On Tue, 16 Feb 2021 at 11:01, Thomas Huth <th...@redhat.com> wrote: > According to the virtio specification, a memory barrier should be > used before incrementing the idx field in the "available" ring. > So far, we did not do this in the s390-ccw bios yet, but recently > Peter Maydell saw problems with the s390-ccw bios when running > the qtests on an aarch64 host (the bios panic'ed with the message: > "SCSI cannot report LUNs: response VS RESP=09"), which could > maybe be related to the missing memory barriers. Thus let's add > those barriers now. Since we've only seen the problem on TCG so far, > a "bcr 14,0" should be sufficient here to trigger the tcg_gen_mb() > in the TCG translate code. > > (Note: The virtio spec also talks about using a memory barrier > *after* incrementing the idx field, but if I understood correctly > this is only required when using notification suppression - which > we don't use in the s390-ccw bios here) > > Signed-off-by: Thomas Huth <th...@redhat.com>
This noticeably increases the reliability of the test for me: it goes from failing one time in 2 or 3 to failing about one time in 5 or 6. However it does still hit the virtio_scsi_parse_req() check on "out_size && in_size" eventually. So my guess is that this is not a sufficient supply of barriers. I'm firmly not a virtio expert but my reading of the spec suggested that some of the way the s390-ccw code is doing things might be in the wrong order and need restructuring beyond merely "add barriers to existing code". thanks -- PMM
