KVM: PPC: Allow for read-only pages backing a Book3S HV guest

With this, if a guest does an H_ENTER with a read/write HPTE on a page
which is currently read-only, we make the actual HPTE inserted be a
read-only version of the HPTE.  We now intercept protection faults as
well as HPTE not found faults, and for a protection fault we work out
whether it should be reflected to the guest (e.g. because the guest HPTE
didn't allow write access to usermode) or handled by switching to
kernel context and calling kvmppc_book3s_hv_page_fault, which will then
request write access to the page and update the actual HPTE.

Signed-off-by: Paul Mackerras <paulus@samba.org>
Signed-off-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Avi Kivity <avi@redhat.com>

1 files changed, 36 insertions, 3 deletions

diff --git a/arch/powerpc/kvm/book3s_64_mmu_hv.c b/arch/powerpc/kvm/book3s_64_mmu_hv.c
index 83761dd8a92..66d6452c108 100644
--- a/arch/powerpc/kvm/book3s_64_mmu_hv.c
+++ b/arch/powerpc/kvm/book3s_64_mmu_hv.c
@@ -503,6 +503,7 @@ int kvmppc_book3s_hv_page_fault(struct kvm_run *run, struct kvm_vcpu *vcpu,
 	struct page *page, *pages[1];
 	long index, ret, npages;
 	unsigned long is_io;
+	unsigned int writing, write_ok;
 	struct vm_area_struct *vma;
 
 	/*
@@ -553,8 +554,11 @@ int kvmppc_book3s_hv_page_fault(struct kvm_run *run, struct kvm_vcpu *vcpu,
 	pfn = 0;
 	page = NULL;
 	pte_size = PAGE_SIZE;
+	writing = (dsisr & DSISR_ISSTORE) != 0;
+	/* If writing != 0, then the HPTE must allow writing, if we get here */
+	write_ok = writing;
 	hva = gfn_to_hva_memslot(memslot, gfn);
-	npages = get_user_pages_fast(hva, 1, 1, pages);
+	npages = get_user_pages_fast(hva, 1, writing, pages);
 	if (npages < 1) {
 		/* Check if it's an I/O mapping */
 		down_read(&current->mm->mmap_sem);
@@ -565,6 +569,7 @@ int kvmppc_book3s_hv_page_fault(struct kvm_run *run, struct kvm_vcpu *vcpu,
 				((hva - vma->vm_start) >> PAGE_SHIFT);
 			pte_size = psize;
 			is_io = hpte_cache_bits(pgprot_val(vma->vm_page_prot));
+			write_ok = vma->vm_flags & VM_WRITE;
 		}
 		up_read(&current->mm->mmap_sem);
 		if (!pfn)
@@ -575,6 +580,24 @@ int kvmppc_book3s_hv_page_fault(struct kvm_run *run, struct kvm_vcpu *vcpu,
 			page = compound_head(page);
 			pte_size <<= compound_order(page);
 		}
+		/* if the guest wants write access, see if that is OK */
+		if (!writing && hpte_is_writable(r)) {
+			pte_t *ptep, pte;
+
+			/*
+			 * We need to protect against page table destruction
+			 * while looking up and updating the pte.
+			 */
+			rcu_read_lock_sched();
+			ptep = find_linux_pte_or_hugepte(current->mm->pgd,
+							 hva, NULL);
+			if (ptep && pte_present(*ptep)) {
+				pte = kvmppc_read_update_linux_pte(ptep, 1);
+				if (pte_write(pte))
+					write_ok = 1;
+			}
+			rcu_read_unlock_sched();
+		}
 		pfn = page_to_pfn(page);
 	}
 
@@ -595,6 +618,8 @@ int kvmppc_book3s_hv_page_fault(struct kvm_run *run, struct kvm_vcpu *vcpu,
 
 	/* Set the HPTE to point to pfn */
 	r = (r & ~(HPTE_R_PP0 - pte_size)) | (pfn << PAGE_SHIFT);
+	if (hpte_is_writable(r) && !write_ok)
+		r = hpte_make_readonly(r);
 	ret = RESUME_GUEST;
 	preempt_disable();
 	while (!try_lock_hpte(hptep, HPTE_V_HVLOCK))
@@ -614,14 +639,22 @@ int kvmppc_book3s_hv_page_fault(struct kvm_run *run, struct kvm_vcpu *vcpu,
 		unlock_rmap(rmap);
 		goto out_unlock;
 	}
-	kvmppc_add_revmap_chain(kvm, rev, rmap, index, 0);
+
+	if (hptep[0] & HPTE_V_VALID) {
+		/* HPTE was previously valid, so we need to invalidate it */
+		unlock_rmap(rmap);
+		hptep[0] |= HPTE_V_ABSENT;
+		kvmppc_invalidate_hpte(kvm, hptep, index);
+	} else {
+		kvmppc_add_revmap_chain(kvm, rev, rmap, index, 0);
+	}
 
 	hptep[1] = r;
 	eieio();
 	hptep[0] = hpte[0];
 	asm volatile("ptesync" : : : "memory");
 	preempt_enable();
-	if (page)
+	if (page && hpte_is_writable(r))
 		SetPageDirty(page);
 
  out_put: