_valid(pte)) {
> + kvm_set_invalid_pte(ptep);
> + kvm_call_hyp(__kvm_tlb_flush_vmid_ipa, data->mmu, addr, level);
> + put_page(page);
> + }
> +
> + kvm_set_table_pte(ptep, childp);
> +
> +out_get_page:
> + get_page(page);
> + return 0;
> +}
> +
> +static int stage2_map_walk_table_post(u64 addr, u64 end, u32 level,
> + kvm_pte_t *ptep,
> + struct stage2_map_data *data)
> +{
> + int ret = 0;
> +
> + if (!data->anchor)
> + return 0;
> +
> + free_page((unsigned long)kvm_pte_follow(*ptep));
> + put_page(virt_to_page(ptep));
> +
> + if (data->anchor == ptep) {
> + data->anchor = NULL;
> + ret = stage2_map_walk_leaf(addr, end, level, ptep, data);
> + }
> +
> + return ret;
> +}
> +
> +static int stage2_map_walker(u64 addr, u64 end, u32 level, kvm_pte_t *ptep,
> + enum kvm_pgtable_walk_flags flag, void * const arg)
> +{
> + struct stage2_map_data *data = arg;
> +
> + switch (flag) {
> + case KVM_PGTABLE_WALK_TABLE_PRE:
> + return stage2_map_walk_table_pre(addr, end, level, ptep, data);
> + case KVM_PGTABLE_WALK_LEAF:
> + return stage2_map_walk_leaf(addr, end, level, ptep, data);
> + case KVM_PGTABLE_WALK_TABLE_POST:
> + return stage2_map_walk_table_post(addr, end, level, ptep, data);
> + }
> +
> + return -EINVAL;
> +}

As I understood the algorithm, each of the pre, leaf and post function do two
different things: 1. free/invalidate the tables/leaf entries if we can create a
block mapping at a previously visited level (stage2_map_data->anchor != NULL); and
create an entry for the range at the correct level. To be honest, to me this
hasn't been obvious from the code and I think some comments to the functions and
especially to the anchor field of stage2_map_data would go a long way to making it
easier for others to understand the code.

With that in mind, the functions look solid to me: every get_page has a
corresponding put_page in stage2_map_walk_leaf or in the unmap walker, and the
algorithm looks sound. I still want to re-read the functions a few times (probably
in the next iteration) because they're definitely not trivial and I don't want to
miss something.

One nitpick below.

> +
> +int kvm_pgtable_stage2_map(struct kvm_pgtable *pgt, u64 addr, u64 size,
> + u64 phys, enum kvm_pgtable_prot prot,
> + struct kvm_mmu_memory_cache *mc)
> +{
> + int ret;
> + struct stage2_map_data map_data = {
> + .phys = ALIGN_DOWN(phys, PAGE_SIZE),
> + .mmu = pgt->mmu,
> + .memcache = mc,
> + };
> + struct kvm_pgtable_walker walker = {
> + .cb = stage2_map_walker,
> + .flags = KVM_PGTABLE_WALK_TABLE_PRE |
> + KVM_PGTABLE_WALK_LEAF |
> + KVM_PGTABLE_WALK_TABLE_POST,
> + .arg = &map_data,
> + };
> +
> + ret = stage2_map_set_prot_attr(prot, &map_data);
> + if (ret)
> + return ret;
> +
> + ret = kvm_pgtable_walk(pgt, addr, size, &walker);
> + dsb(ishst);
> + return ret;
> +}
> +
> +static void stage2_flush_dcache(void *addr, u64 size)
> +{
> + if (cpus_have_const_cap(ARM64_HAS_STAGE2_FWB))
> + return;
> +
> + __flush_dcache_area(addr, size);
> +}
> +
> +static bool stage2_pte_cacheable(kvm_pte_t pte)
> +{
> + u64 memattr = FIELD_GET(KVM_PTE_LEAF_ATTR_LO_S2_MEMATTR, pte);
> + return memattr == PAGE_S2_MEMATTR(NORMAL);
> +}
> +
> +static int stage2_unmap_walker(u64 addr, u64 end, u32 level, kvm_pte_t *ptep,
> + enum kvm_pgtable_walk_flags flag,
> + void * const arg)
> +{
> + struct kvm_s2_mmu *mmu = arg;
> + kvm_pte_t pte = *ptep, *childp = NULL;
> + bool need_flush = false;
> +
> + if (!kvm_pte_valid(pte))
> + return 0;
> +
> + if (kvm_pte_table(pte, level)) {
> + childp = kvm_pte_follow(pte);
> +
> + if (page_count(virt_to_page(childp)) != 1)
> + return 0;
> + } else if (stage2_pte_cacheable(pte)) {
> + need_flush = true;
> + }
> +
> + /*
> + * This is similar to the map() path in that we unmap the entire
> + * block entry and rely on the remaining portions being faulted
> + * back lazily.
> + */
> + kvm_set_invalid_pte(ptep);
> + kvm_call_hyp(__kvm_tlb_flush_vmid_ipa, mmu, addr, level);
> + put_page(virt_to_page(ptep));
> +
> + if (need_flush) {
> + stage2_flush_dcache(kvm_pte_follow(pte),
> + kvm_granule_size(level));
> + }

The curly braces are unnecessary; I'm only mentioning it because you don't use
them in this function for the rest of the one line if statements.

Thanks,

Alex

> +
> + if (childp)
> + free_page((unsigned long)childp);
> +
> + return 0;
> +}
> +
> +int kvm_pgtable_stage2_unmap(struct kvm_pgtable *pgt, u64 addr, u64 size)
> +{
> + struct kvm_pgtable_walker walker = {
> + .cb = stage2_unmap_walker,
> + .arg = pgt->mmu,
> + .flags = KVM_PGTABLE_WALK_LEAF | KVM_PGTABLE_WALK_TABLE_POST,
> + };
> +
> + return kvm_pgtable_walk(pgt, addr, size, &walker);
> +}
> +
> int kvm_pgtable_stage2_init(struct kvm_pgtable *pgt, struct kvm *kvm)
> {
> size_t pgd_sz;