[PATCH v4 2/5] KVM: MMU: fix infinite fault access retry

Xiao Guangrong
2013-01-04T13:55:11Z

We have two issues in current code:
- if target gfn is used as its page table, guest will refault then kvm will use
small page size to map it. We need two #PF to fix its shadow page table

- sometimes, say a exception is triggered during vm-exit caused by #PF
(see handle_exception() in vmx.c), we remove all the shadow pages shadowed
by the target gfn before go into page fault path, it will cause infinite
loop:
delete shadow pages shadowed by the gfn -> try to use large page size to map
the gfn -> retry the access ->...

To fix these, we can adjust page size early if the target gfn is used as page
table

Signed-off-by: Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>
---
arch/x86/kvm/mmu.c | 13 ++++---------
arch/x86/kvm/paging_tmpl.h | 35 ++++++++++++++++++++++++++++++++++-
2 files changed, 38 insertions(+), 10 deletions(-)

diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
index 2a3c890..54fc61e 100644
--- a/arch/x86/kvm/mmu.c
+++ b/arch/x86/kvm/mmu.c
@@ -2380,15 +2380,10 @@ static int set_spte(struct kvm_vcpu *vcpu, u64 *sptep,
if (pte_access & ACC_WRITE_MASK) {

/*
- * There are two cases:
- * - the one is other vcpu creates new sp in the window
- * between mapping_level() and acquiring mmu-lock.
- * - the another case is the new sp is created by itself
- * (page-fault path) when guest uses the target gfn as
- * its page table.
- * Both of these cases can be fixed by allowing guest to
- * retry the access, it will refault, then we can establish
- * the mapping by using small page.
+ * Other vcpu creates new sp in the window between
+ * mapping_level() and acquiring mmu-lock. We can
+ * allow guest to retry the access, the mapping can
+ * be fixed if guest refault.
*/
if (level > PT_PAGE_TABLE_LEVEL &&
has_wrprotected_page(vcpu->kvm, gfn, level))
diff --git a/arch/x86/kvm/paging_tmpl.h b/arch/x86/kvm/paging_tmpl.h
index c1e01b6..0453fa0 100644
--- a/arch/x86/kvm/paging_tmpl.h
+++ b/arch/x86/kvm/paging_tmpl.h
@@ -491,6 +491,38 @@ out_gpte_changed:
return 0;
}

+ /*
+ * To see whether the mapped gfn can write its page table in the current
+ * mapping.
+ *
+ * It is the helper function of FNAME(page_fault). When guest uses large page
+ * size to map the writable gfn which is used as current page table, we should
+ * force kvm to use small page size to map it because new shadow page will be
+ * created when kvm establishes shadow page table that stop kvm using large
+ * page size. Do it early can avoid unnecessary #PF and emulation.
+ *
+ * Note: the PDPT page table is not checked for PAE-32 bit guest. It is ok
+ * since the PDPT is always shadowed, that means, we can not use large page
+ * size to map the gfn which is used as PDPT.
+ */
+static bool
+FNAME(is_self_change_mapping)(struct kvm_vcpu *vcpu,
+ struct guest_walker *walker, int user_fault)
+{
+ int level;
+ gfn_t mask = ~(KVM_PAGES_PER_HPAGE(walker->level) - 1);
+
+ if (!(walker->pte_access & ACC_WRITE_MASK ||
+ (!is_write_protection(vcpu) && !user_fault)))
+ return false;
+
+ for (level = walker->level; level <= walker->max_level; level++)
+ if (!((walker->gfn ^ walker->table_gfn[level - 1]) & mask))
+ return true;
+
+ return false;
+}
+
/*
* Page fault handler. There are several causes for a page fault:
* - there is no shadow pte for the guest pte
@@ -545,7 +577,8 @@ static int FNAME(page_fault)(struct kvm_vcpu *vcpu, gva_t addr, u32 error_code,
}

if (walker.level >= PT_DIRECTORY_LEVEL)
- force_pt_level = mapping_level_dirty_bitmap(vcpu, walker.gfn);
+ force_pt_level = mapping_level_dirty_bitmap(vcpu, walker.gfn)
+ || FNAME(is_self_change_mapping)(vcpu, &walker, user_fault);
else
force_pt_level = 1;
if (!force_pt_level) {
--
1.7.7.6