On 2020-04-20 08:29, Cornelia Huck wrote:
On Mon, 20 Apr 2020 14:26:17 +0200
Cornelia Huck <coh...@redhat.com> wrote:
On Fri, 17 Apr 2020 14:38:38 -0400
Jared Rossi <jro...@linux.ibm.com> wrote:
> Remove the explicit prefetch check when using vfio-ccw devices.
> This check is not needed as all Linux channel programs are intended
> to use prefetch and will be executed in the same way regardless.
As already commented on the Linux patch: Can we log something, so this
is debuggable if this statement does not hold true in the future?
Agreed. I will work on debugging improvements so that any future issues
related to unintended prefetching are more clearly logged.
>
> Signed-off-by: Jared Rossi <jro...@linux.ibm.com>
> ---
> hw/vfio/ccw.c | 13 +++----------
> 1 file changed, 3 insertions(+), 10 deletions(-)
>
> diff --git a/hw/vfio/ccw.c b/hw/vfio/ccw.c
> index 50cc2ec75c..e649377b68 100644
> --- a/hw/vfio/ccw.c
> +++ b/hw/vfio/ccw.c
> @@ -74,16 +74,9 @@ static IOInstEnding vfio_ccw_handle_request(SubchDev *sch)
> struct ccw_io_region *region = vcdev->io_region;
> int ret;
>
> - if (!(sch->orb.ctrl0 & ORB_CTRL0_MASK_PFCH)) {
> - if (!(vcdev->force_orb_pfch)) {
> - warn_once_pfch(vcdev, sch, "requires PFCH flag set");
> - sch_gen_unit_exception(sch);
> - css_inject_io_interrupt(sch);
> - return IOINST_CC_EXPECTED;
> - } else {
> - sch->orb.ctrl0 |= ORB_CTRL0_MASK_PFCH;
> - warn_once_pfch(vcdev, sch, "PFCH flag forced");
> - }
> + if (!(sch->orb.ctrl0 & ORB_CTRL0_MASK_PFCH) && vcdev->force_orb_pfch) {
> + sch->orb.ctrl0 |= ORB_CTRL0_MASK_PFCH;
> + warn_once_pfch(vcdev, sch, "PFCH flag forced");
> }
What happens when you run it with an old kernel? I guess the I/O is
only rejected later (after a trip into the kernel), but has that path
ever been tested?
Yes, this was tested and you are correct that the kernel will reject the
I/O unless
the corresponding patch is also applied there. I will revisit this path
while I'm
updating the logging to ensure that any potential interactions are
appropriately
considered.
>
> QEMU_BUILD_BUG_ON(sizeof(region->orb_area) != sizeof(ORB));
Oh, and do we want to deprecate the force prefetch interface in the
future? We probably need to wait a bit, until the kernel changes have
become widely available.
Yes, I think we will want to deprecate it at an appropriate time in the
future.
