Re: [PATCH v4 14/25] KVM: arm64: Add per-cpu fixmap infrastructure at EL2

Mark Rutland
2022-10-18T11:07:48Z

On Mon, Oct 17, 2022 at 12:51:58PM +0100, Will Deacon wrote:
> From: Quentin Perret <qperret@google.com>
>
> Mapping pages in a guest page-table from within the pKVM hypervisor at
> EL2 may require cache maintenance to ensure that the initialised page
> contents is visible even to non-cacheable (e.g. MMU-off) accesses from
> the guest.
>
> In preparation for performing this maintenance at EL2, introduce a
> per-vCPU fixmap which allows the pKVM hypervisor to map guest pages
> temporarily into its stage-1 page-table for the purposes of cache
> maintenance and, in future, poisoning on the reclaim path. The use of a
> fixmap avoids the need for memory allocation or locking on the map()
> path.
>
> Tested-by: Vincent Donnefort <vdonnefort@google.com>
> Signed-off-by: Quentin Perret <qperret@google.com>
> Signed-off-by: Will Deacon <will@kernel.org>
> ---
> arch/arm64/include/asm/kvm_pgtable.h | 14 +++
> arch/arm64/kvm/hyp/include/nvhe/mem_protect.h | 2 +
> arch/arm64/kvm/hyp/include/nvhe/mm.h | 4 +
> arch/arm64/kvm/hyp/nvhe/mem_protect.c | 1 -
> arch/arm64/kvm/hyp/nvhe/mm.c | 94 +++++++++++++++++++
> arch/arm64/kvm/hyp/nvhe/setup.c | 4 +
> arch/arm64/kvm/hyp/pgtable.c | 12 ---
> 7 files changed, 118 insertions(+), 13 deletions(-)
>
> diff --git a/arch/arm64/include/asm/kvm_pgtable.h b/arch/arm64/include/asm/kvm_pgtable.h
> index 4f6d79fe4352..b2a886c9e78d 100644
> --- a/arch/arm64/include/asm/kvm_pgtable.h
> +++ b/arch/arm64/include/asm/kvm_pgtable.h
> @@ -30,6 +30,8 @@ typedef u64 kvm_pte_t;
> #define KVM_PTE_ADDR_MASK GENMASK(47, PAGE_SHIFT)
> #define KVM_PTE_ADDR_51_48 GENMASK(15, 12)
>
> +#define KVM_PHYS_INVALID (-1ULL)
> +
> static inline bool kvm_pte_valid(kvm_pte_t pte)
> {
> return pte & KVM_PTE_VALID;
> @@ -45,6 +47,18 @@ static inline u64 kvm_pte_to_phys(kvm_pte_t pte)
> return pa;
> }
>
> +static inline kvm_pte_t kvm_phys_to_pte(u64 pa)
> +{
> + kvm_pte_t pte = pa & KVM_PTE_ADDR_MASK;
> +
> + if (PAGE_SHIFT == 16) {
> + pa &= GENMASK(51, 48);
> + pte |= FIELD_PREP(KVM_PTE_ADDR_51_48, pa >> 48);
> + }
> +
> + return pte;
> +}
> +
> static inline u64 kvm_granule_shift(u32 level)
> {
> /* Assumes KVM_PGTABLE_MAX_LEVELS is 4 */
> diff --git a/arch/arm64/kvm/hyp/include/nvhe/mem_protect.h b/arch/arm64/kvm/hyp/include/nvhe/mem_protect.h
> index ce9a796a85ee..ef31a1872c93 100644
> --- a/arch/arm64/kvm/hyp/include/nvhe/mem_protect.h
> +++ b/arch/arm64/kvm/hyp/include/nvhe/mem_protect.h
> @@ -59,6 +59,8 @@ enum pkvm_component_id {
> PKVM_ID_HYP,
> };
>
> +extern unsigned long hyp_nr_cpus;
> +
> int __pkvm_prot_finalize(void);
> int __pkvm_host_share_hyp(u64 pfn);
> int __pkvm_host_unshare_hyp(u64 pfn);
> diff --git a/arch/arm64/kvm/hyp/include/nvhe/mm.h b/arch/arm64/kvm/hyp/include/nvhe/mm.h
> index b2ee6d5df55b..d5ec972b5c1e 100644
> --- a/arch/arm64/kvm/hyp/include/nvhe/mm.h
> +++ b/arch/arm64/kvm/hyp/include/nvhe/mm.h
> @@ -13,6 +13,10 @@
> extern struct kvm_pgtable pkvm_pgtable;
> extern hyp_spinlock_t pkvm_pgd_lock;
>
> +int hyp_create_pcpu_fixmap(void);
> +void *hyp_fixmap_map(phys_addr_t phys);
> +void hyp_fixmap_unmap(void);
> +
> int hyp_create_idmap(u32 hyp_va_bits);
> int hyp_map_vectors(void);
> int hyp_back_vmemmap(phys_addr_t back);
> diff --git a/arch/arm64/kvm/hyp/nvhe/mem_protect.c b/arch/arm64/kvm/hyp/nvhe/mem_protect.c
> index 2ef6aaa21ba5..1c38451050e5 100644
> --- a/arch/arm64/kvm/hyp/nvhe/mem_protect.c
> +++ b/arch/arm64/kvm/hyp/nvhe/mem_protect.c
> @@ -21,7 +21,6 @@
>
> #define KVM_HOST_S2_FLAGS (KVM_PGTABLE_S2_NOFWB | KVM_PGTABLE_S2_IDMAP)
>
> -extern unsigned long hyp_nr_cpus;
> struct host_mmu host_mmu;
>
> static struct hyp_pool host_s2_pool;
> diff --git a/arch/arm64/kvm/hyp/nvhe/mm.c b/arch/arm64/kvm/hyp/nvhe/mm.c
> index d3a3b47181de..b77215630d5c 100644
> --- a/arch/arm64/kvm/hyp/nvhe/mm.c
> +++ b/arch/arm64/kvm/hyp/nvhe/mm.c
> @@ -14,6 +14,7 @@
> #include <nvhe/early_alloc.h>
> #include <nvhe/gfp.h>
> #include <nvhe/memory.h>
> +#include <nvhe/mem_protect.h>
> #include <nvhe/mm.h>
> #include <nvhe/spinlock.h>
>
> @@ -25,6 +26,12 @@ unsigned int hyp_memblock_nr;
>
> static u64 __io_map_base;
>
> +struct hyp_fixmap_slot {
> + u64 addr;
> + kvm_pte_t *ptep;
> +};
> +static DEFINE_PER_CPU(struct hyp_fixmap_slot, fixmap_slots);
> +
> static int __pkvm_create_mappings(unsigned long start, unsigned long size,
> unsigned long phys, enum kvm_pgtable_prot prot)
> {
> @@ -212,6 +219,93 @@ int hyp_map_vectors(void)
> return 0;
> }
>
> +void *hyp_fixmap_map(phys_addr_t phys)
> +{
> + struct hyp_fixmap_slot *slot = this_cpu_ptr(&fixmap_slots);
> + kvm_pte_t pte, *ptep = slot->ptep;
> +
> + pte = *ptep;
> + pte &= ~kvm_phys_to_pte(KVM_PHYS_INVALID);
> + pte |= kvm_phys_to_pte(phys) | KVM_PTE_VALID;
> + WRITE_ONCE(*ptep, pte);
> + dsb(nshst);
> +
> + return (void *)slot->addr;
> +}
> +
> +static void fixmap_clear_slot(struct hyp_fixmap_slot *slot)
> +{
> + kvm_pte_t *ptep = slot->ptep;
> + u64 addr = slot->addr;
> +
> + WRITE_ONCE(*ptep, *ptep & ~KVM_PTE_VALID);
> + dsb(nshst);
> + __tlbi_level(vale2, __TLBI_VADDR(addr, 0), (KVM_PGTABLE_MAX_LEVELS - 1));
> + dsb(nsh);
> + isb();
> +}

Does each CPU have independent Stage-1 tables at EL2? i.e. each has a distinct
root table?

If the tables are shared, you need broadcast maintenance and ISH barriers here,
or you risk the usual issues with asynchronous MMU behaviour.

If those are per-cpu, sorry for the noise!

Thanks,
Mark.

> +
> +void hyp_fixmap_unmap(void)
> +{
> + fixmap_clear_slot(this_cpu_ptr(&fixmap_slots));
> +}
> +
> +static int __create_fixmap_slot_cb(u64 addr, u64 end, u32 level, kvm_pte_t *ptep,
> + enum kvm_pgtable_walk_flags flag,
> + void * const arg)
> +{
> + struct hyp_fixmap_slot *slot = per_cpu_ptr(&fixmap_slots, (u64)arg);
> +
> + if (!kvm_pte_valid(*ptep) || level != KVM_PGTABLE_MAX_LEVELS - 1)
> + return -EINVAL;
> +
> + slot->addr = addr;
> + slot->ptep = ptep;
> +
> + /*
> + * Clear the PTE, but keep the page-table page refcount elevated to
> + * prevent it from ever being freed. This lets us manipulate the PTEs
> + * by hand safely without ever needing to allocate memory.
> + */
> + fixmap_clear_slot(slot);
> +
> + return 0;
> +}
> +
> +static int create_fixmap_slot(u64 addr, u64 cpu)
> +{
> + struct kvm_pgtable_walker walker = {
> + .cb = __create_fixmap_slot_cb,
> + .flags = KVM_PGTABLE_WALK_LEAF,
> + .arg = (void *)cpu,
> + };
> +
> + return kvm_pgtable_walk(&pkvm_pgtable, addr, PAGE_SIZE, &walker);
> +}
> +
> +int hyp_create_pcpu_fixmap(void)
> +{
> + unsigned long addr, i;
> + int ret;
> +
> + for (i = 0; i < hyp_nr_cpus; i++) {
> + ret = pkvm_alloc_private_va_range(PAGE_SIZE, &addr);
> + if (ret)
> + return ret;
> +
> + ret = kvm_pgtable_hyp_map(&pkvm_pgtable, addr, PAGE_SIZE,
> + __hyp_pa(__hyp_bss_start), PAGE_HYP);
> + if (ret)
> + return ret;
> +
> + ret = create_fixmap_slot(addr, i);
> + if (ret)
> + return ret;
> + }
> +
> + return 0;
> +}
> +
> int hyp_create_idmap(u32 hyp_va_bits)
> {
> unsigned long start, end;
> diff --git a/arch/arm64/kvm/hyp/nvhe/setup.c b/arch/arm64/kvm/hyp/nvhe/setup.c
> index 2be72fbe7279..0f69c1393416 100644
> --- a/arch/arm64/kvm/hyp/nvhe/setup.c
> +++ b/arch/arm64/kvm/hyp/nvhe/setup.c
> @@ -321,6 +321,10 @@ void __noreturn __pkvm_init_finalise(void)
> if (ret)
> goto out;
>
> + ret = hyp_create_pcpu_fixmap();
> + if (ret)
> + goto out;
> +
> pkvm_hyp_vm_table_init(vm_table_base);
> out:
> /*
> diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c
> index a1a27f88a312..2bcb2d5903ba 100644
> --- a/arch/arm64/kvm/hyp/pgtable.c
> +++ b/arch/arm64/kvm/hyp/pgtable.c
> @@ -57,8 +57,6 @@ struct kvm_pgtable_walk_data {
> u64 end;
> };
>
> -#define KVM_PHYS_INVALID (-1ULL)
> -
> static bool kvm_phys_is_valid(u64 phys)
> {
> return phys < BIT(id_aa64mmfr0_parange_to_phys_shift(ID_AA64MMFR0_EL1_PARANGE_MAX));
> @@ -122,16 +120,6 @@ static bool kvm_pte_table(kvm_pte_t pte, u32 level)
> return FIELD_GET(KVM_PTE_TYPE, pte) == KVM_PTE_TYPE_TABLE;
> }
>
> -static kvm_pte_t kvm_phys_to_pte(u64 pa)
> -{
> - kvm_pte_t pte = pa & KVM_PTE_ADDR_MASK;
> -
> - if (PAGE_SHIFT == 16)
> - pte |= FIELD_PREP(KVM_PTE_ADDR_51_48, pa >> 48);
> -
> - return pte;
> -}
> -
> static kvm_pte_t *kvm_pte_follow(kvm_pte_t pte, struct kvm_pgtable_mm_ops *mm_ops)
> {
> return mm_ops->phys_to_virt(kvm_pte_to_phys(pte));
> --
> 2.38.0.413.g74048e4d9e-goog
>