On Tue, 12 May 2020 14:15:35 -0400
Jared Rossi <jro...@linux.ibm.com> wrote:
> Remove the explicit prefetch check when using vfio-ccw devices.
> This check does not trigger in practice as all Linux channel programs
> are intended to use prefetch.
>
> It is no longer required to force the PFCH flag when using vfio-ccw
> devices.
That's not quite true: Only kernels that include the currently-queued
patch do not require it. Maybe
"Newer Linux kernel versions do not require to force the PFCH flag with
vfio-ccw devices anymore."
?
>
> 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");
> }
>
> QEMU_BUILD_BUG_ON(sizeof(region->orb_area) != sizeof(ORB));
Let me spell out what happens:
- PFCH bit set -> no change
- PFCH bit not set, but force_orb_pfch set -> no change
- neither PFCH bit nor force_orb_pfch set:
- older kernels: QEMU makes the request, the kernel rejects it, guest
gets a unit exception (same result for the guest as before, only a
different code flow)
- newer kernels: QEMU makes the request, the kernel forwards the
request (logging a rate-limited warning); the result depends on
whether the guest actually tries to rewrite the channel program or
not
I think that is what we want, and I think I'll queue this patch with
the tweaked commit message, but I'd like a second opinion.
(We should also deprecate force_orb_pfch in the future.)
