[Xen-devel] [PATCH RFC] ioreq-server: handle the lack of a default emulator properly

[Paul Durrant <paul.durrant@xxxxxxxxxx>]

I started porting QEMU over to use the new ioreq server API and hit a
problem with PCI bus enumeration. Because, with my patches, QEMU only
registers to handle config space accesses for the PCI device it implements
all other attempts by the guest to access 0xcfc go nowhere and this was
causing the vcpu to wedge up because nothing was completing the I/O.

This patch introduces an I/O completion handler into the hypervisor for the
case where no ioreq server matches a particular request. Read requests are
completed with 0xf's in the data buffer, writes and all other I/O req types
are ignored.

Signed-off-by: Paul Durrant <paul.durrant@xxxxxxxxxx>
Cc: Keir Fraser <keir@xxxxxxx>
Cc: Jan Beulich <jbeulich@xxxxxxxx>

---

This fix should probably go in 4.5 but this patch is RFC for the moment
because of uncertainty about what to do for unemulated MMIO accesses.
Originally I forced a domain crash in this case, but hvmloader actually
hit the crash because the code that deals with building the ACPI TPM
info reads from 0xFED40F00 looking for a signature value and there is
nothing backing this access in my configuration. So, the question is
whether to whitelist this access in some way or make building that
table optional in some way so that it is only invoked if an emulated
TPM is definitely present.

 xen/arch/x86/hvm/hvm.c |   41 ++++++++++++++++++++++++++++++++++++++---
 1 file changed, 38 insertions(+), 3 deletions(-)

diff --git a/xen/arch/x86/hvm/hvm.c b/xen/arch/x86/hvm/hvm.c
index bb45593..5d653d8 100644
--- a/xen/arch/x86/hvm/hvm.c
+++ b/xen/arch/x86/hvm/hvm.c
@@ -2386,8 +2386,7 @@ static struct hvm_ioreq_server 
*hvm_select_ioreq_server(struct domain *d,
     if ( list_empty(&d->arch.hvm_domain.ioreq_server.list) )
         return NULL;
 
-    if ( list_is_singular(&d->arch.hvm_domain.ioreq_server.list) ||
-         (p->type != IOREQ_TYPE_COPY && p->type != IOREQ_TYPE_PIO) )
+    if ( p->type != IOREQ_TYPE_COPY && p->type != IOREQ_TYPE_PIO )
         return d->arch.hvm_domain.default_ioreq_server;
 
     cf8 = d->arch.hvm_domain.pci_cf8;
@@ -2618,12 +2617,48 @@ bool_t hvm_send_assist_req_to_ioreq_server(struct 
hvm_ioreq_server *s,
     return 0;
 }
 
+static bool_t hvm_complete_assist_req(ioreq_t *p)
+{
+    switch (p->type)
+    {
+    case IOREQ_TYPE_COPY:
+        gdprintk(XENLOG_WARNING, "unemulated MMIO (%s %d bytes @ %lx) %u 
reps\n",
+                 (p->dir == IOREQ_READ) ? "read" : "write",
+                 p->size,
+                 p->addr,
+                 p->count);
+        /* FALLTHRU */
+    case IOREQ_TYPE_PIO:
+        if ( p->dir == IOREQ_READ )
+        {
+            if ( !p->data_is_ptr )
+                p->data = ~0ul;
+            else
+            {
+                int i, sign = p->df ? -1 : 1;
+                uint32_t data = ~0;
+
+                for ( i = 0; i < p->count; i++ )
+                    hvm_copy_to_guest_phys(p->data + sign * i * p->size, &data,
+                                           p->size);
+            }
+        }
+        /* FALLTHRU */
+    default:
+        p->state = STATE_IORESP_READY;
+        hvm_io_assist(p);
+        break;
+    }
+
+    return 1;
+}
+
 bool_t hvm_send_assist_req(ioreq_t *p)
 {
     struct hvm_ioreq_server *s = hvm_select_ioreq_server(current->domain, p);
 
     if ( !s )
-        return 0;
+        return hvm_complete_assist_req(p);
 
     return hvm_send_assist_req_to_ioreq_server(s, p);
 }
-- 
1.7.10.4


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