Commit 815d1799c3 for qemu.org

commit 815d1799c3d3d6cd058cb4634392ca5a34830450
Author: Christian Schoenebeck <qemu_oss@crudebyte.com>
Date:   Fri Jun 12 20:22:52 2026 +0200

    9pfs/xen: implement msize_limit callback

    Add and implement the msize_limit callback for the Xen transport.

    The limit is calculated using XEN_FLEX_RING_SIZE() based on the
    negotiated ring_order. For the theoretical maximum ring_order of 9,
    this results in a maximum 'msize' of 1048576 bytes (1 MiB).

    The minimum limit of all rings is picked, because multiple rings
    could theoretically have different ring_orders.

    Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
    Link: https://lore.kernel.org/qemu-devel/9471786bc47b93e822f6c6233a83f2b9f61e6c82.1781287774.git.qemu_oss@crudebyte.com
    Signed-off-by: Christian Schoenebeck <qemu_oss@crudebyte.com>

diff --git a/hw/9pfs/xen-9p-backend.c b/hw/9pfs/xen-9p-backend.c
index ca0fff5fa9..e31124bcf5 100644
--- a/hw/9pfs/xen-9p-backend.c
+++ b/hw/9pfs/xen-9p-backend.c
@@ -250,12 +250,31 @@ static void xen_9pfs_push_and_notify(V9fsPDU *pdu)
     qemu_bh_schedule(ring->bh);
 }

+static size_t xen_9p_msize_limit(V9fsState *s)
+{
+    Xen9pfsDev *xen_9pfs = container_of(s, Xen9pfsDev, state);
+    size_t limit;
+    int i;
+
+    if (!xen_9pfs->num_rings) {
+        return 0;
+    }
+
+    limit = XEN_FLEX_RING_SIZE(xen_9pfs->rings[0].ring_order);
+    for (i = 1; i < xen_9pfs->num_rings; i++) {
+        limit = MIN(limit, XEN_FLEX_RING_SIZE(xen_9pfs->rings[i].ring_order));
+    }
+
+    return limit;
+}
+
 static const V9fsTransport xen_9p_transport = {
     .pdu_vmarshal = xen_9pfs_pdu_vmarshal,
     .pdu_vunmarshal = xen_9pfs_pdu_vunmarshal,
     .init_in_iov_from_pdu = xen_9pfs_init_in_iov_from_pdu,
     .init_out_iov_from_pdu = xen_9pfs_init_out_iov_from_pdu,
     .push_and_notify = xen_9pfs_push_and_notify,
+    .msize_limit = xen_9p_msize_limit,
 };

 static int xen_9pfs_init(struct XenLegacyDevice *xendev)