ARM: KVM: switch to a dual-step HYP init code

Our HYP init code suffers from two major design issues:
- it cannot support CPU hotplug, as we tear down the idmap very early
- it cannot perform a TLB invalidation when switching from init to
  runtime mappings, as pages are manipulated from PL1 exclusively

The hotplug problem mandates that we keep two sets of page tables
(boot and runtime). The TLB problem mandates that we're able to
transition from one PGD to another while in HYP, invalidating the TLBs
in the process.

To be able to do this, we need to share a page between the two page
tables. A page that will have the same VA in both configurations. All we
need is a VA that has the following properties:
- This VA can't be used to represent a kernel mapping.
- This VA will not conflict with the physical address of the kernel text

The vectors page seems to satisfy this requirement:
- The kernel never maps anything else there
- The kernel text being copied at the beginning of the physical memory,
  it is unlikely to use the last 64kB (I doubt we'll ever support KVM
  on a system with something like 4MB of RAM, but patches are very
  welcome).

Let's call this VA the trampoline VA.

Now, we map our init page at 3 locations:
- idmap in the boot pgd
- trampoline VA in the boot pgd
- trampoline VA in the runtime pgd

The init scenario is now the following:
- We jump in HYP with four parameters: boot HYP pgd, runtime HYP pgd,
  runtime stack, runtime vectors
- Enable the MMU with the boot pgd
- Jump to a target into the trampoline page (remember, this is the same
  physical page!)
- Now switch to the runtime pgd (same VA, and still the same physical
  page!)
- Invalidate TLBs
- Set stack and vectors
- Profit! (or eret, if you only care about the code).

Note that we keep the boot mapping permanently (it is not strictly an
idmap anymore) to allow for CPU hotplug in later patches.

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Christoffer Dall <cdall@cs.columbia.edu>

1 files changed, 94 insertions, 38 deletions

diff --git a/arch/arm/kvm/mmu.c b/arch/arm/kvm/mmu.c
index 7464824c17ef..4646c17f571c 100644
--- a/arch/arm/kvm/mmu.c
+++ b/arch/arm/kvm/mmu.c
@@ -32,9 +32,15 @@
 
 extern char  __hyp_idmap_text_start[], __hyp_idmap_text_end[];
 
+static pgd_t *boot_hyp_pgd;
 static pgd_t *hyp_pgd;
 static DEFINE_MUTEX(kvm_hyp_pgd_mutex);
 
+static void *init_bounce_page;
+static unsigned long hyp_idmap_start;
+static unsigned long hyp_idmap_end;
+static phys_addr_t hyp_idmap_vector;
+
 static void kvm_tlb_flush_vmid_ipa(struct kvm *kvm, phys_addr_t ipa)
 {
 	kvm_call_hyp(__kvm_tlb_flush_vmid_ipa, kvm, ipa);
@@ -152,9 +158,12 @@ static void unmap_range(pgd_t *pgdp, unsigned long long start, u64 size)
 /**
  * free_hyp_pgds - free Hyp-mode page tables
  *
- * Assumes hyp_pgd is a page table used strictly in Hyp-mode and therefore contains
- * either mappings in the kernel memory area (above PAGE_OFFSET), or
- * device mappings in the vmalloc range (from VMALLOC_START to VMALLOC_END).
+ * Assumes hyp_pgd is a page table used strictly in Hyp-mode and
+ * therefore contains either mappings in the kernel memory area (above
+ * PAGE_OFFSET), or device mappings in the vmalloc range (from
+ * VMALLOC_START to VMALLOC_END).
+ *
+ * boot_hyp_pgd should only map two pages for the init code.
  */
 void free_hyp_pgds(void)
 {
@@ -162,6 +171,12 @@ void free_hyp_pgds(void)
 
 	mutex_lock(&kvm_hyp_pgd_mutex);
 
+	if (boot_hyp_pgd) {
+		unmap_range(boot_hyp_pgd, hyp_idmap_start, PAGE_SIZE);
+		unmap_range(boot_hyp_pgd, TRAMPOLINE_VA, PAGE_SIZE);
+		kfree(boot_hyp_pgd);
+	}
+
 	if (hyp_pgd) {
 		for (addr = PAGE_OFFSET; virt_addr_valid(addr); addr += PGDIR_SIZE)
 			unmap_range(hyp_pgd, KERN_TO_HYP(addr), PGDIR_SIZE);
@@ -170,6 +185,7 @@ void free_hyp_pgds(void)
 		kfree(hyp_pgd);
 	}
 
+	kfree(init_bounce_page);
 	mutex_unlock(&kvm_hyp_pgd_mutex);
 }
 
@@ -185,6 +201,7 @@ static void create_hyp_pte_mappings(pmd_t *pmd, unsigned long start,
 		pte = pte_offset_kernel(pmd, addr);
 		kvm_set_pte(pte, pfn_pte(pfn, prot));
 		get_page(virt_to_page(pte));
+		kvm_flush_dcache_to_poc(pte, sizeof(*pte));
 		pfn++;
 	} while (addr += PAGE_SIZE, addr != end);
 }
@@ -211,6 +228,7 @@ static int create_hyp_pmd_mappings(pud_t *pud, unsigned long start,
 			}
 			pmd_populate_kernel(NULL, pmd, pte);
 			get_page(virt_to_page(pmd));
+			kvm_flush_dcache_to_poc(pmd, sizeof(*pmd));
 		}
 
 		next = pmd_addr_end(addr, end);
@@ -248,6 +266,7 @@ static int __create_hyp_mappings(pgd_t *pgdp,
 			}
 			pud_populate(NULL, pud, pmd);
 			get_page(virt_to_page(pud));
+			kvm_flush_dcache_to_poc(pud, sizeof(*pud));
 		}
 
 		next = pgd_addr_end(addr, end);
@@ -689,18 +708,64 @@ void kvm_mmu_free_memory_caches(struct kvm_vcpu *vcpu)
 
 phys_addr_t kvm_mmu_get_httbr(void)
 {
-	VM_BUG_ON(!virt_addr_valid(hyp_pgd));
 	return virt_to_phys(hyp_pgd);
 }
 
+phys_addr_t kvm_mmu_get_boot_httbr(void)
+{
+	return virt_to_phys(boot_hyp_pgd);
+}
+
+phys_addr_t kvm_get_idmap_vector(void)
+{
+	return hyp_idmap_vector;
+}
+
 int kvm_mmu_init(void)
 {
-	unsigned long hyp_idmap_start = virt_to_phys(__hyp_idmap_text_start);
-	unsigned long hyp_idmap_end = virt_to_phys(__hyp_idmap_text_end);
 	int err;
 
+	hyp_idmap_start = virt_to_phys(__hyp_idmap_text_start);
+	hyp_idmap_end = virt_to_phys(__hyp_idmap_text_end);
+	hyp_idmap_vector = virt_to_phys(__kvm_hyp_init);
+
+	if ((hyp_idmap_start ^ hyp_idmap_end) & PAGE_MASK) {
+		/*
+		 * Our init code is crossing a page boundary. Allocate
+		 * a bounce page, copy the code over and use that.
+		 */
+		size_t len = __hyp_idmap_text_end - __hyp_idmap_text_start;
+		phys_addr_t phys_base;
+
+		init_bounce_page = kmalloc(PAGE_SIZE, GFP_KERNEL);
+		if (!init_bounce_page) {
+			kvm_err("Couldn't allocate HYP init bounce page\n");
+			err = -ENOMEM;
+			goto out;
+		}
+
+		memcpy(init_bounce_page, __hyp_idmap_text_start, len);
+		/*
+		 * Warning: the code we just copied to the bounce page
+		 * must be flushed to the point of coherency.
+		 * Otherwise, the data may be sitting in L2, and HYP
+		 * mode won't be able to observe it as it runs with
+		 * caches off at that point.
+		 */
+		kvm_flush_dcache_to_poc(init_bounce_page, len);
+
+		phys_base = virt_to_phys(init_bounce_page);
+		hyp_idmap_vector += phys_base - hyp_idmap_start;
+		hyp_idmap_start = phys_base;
+		hyp_idmap_end = phys_base + len;
+
+		kvm_info("Using HYP init bounce page @%lx\n",
+			 (unsigned long)phys_base);
+	}
+
 	hyp_pgd = kzalloc(PTRS_PER_PGD * sizeof(pgd_t), GFP_KERNEL);
-	if (!hyp_pgd) {
+	boot_hyp_pgd = kzalloc(PTRS_PER_PGD * sizeof(pgd_t), GFP_KERNEL);
+	if (!hyp_pgd || !boot_hyp_pgd) {
 		kvm_err("Hyp mode PGD not allocated\n");
 		err = -ENOMEM;
 		goto out;
@@ -718,39 +783,30 @@ int kvm_mmu_init(void)
 		goto out;
 	}
 
+	/* Map the very same page at the trampoline VA */
+	err = 	__create_hyp_mappings(boot_hyp_pgd,
+				      TRAMPOLINE_VA, TRAMPOLINE_VA + PAGE_SIZE,
+				      __phys_to_pfn(hyp_idmap_start),
+				      PAGE_HYP);
+	if (err) {
+		kvm_err("Failed to map trampoline @%lx into boot HYP pgd\n",
+			TRAMPOLINE_VA);
+		goto out;
+	}
+
+	/* Map the same page again into the runtime page tables */
+	err = 	__create_hyp_mappings(hyp_pgd,
+				      TRAMPOLINE_VA, TRAMPOLINE_VA + PAGE_SIZE,
+				      __phys_to_pfn(hyp_idmap_start),
+				      PAGE_HYP);
+	if (err) {
+		kvm_err("Failed to map trampoline @%lx into runtime HYP pgd\n",
+			TRAMPOLINE_VA);
+		goto out;
+	}
+
 	return 0;
 out:
 	free_hyp_pgds();
 	return err;
 }
-
-/**
- * kvm_clear_idmap - remove all idmaps from the hyp pgd
- *
- * Free the underlying pmds for all pgds in range and clear the pgds (but
- * don't free them) afterwards.
- */
-void kvm_clear_hyp_idmap(void)
-{
-	unsigned long addr, end;
-	unsigned long next;
-	pgd_t *pgd = hyp_pgd;
-	pud_t *pud;
-	pmd_t *pmd;
-
-	addr = virt_to_phys(__hyp_idmap_text_start);
-	end = virt_to_phys(__hyp_idmap_text_end);
-
-	pgd += pgd_index(addr);
-	do {
-		next = pgd_addr_end(addr, end);
-		if (pgd_none_or_clear_bad(pgd))
-			continue;
-		pud = pud_offset(pgd, addr);
-		pmd = pmd_offset(pud, addr);
-
-		pud_clear(pud);
-		kvm_clean_pmd_entry(pmd);
-		pmd_free(NULL, (pmd_t *)((unsigned long)pmd & PAGE_MASK));
-	} while (pgd++, addr = next, addr < end);
-}