util/vfio-helpers.c: Use ram_block_discard_disable() in qemu_vfio_open_pci()

Currently, when using "nvme://" for a block device, like
    -drive file=nvme://0000:01:00.0/1,if=none,id=drive0 \
    -device virtio-blk,drive=drive0 \

VFIO may pin all guest memory, and discarding of RAM no longer works as
expected. I was able to reproduce this easily with my
    01:00.0 Non-Volatile memory controller: Samsung Electronics Co Ltd
            NVMe SSD Controller SM981/PM981/PM983

Similar to common VFIO, we have to disable it, making sure that:
a) virtio-balloon won't discard any memory ("silently disabled")
b) virtio-mem and nvme:// run mutually exclusive

Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Alex Williamson <alex.williamson@redhat.com>
Cc: Wei Yang <richardw.yang@linux.intel.com>
Cc: Dr. David Alan Gilbert <dgilbert@redhat.com>
Cc: Igor Mammedov <imammedo@redhat.com>
Cc: Pankaj Gupta <pankaj.gupta.linux@gmail.com>
Cc: Peter Xu <peterx@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20201116105947.9194-1-david@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

1 files changed, 14 insertions, 0 deletions

diff --git a/util/vfio-helpers.c b/util/vfio-helpers.c
index c469beb..2bec48e 100644
--- a/util/vfio-helpers.c
+++ b/util/vfio-helpers.c
@@ -16,6 +16,7 @@
 #include "qapi/error.h"
 #include "exec/ramlist.h"
 #include "exec/cpu-common.h"
+#include "exec/memory.h"
 #include "trace.h"
 #include "qemu/error-report.h"
 #include "standard-headers/linux/pci_regs.h"
@@ -494,8 +495,20 @@ QEMUVFIOState *qemu_vfio_open_pci(const char *device, Error **errp)
     int r;
     QEMUVFIOState *s = g_new0(QEMUVFIOState, 1);
 
+    /*
+     * VFIO may pin all memory inside mappings, resulting it in pinning
+     * all memory inside RAM blocks unconditionally.
+     */
+    r = ram_block_discard_disable(true);
+    if (r) {
+        error_setg_errno(errp, -r, "Cannot set discarding of RAM broken");
+        g_free(s);
+        return NULL;
+    }
+
     r = qemu_vfio_init_pci(s, device, errp);
     if (r) {
+        ram_block_discard_disable(false);
         g_free(s);
         return NULL;
     }
@@ -837,4 +850,5 @@ void qemu_vfio_close(QEMUVFIOState *s)
     close(s->device);
     close(s->group);
     close(s->container);
+    ram_block_discard_disable(false);
 }