[PATCH] x86/svm: do not try to handle recalc NPT faults immediately

[Igor Druzhinin <igor.druzhinin@xxxxxxxxxx>]

A recalculation NPT fault doesn't always require additional handling
in hvm_hap_nested_page_fault(), moreover in general case if there is no
explicit handling done there - the fault is wrongly considered fatal.

Instead of trying to be opportunistic - use safer approach and handle
P2M recalculation in a separate NPT fault by attempting to retry after
making the necessary adjustments. This is aligned with Intel behavior
where there are separate VMEXITs for recalculation and EPT violations
(faults) and only faults are handled in hvm_hap_nested_page_fault().
Do it by also unifying do_recalc return code with Intel implementation
where returning 1 means P2M was actually changed.

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.

Signed-off-by: Igor Druzhinin <igor.druzhinin@xxxxxxxxxx>
---
This is a safer alternative to:
https://lists.xenproject.org/archives/html/xen-devel/2020-05/msg01662.html
and more correct approach from my PoV.
---
 xen/arch/x86/hvm/svm/svm.c | 5 +++--
 xen/arch/x86/mm/p2m-pt.c   | 8 ++++++--
 2 files changed, 9 insertions(+), 4 deletions(-)

diff --git a/xen/arch/x86/hvm/svm/svm.c b/xen/arch/x86/hvm/svm/svm.c
index 46a1aac..7f6f578 100644
--- a/xen/arch/x86/hvm/svm/svm.c
+++ b/xen/arch/x86/hvm/svm/svm.c
@@ -2923,9 +2923,10 @@ void svm_vmexit_handler(struct cpu_user_regs *regs)
             v->arch.hvm.svm.cached_insn_len = vmcb->guest_ins_len & 0xf;
         rc = vmcb->exitinfo1 & PFEC_page_present
              ? p2m_pt_handle_deferred_changes(vmcb->exitinfo2) : 0;
-        if ( rc >= 0 )
+        if ( rc == 0 )
+            /* If no recal adjustments were being made - handle this fault */
             svm_do_nested_pgfault(v, regs, vmcb->exitinfo1, vmcb->exitinfo2);
-        else
+        else if ( rc < 0 )
         {
             printk(XENLOG_G_ERR
                    "%pv: Error %d handling NPF (gpa=%08lx ec=%04lx)\n",
diff --git a/xen/arch/x86/mm/p2m-pt.c b/xen/arch/x86/mm/p2m-pt.c
index 5c05017..377565b 100644
--- a/xen/arch/x86/mm/p2m-pt.c
+++ b/xen/arch/x86/mm/p2m-pt.c
@@ -340,7 +340,7 @@ static int do_recalc(struct p2m_domain *p2m, unsigned long 
gfn)
     unsigned long gfn_remainder = gfn;
     unsigned int level = 4;
     l1_pgentry_t *pent;
-    int err = 0;
+    int err = 0, rc = 0;
 
     table = map_domain_page(pagetable_get_mfn(p2m_get_pagetable(p2m)));
     while ( --level )
@@ -402,6 +402,8 @@ static int do_recalc(struct p2m_domain *p2m, unsigned long 
gfn)
                 clear_recalc(l1, e);
                 err = p2m->write_p2m_entry(p2m, gfn, pent, e, level + 1);
                 ASSERT(!err);
+
+                rc = 1;
             }
         }
         unmap_domain_page((void *)((unsigned long)pent & PAGE_MASK));
@@ -448,12 +450,14 @@ static int do_recalc(struct p2m_domain *p2m, unsigned 
long gfn)
             clear_recalc(l1, e);
         err = p2m->write_p2m_entry(p2m, gfn, pent, e, level + 1);
         ASSERT(!err);
+
+        rc = 1;
     }
 
  out:
     unmap_domain_page(table);
 
-    return err;
+    return err ? err : rc;
 }
 
 int p2m_pt_handle_deferred_changes(uint64_t gpa)
-- 
2.7.4