[PATCH] hw/usb/hcd-ehci: fix writeback order

[Arnout Engelen]

The 'active' bit passes control over a qTD between the guest and the
controller: set to 1 by guest to enable execution by the controller,
and the controller sets it to '0' to hand back control to the guest.

ehci_state_writeback write two dwords to main memory using DMA:
the third dword of the qTD (containing dt, total bytes to transfer,
cpage, cerr and status) and the fourth dword of the qTD (containing
the offset).

This commit makes sure the fourth dword is written before the third,
avoiding a race condition where a new offset written into the qTD
by the guest after it observed the status going to go to '0' gets
overwritten by a 'late' DMA writeback of the previous offset.

This race condition could lead to 'cpage out of range (5)' errors,
and reproduced by:

./qemu-system-x86_64 -enable-kvm -bios $SEABIOS/bios.bin -m 4096 -device 
usb-ehci -blockdev 
driver=file,read-only=on,filename=/home/aengelen/Downloads/openSUSE-Tumbleweed-DVD-i586-Snapshot20220428-Media.iso,node-name=iso
 -device usb-storage,drive=iso,bootindex=0 -chardev 
pipe,id=shell,path=/tmp/pipe -device virtio-serial -device 
virtconsole,chardev=shell -device virtio-rng-pci -serial mon:stdio -nographic

(press a key, select 'Installation' (2), and accept the default
values. On my machine the 'cpage out of range' is reproduced while
loading the Linux Kernel about once per 7 attempts. With the fix in
this commit it no longer fails)

This problem was previously reported as a seabios problem in
https://mail.coreboot.org/hyperkitty/list/seabios@seabios.org/thread/OUTHT5ISSQJGXPNTUPY3O5E5EPZJCHM3/
and as a nixos CI build failure in
https://github.com/NixOS/nixpkgs/issues/170803

Signed-off-by: Arnout Engelen <arnout@bzzt.net>
---
 hw/usb/hcd-ehci.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/hw/usb/hcd-ehci.c b/hw/usb/hcd-ehci.c
index 33a8a377bd..d4da8dcb8d 100644
--- a/hw/usb/hcd-ehci.c
+++ b/hw/usb/hcd-ehci.c
@@ -2011,7 +2011,10 @@ static int ehci_state_writeback(EHCIQueue *q)
     ehci_trace_qtd(q, NLPTR_GET(p->qtdaddr), (EHCIqtd *) &q->qh.next_qtd);
     qtd = (uint32_t *) &q->qh.next_qtd;
     addr = NLPTR_GET(p->qtdaddr);
-    put_dwords(q->ehci, addr + 2 * sizeof(uint32_t), qtd + 2, 2);
+    /* First write back the offset */
+    put_dwords(q->ehci, addr + 3 * sizeof(uint32_t), qtd + 3, 1);
+    /* Then write back the token, clearing the 'active' bit */
+    put_dwords(q->ehci, addr + 2 * sizeof(uint32_t), qtd + 2, 1);
     ehci_free_packet(p);
 
     /*
-- 
2.35.3