[PULL 16/17] vhost: Only align sections for vhost-user

["Michael S. Tsirkin" <mst@redhat.com>]

From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>

I added hugepage alignment code in c1ece84e7c9 to deal with
vhost-user + postcopy which needs aligned pages when using userfault.
However, on x86 the lower 2MB of address space tends to be shotgun'd
with small fragments around the 512-640k range - e.g. video RAM, and
with HyperV synic pages tend to sit around there - again splitting
it up.  The alignment code complains with a 'Section rounded to ...'
error and gives up.

Since vhost-user already filters out devices without an fd
(see vhost-user.c vhost_user_mem_section_filter) it shouldn't be
affected by those overlaps.

Turn the alignment off on vhost-kernel so that it doesn't try
and align, and thus won't hit the rounding issues.

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Message-Id: <20200116202414.157959-3-dgilbert@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
---
 hw/virtio/vhost.c | 32 +++++++++++++++++---------------
 1 file changed, 17 insertions(+), 15 deletions(-)

diff --git a/hw/virtio/vhost.c b/hw/virtio/vhost.c
index 774d87d98e..25fd469179 100644
--- a/hw/virtio/vhost.c
+++ b/hw/virtio/vhost.c
@@ -547,26 +547,28 @@ static void vhost_region_add_section(struct vhost_dev *dev,
     uintptr_t mrs_host = (uintptr_t)memory_region_get_ram_ptr(section->mr) +
                          section->offset_within_region;
     RAMBlock *mrs_rb = section->mr->ram_block;
-    size_t mrs_page = qemu_ram_pagesize(mrs_rb);
 
     trace_vhost_region_add_section(section->mr->name, mrs_gpa, mrs_size,
                                    mrs_host);
 
-    /* Round the section to it's page size */
-    /* First align the start down to a page boundary */
-    uint64_t alignage = mrs_host & (mrs_page - 1);
-    if (alignage) {
-        mrs_host -= alignage;
-        mrs_size += alignage;
-        mrs_gpa  -= alignage;
+    if (dev->vhost_ops->backend_type == VHOST_BACKEND_TYPE_USER) {   
+        /* Round the section to it's page size */
+        /* First align the start down to a page boundary */
+        size_t mrs_page = qemu_ram_pagesize(mrs_rb);
+        uint64_t alignage = mrs_host & (mrs_page - 1);
+        if (alignage) {
+            mrs_host -= alignage;
+            mrs_size += alignage;
+            mrs_gpa  -= alignage;
+        }
+        /* Now align the size up to a page boundary */
+        alignage = mrs_size & (mrs_page - 1);
+        if (alignage) {
+            mrs_size += mrs_page - alignage;
+        }
+        trace_vhost_region_add_section_aligned(section->mr->name, mrs_gpa, mrs_size,
+                                               mrs_host);
     }
-    /* Now align the size up to a page boundary */
-    alignage = mrs_size & (mrs_page - 1);
-    if (alignage) {
-        mrs_size += mrs_page - alignage;
-    }
-    trace_vhost_region_add_section_aligned(section->mr->name, mrs_gpa, mrs_size,
-                                           mrs_host);
 
     if (dev->n_tmp_sections) {
         /* Since we already have at least one section, lets see if
-- 
MST