Forwarding entire batches to the device model when an individual
iteration of them got rejected by internal device emulation handlers
with X86EMUL_UNHANDLEABLE is wrong: The device model would then handle
all iterations, without the internal handler getting to see any past
the one it returned failure for. This causes misbehavior in at least
the MSI-X and VGA code, which want to see all such requests for
internal tracking/caching purposes. But note that this does not apply
to buffered I/O requests.

This in turn means that the condition in hvm_process_io_intercept() of
when to crash the domain was wrong: Since X86EMUL_UNHANDLEABLE can
validly be returned by the individual device handlers, we mustn't
blindly crash the domain if such occurs on other than the initial
iteration. Instead we need to distinguish hvm_copy_*_guest_phys()
failures from device specific ones, and then the former need to always
be fatal to the domain (i.e. also on the first iteration), since
otherwise we again would end up forwarding a request to qemu which the
internal handler didn't get to see.

Also commit 4faffc41d ("x86/hvm: limit reps to avoid the need to handle
retry") went too far in removing code from hvm_process_io_intercept():
When there were successfully handled iterations, the function should
continue to return success with a clipped repeat count.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Cc: Chang Jianzhong <changjzh@gmail.com>
---
I assume this also addresses the issue which
http://lists.xenproject.org/archives/html/xen-devel/2016-01/msg03189.html 
attempted to deal with in a not really acceptable way.

--- a/xen/arch/x86/hvm/emulate.c
+++ b/xen/arch/x86/hvm/emulate.c
@@ -95,7 +95,7 @@ static const struct hvm_io_handler null_
 };
 
 static int hvmemul_do_io(
-    bool_t is_mmio, paddr_t addr, unsigned long reps, unsigned int size,
+    bool_t is_mmio, paddr_t addr, unsigned long *reps, unsigned int size,
     uint8_t dir, bool_t df, bool_t data_is_addr, uintptr_t data)
 {
     struct vcpu *curr = current;
@@ -104,7 +104,7 @@ static int hvmemul_do_io(
         .type = is_mmio ? IOREQ_TYPE_COPY : IOREQ_TYPE_PIO,
         .addr = addr,
         .size = size,
-        .count = reps,
+        .count = *reps,
         .dir = dir,
         .df = df,
         .data = data,
@@ -136,7 +136,7 @@ static int hvmemul_do_io(
         if ( (p.type != is_mmio ? IOREQ_TYPE_COPY : IOREQ_TYPE_PIO) ||
              (p.addr != addr) ||
              (p.size != size) ||
-             (p.count != reps) ||
+             (p.count != *reps) ||
              (p.dir != dir) ||
              (p.df != df) ||
              (p.data_is_ptr != data_is_addr) )
@@ -214,7 +214,7 @@ static int hvmemul_do_io_buffer(
 
     BUG_ON(buffer == NULL);
 
-    rc = hvmemul_do_io(is_mmio, addr, *reps, size, dir, df, 0,
+    rc = hvmemul_do_io(is_mmio, addr, reps, size, dir, df, 0,
                        (uintptr_t)buffer);
     if ( rc == X86EMUL_UNHANDLEABLE && dir == IOREQ_READ )
         memset(buffer, 0xff, size);
@@ -305,13 +305,13 @@ static int hvmemul_do_io_addr(
         count = 1;
     }
 
-    rc = hvmemul_do_io(is_mmio, addr, count, size, dir, df, 1,
+    rc = hvmemul_do_io(is_mmio, addr, &count, size, dir, df, 1,
                        ram_gpa);
+
     if ( rc == X86EMUL_OKAY )
-    {
         v->arch.hvm_vcpu.hvm_io.mmio_retry = (count < *reps);
-        *reps = count;
-    }
+
+    *reps = count;
 
  out:
     while ( nr_pages )
--- a/xen/arch/x86/hvm/intercept.c
+++ b/xen/arch/x86/hvm/intercept.c
@@ -148,7 +148,7 @@ int hvm_process_io_intercept(const struc
                     ASSERT_UNREACHABLE();
                     /* fall through */
                 default:
-                    rc = X86EMUL_UNHANDLEABLE;
+                    rc = -1; /* != any X86EMUL_* value. */
                     break;
                 }
                 if ( rc != X86EMUL_OKAY )
@@ -178,7 +178,7 @@ int hvm_process_io_intercept(const struc
                     ASSERT_UNREACHABLE();
                     /* fall through */
                 default:
-                    rc = X86EMUL_UNHANDLEABLE;
+                    rc = -1; /* != any X86EMUL_* value. */
                     break;
                 }
                 if ( rc != X86EMUL_OKAY )
@@ -196,8 +196,22 @@ int hvm_process_io_intercept(const struc
         }
     }
 
-    if ( i != 0 && rc == X86EMUL_UNHANDLEABLE )
+    if ( unlikely(rc < 0) )
         domain_crash(current->domain);
+    else if ( i )
+    {
+        p->count = i;
+        rc = X86EMUL_OKAY;
+    }
+    else if ( rc == X86EMUL_UNHANDLEABLE )
+    {
+        /*
+         * Don't forward entire batches to the device model: This would
+         * prevent the internal handlers to see subsequent iterations of
+         * the request.
+         */
+        p->count = 1;
+    }
 
     return rc;
 }