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

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.

This covers a specific case of migration with vGPU assigned which
uses direct MMIO mappings made by XEN_DOMCTL_memory_mapping hypercall:
at a moment log-dirty is enabled globally, recalculation is requested
for the whole guest memory including those mapped MMIO regions
which causes a page fault being raised at the first access to them;
but due to MMIO P2M type not having any explicit handling in
hvm_hap_nested_page_fault() a domain is erroneously crashed with unhandled
SVM violation.

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.

Since there was no case previously where p2m_pt_handle_deferred_changes()
could return a positive value - it's safe to replace ">= 0" with just "== 0"
in VMEXIT_NPF handler. finish_type_change() is also not affected by the
change as being able to deal with >0 return value of p2m->recalc from
EPT implementation.

Signed-off-by: Igor Druzhinin <igor.druzhinin@citrix.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
master commit: 51ca66c37371b10b378513af126646de22eddb17
master date: 2020-06-05 17:12:11 +0200

diff --git a/xen/arch/x86/hvm/svm/svm.c b/xen/arch/x86/hvm/svm/svm.c
index 0cec511b3af552bd7cc3c87dbf609e66bb267db7..8317ef8fb0bc9f7cdf9c2c1e3de1bebecaeb4750 100644 (file)
--- a/xen/arch/x86/hvm/svm/svm.c
+++ b/xen/arch/x86/hvm/svm/svm.c
@@ -2947,9 +2947,10 @@ 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 eb660774964da97cf560292a36cfae47e80ee5a3..2b74726e39e7d2bc2ce237f158a063203b170bd9 100644 (file)
--- a/xen/arch/x86/mm/p2m-pt.c
+++ b/xen/arch/x86/mm/p2m-pt.c
@@ -341,6 +341,7 @@ static int do_recalc(struct p2m_domain *p2m, unsigned long gfn)
     unsigned int level = 4;
     l1_pgentry_t *pent;
     int err = 0;
+    bool recalc_done = false;
 
     table = map_domain_page(pagetable_get_mfn(p2m_get_pagetable(p2m)));
     while ( --level )
@@ -402,6 +403,8 @@ 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);
+
+                recalc_done = true;
             }
         }
         unmap_domain_page((void *)((unsigned long)pent & PAGE_MASK));
@@ -448,12 +451,14 @@ 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);
+
+        recalc_done = true;
     }
 
  out:
     unmap_domain_page(table);
 
-    return err;
+    return err ?: recalc_done;
 }
 
 int p2m_pt_handle_deferred_changes(uint64_t gpa)

diff --git a/xen/arch/x86/mm/p2m.c b/xen/arch/x86/mm/p2m.c
index ba126f790a13ca49c027e98e40ba75a487a75502..40396307614907fad8320a21935b99e46f71f81d 100644 (file)
--- a/xen/arch/x86/mm/p2m.c
+++ b/xen/arch/x86/mm/p2m.c
@@ -1194,7 +1194,7 @@ static int finish_type_change(struct p2m_domain *p2m,
         rc = p2m->recalc(p2m, gfn);
         /*
          * ept->recalc could return 0/1/-ENOMEM. pt->recalc could return
-         * 0/-ENOMEM/-ENOENT, -ENOENT isn't an error as we are looping
+         * 0/1/-ENOMEM/-ENOENT, -ENOENT isn't an error as we are looping
          * gfn here. If rc is 1 we need to have it 0 for success.
          */
         if ( rc == -ENOENT || rc > 0 )