[PATCH v2] x86/svm: retry after unhandled NPT fault if gfn was marked for recalculation

[Igor Druzhinin <igor.druzhinin@citrix.com>]

If a recalculation NPT fault hasn't been handled explicitly in
hvm_hap_nested_page_fault() then it's potentially safe to retry -
US bit has been re-instated in PTE and any real fault would be correctly
re-raised next time. Do it by allowing hvm_hap_nested_page_fault to
fall through in that case.

This covers a specific case of migration with vGPU assigned on AMD:
global log-dirty is enabled and causes immediate recalculation NPT
fault in MMIO area upon access. This type of fault isn't described
explicitly in hvm_hap_nested_page_fault (this isn't called on
EPT misconfig exit on Intel) which results in domain crash.

Signed-off-by: Igor Druzhinin <igor.druzhinin@citrix.com>
---
Changes in v2:
- don't gamble with retrying every recal fault and instead let
  hvm_hap_nested_page_fault know it's allowed to fall through in default case
---
 xen/arch/x86/hvm/hvm.c        | 6 +++---
 xen/arch/x86/hvm/svm/svm.c    | 7 ++++++-
 xen/arch/x86/hvm/vmx/vmx.c    | 2 +-
 xen/include/asm-x86/hvm/hvm.h | 2 +-
 4 files changed, 11 insertions(+), 6 deletions(-)

diff --git a/xen/arch/x86/hvm/hvm.c b/xen/arch/x86/hvm/hvm.c
index 74c9f84..42bd720 100644
--- a/xen/arch/x86/hvm/hvm.c
+++ b/xen/arch/x86/hvm/hvm.c
@@ -1731,7 +1731,7 @@ void hvm_inject_event(const struct x86_event *event)
 }
 
 int hvm_hap_nested_page_fault(paddr_t gpa, unsigned long gla,
-                              struct npfec npfec)
+                              struct npfec npfec, bool fall_through)
 {
     unsigned long gfn = gpa >> PAGE_SHIFT;
     p2m_type_t p2mt;
@@ -1740,7 +1740,7 @@ int hvm_hap_nested_page_fault(paddr_t gpa, unsigned long gla,
     struct vcpu *curr = current;
     struct domain *currd = curr->domain;
     struct p2m_domain *p2m, *hostp2m;
-    int rc, fall_through = 0, paged = 0;
+    int rc, paged = 0;
     bool sharing_enomem = false;
     vm_event_request_t *req_ptr = NULL;
     bool sync = false;
@@ -1905,7 +1905,7 @@ int hvm_hap_nested_page_fault(paddr_t gpa, unsigned long gla,
             sync = p2m_mem_access_check(gpa, gla, npfec, &req_ptr);
 
             if ( !sync )
-                fall_through = 1;
+                fall_through = true;
             else
             {
                 /* Rights not promoted (aka. sync event), work here is done */
diff --git a/xen/arch/x86/hvm/svm/svm.c b/xen/arch/x86/hvm/svm/svm.c
index 46a1aac..8ef3fed 100644
--- a/xen/arch/x86/hvm/svm/svm.c
+++ b/xen/arch/x86/hvm/svm/svm.c
@@ -1695,7 +1695,12 @@ static void svm_do_nested_pgfault(struct vcpu *v,
     else if ( pfec & NPT_PFEC_in_gpt )
         npfec.kind = npfec_kind_in_gpt;
 
-    ret = hvm_hap_nested_page_fault(gpa, ~0ul, npfec);
+    /*
+     * US bit being set in error code indicates P2M type recalculation has
+     * just been done meaning that it's possible there is nothing else to handle
+     * and we can just fall through and retry.
+     */
+    ret = hvm_hap_nested_page_fault(gpa, ~0ul, npfec, !!(pfec & PFEC_user_mode));
 
     if ( tb_init_done )
     {
diff --git a/xen/arch/x86/hvm/vmx/vmx.c b/xen/arch/x86/hvm/vmx/vmx.c
index 11a4dd9..10f1eeb 100644
--- a/xen/arch/x86/hvm/vmx/vmx.c
+++ b/xen/arch/x86/hvm/vmx/vmx.c
@@ -3398,7 +3398,7 @@ static void ept_handle_violation(ept_qual_t q, paddr_t gpa)
     else
         gla = ~0ull;
 
-    ret = hvm_hap_nested_page_fault(gpa, gla, npfec);
+    ret = hvm_hap_nested_page_fault(gpa, gla, npfec, false);
     switch ( ret )
     {
     case 0:         // Unhandled L1 EPT violation
diff --git a/xen/include/asm-x86/hvm/hvm.h b/xen/include/asm-x86/hvm/hvm.h
index 1eb377d..03e5f1d 100644
--- a/xen/include/asm-x86/hvm/hvm.h
+++ b/xen/include/asm-x86/hvm/hvm.h
@@ -329,7 +329,7 @@ void hvm_fast_singlestep(struct vcpu *v, uint16_t p2midx);
 
 struct npfec;
 int hvm_hap_nested_page_fault(paddr_t gpa, unsigned long gla,
-                              struct npfec npfec);
+                              struct npfec npfec, bool fall_through);
 
 /* Check CR4/EFER values */
 const char *hvm_efer_valid(const struct vcpu *v, uint64_t value,
-- 
2.7.4