[PATCH v6 17/26] KVM: arm64: Add generic hyp_memcache helpers

[Will Deacon]

From: Quentin Perret <qperret at google.com>

The host at EL1 and the pKVM hypervisor at EL2 will soon need to
exchange memory pages dynamically for creating and destroying VM state.

Indeed, the hypervisor will rely on the host to donate memory pages it
can use to create guest stage-2 page-tables and to store VM and vCPU
metadata. In order to ease this process, introduce a
'struct hyp_memcache' which is essentially a linked list of available
pages, indexed by physical addresses so that it can be passed
meaningfully between the different virtual address spaces configured at
EL1 and EL2.

Tested-by: Vincent Donnefort <vdonnefort at google.com>
Signed-off-by: Quentin Perret <qperret at google.com>
Signed-off-by: Will Deacon <will at kernel.org>
---
 arch/arm64/include/asm/kvm_host.h             | 57 +++++++++++++++++++
 arch/arm64/kvm/hyp/include/nvhe/mem_protect.h |  2 +
 arch/arm64/kvm/hyp/nvhe/mm.c                  | 33 +++++++++++
 arch/arm64/kvm/mmu.c                          | 26 +++++++++
 4 files changed, 118 insertions(+)

diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
index 467393e7331f..835987e0f868 100644
--- a/arch/arm64/include/asm/kvm_host.h
+++ b/arch/arm64/include/asm/kvm_host.h
@@ -73,6 +73,63 @@ u32 __attribute_const__ kvm_target_cpu(void);
 int kvm_reset_vcpu(struct kvm_vcpu *vcpu);
 void kvm_arm_vcpu_destroy(struct kvm_vcpu *vcpu);
 
+struct kvm_hyp_memcache {
+	phys_addr_t head;
+	unsigned long nr_pages;
+};
+
+static inline void push_hyp_memcache(struct kvm_hyp_memcache *mc,
+				     phys_addr_t *p,
+				     phys_addr_t (*to_pa)(void *virt))
+{
+	*p = mc->head;
+	mc->head = to_pa(p);
+	mc->nr_pages++;
+}
+
+static inline void *pop_hyp_memcache(struct kvm_hyp_memcache *mc,
+				     void *(*to_va)(phys_addr_t phys))
+{
+	phys_addr_t *p = to_va(mc->head);
+
+	if (!mc->nr_pages)
+		return NULL;
+
+	mc->head = *p;
+	mc->nr_pages--;
+
+	return p;
+}
+
+static inline int __topup_hyp_memcache(struct kvm_hyp_memcache *mc,
+				       unsigned long min_pages,
+				       void *(*alloc_fn)(void *arg),
+				       phys_addr_t (*to_pa)(void *virt),
+				       void *arg)
+{
+	while (mc->nr_pages < min_pages) {
+		phys_addr_t *p = alloc_fn(arg);
+
+		if (!p)
+			return -ENOMEM;
+		push_hyp_memcache(mc, p, to_pa);
+	}
+
+	return 0;
+}
+
+static inline void __free_hyp_memcache(struct kvm_hyp_memcache *mc,
+				       void (*free_fn)(void *virt, void *arg),
+				       void *(*to_va)(phys_addr_t phys),
+				       void *arg)
+{
+	while (mc->nr_pages)
+		free_fn(pop_hyp_memcache(mc, to_va), arg);
+}
+
+void free_hyp_memcache(struct kvm_hyp_memcache *mc);
+int topup_hyp_memcache(struct kvm_hyp_memcache *mc, unsigned long min_pages);
+
 struct kvm_vmid {
 	atomic64_t id;
 };
diff --git a/arch/arm64/kvm/hyp/include/nvhe/mem_protect.h b/arch/arm64/kvm/hyp/include/nvhe/mem_protect.h
index ef31a1872c93..420b87e755a4 100644
--- a/arch/arm64/kvm/hyp/include/nvhe/mem_protect.h
+++ b/arch/arm64/kvm/hyp/include/nvhe/mem_protect.h
@@ -77,6 +77,8 @@ void handle_host_mem_abort(struct kvm_cpu_context *host_ctxt);
 int hyp_pin_shared_mem(void *from, void *to);
 void hyp_unpin_shared_mem(void *from, void *to);
 void reclaim_guest_pages(struct pkvm_hyp_vm *vm);
+int refill_memcache(struct kvm_hyp_memcache *mc, unsigned long min_pages,
+		    struct kvm_hyp_memcache *host_mc);
 
 static __always_inline void __load_host_stage2(void)
 {
diff --git a/arch/arm64/kvm/hyp/nvhe/mm.c b/arch/arm64/kvm/hyp/nvhe/mm.c
index 5648ac21e62d..c80b2c007619 100644
--- a/arch/arm64/kvm/hyp/nvhe/mm.c
+++ b/arch/arm64/kvm/hyp/nvhe/mm.c
@@ -340,3 +340,36 @@ int hyp_create_idmap(u32 hyp_va_bits)
 
 	return __pkvm_create_mappings(start, end - start, start, PAGE_HYP_EXEC);
 }
+
+static void *admit_host_page(void *arg)
+{
+	struct kvm_hyp_memcache *host_mc = arg;
+
+	if (!host_mc->nr_pages)
+		return NULL;
+
+	/*
+	 * The host still owns the pages in its memcache, so we need to go
+	 * through a full host-to-hyp donation cycle to change it. Fortunately,
+	 * __pkvm_host_donate_hyp() takes care of races for us, so if it
+	 * succeeds we're good to go.
+	 */
+	if (__pkvm_host_donate_hyp(hyp_phys_to_pfn(host_mc->head), 1))
+		return NULL;
+
+	return pop_hyp_memcache(host_mc, hyp_phys_to_virt);
+}
+
+/* Refill our local memcache by poping pages from the one provided by the host. */
+int refill_memcache(struct kvm_hyp_memcache *mc, unsigned long min_pages,
+		    struct kvm_hyp_memcache *host_mc)
+{
+	struct kvm_hyp_memcache tmp = *host_mc;
+	int ret;
+
+	ret =  __topup_hyp_memcache(mc, min_pages, admit_host_page,
+				    hyp_virt_to_phys, &tmp);
+	*host_mc = tmp;
+
+	return ret;
+}
diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
index 34c5feed9dc1..8b52566d1cb9 100644
--- a/arch/arm64/kvm/mmu.c
+++ b/arch/arm64/kvm/mmu.c
@@ -800,6 +800,32 @@ void kvm_free_stage2_pgd(struct kvm_s2_mmu *mmu)
 	}
 }
 
+static void hyp_mc_free_fn(void *addr, void *unused)
+{
+	free_page((unsigned long)addr);
+}
+
+static void *hyp_mc_alloc_fn(void *unused)
+{
+	return (void *)__get_free_page(GFP_KERNEL_ACCOUNT);
+}
+
+void free_hyp_memcache(struct kvm_hyp_memcache *mc)
+{
+	if (is_protected_kvm_enabled())
+		__free_hyp_memcache(mc, hyp_mc_free_fn,
+				    kvm_host_va, NULL);
+}
+
+int topup_hyp_memcache(struct kvm_hyp_memcache *mc, unsigned long min_pages)
+{
+	if (!is_protected_kvm_enabled())
+		return 0;
+
+	return __topup_hyp_memcache(mc, min_pages, hyp_mc_alloc_fn,
+				    kvm_host_pa, NULL);
+}
+
 /**
  * kvm_phys_addr_ioremap - map a device range to guest IPA
  *
-- 
2.38.1.431.g37b22c650d-goog