Re: [PATCH v2] s390x/css: revert SCSW ctrl/flag bits on error

[Peter Jin]

On 10/28/22 16:22, Eric Farman wrote:
> On Thu, 2022-10-27 at 23:23 +0200, Peter Jin wrote:
> > Revert the control and flag bits in the subchannel status word in
> > case
> > the SSCH operation fails with non-zero CC (ditto for CSCH and HSCH).
> > According to POPS, the control and flag bits are only changed if
> > SSCH,
> > CSCH, and HSCH return CC 0, and no other action should be taken
> > otherwise.
> > In order to simulate that after the fact, the bits need to be
> > reverted on
> > non-zero CC.
> I'm okay to this point...
>
> > This change is necessary due to the fact that the pwrite() in vfio-
> > ccw
> > which triggers the SSCH can fail at any time. Previously, there was
> > only virtio-ccw, whose do_subchannel_work function was only able to
> > return CC0. However, once vfio-ccw went into the mix, it has become
> > necessary to handle errors in code paths that were previously assumed
> > to always return success.
> >
> > In our case, we found that in case of pwrite() failure (which was
> > discovered by strace injection), the subchannel could be stuck in
> > start
> > pending state, which could be problematic if the pwrite() call
> > returns
> > CC2. Experimentation shows that the guest tries to retry the SSCH
> > call as
> > normal for CC2, but it actually continously fails due to the fact
> > that
> > the subchannel is stuck in start pending state even though no start
> > function is actually taking place.
> ...but the two paragraphs above are a bit cumbersome to digest. Maybe
> it's just too late in the week for me. What about something like this?
>
> """
> While the do_subchannel_work logic for virtual (virtio) devices will
> return condition code 0, passthrough (vfio) devices may encounter
> errors from either the host kernel or real hardware that need to be
> accounted for after this point. This includes restoring the state of
> the Subchannel Status Word to reflect the subchannel, as these bits
> would not be set in the event of a non-zero condition code from the
> affected instructions.
>
> Experimentation has shown that a failure on a START SUBCHANNEL (SSCH)
> to a passthrough device would leave the subchannel with the START
> PENDING activity control bit set, thus blocking subsequent SSCH
> operations in css_do_ssch() until some form of error recovery was
> undertaken since no interrupt would be expected.
> """
>
> > Signed-off-by: Peter Jin <pjin@linux.ibm.com>
> We've talked previously about clearing this within the
> do_subchannel_work_passthrough routine in order to keep the _virtual
> paths untouched, but this seems like a reasonable approach to me.
>
> The commit message is probably fine either way, but as far as the code
> goes:
>
> Reviewed-by: Eric Farman <farman@linux.ibm.com>
Okay, that sounds much better :)
> > ---
> >   hw/s390x/css.c | 51 +++++++++++++++++++++++++++++++++++++++++++++++-
> > --
> >   1 file changed, 48 insertions(+), 3 deletions(-)
> >
> > diff --git a/hw/s390x/css.c b/hw/s390x/css.c
> > index 7d9523f811..95d1b3a3ce 100644
> > --- a/hw/s390x/css.c
> > +++ b/hw/s390x/css.c
> > @@ -1522,21 +1522,37 @@ IOInstEnding css_do_xsch(SubchDev *sch)
> >   IOInstEnding css_do_csch(SubchDev *sch)
> >   {
> >       SCHIB *schib = &sch->curr_status;
> > +    uint16_t old_scsw_ctrl;
> > +    IOInstEnding ccode;
> >   
> >       if (~(schib->pmcw.flags) & (PMCW_FLAGS_MASK_DNV |
> > PMCW_FLAGS_MASK_ENA)) {
> >           return IOINST_CC_NOT_OPERATIONAL;
> >       }
> >   
> > +    /*
> > +     * Save the current scsw.ctrl in case CSCH fails and we need
> > +     * to revert the scsw to the status quo ante.
> > +     */
> > +    old_scsw_ctrl = schib->scsw.ctrl;
> > +
> >       /* Trigger the clear function. */
> >       schib->scsw.ctrl &= ~(SCSW_CTRL_MASK_FCTL |
> > SCSW_CTRL_MASK_ACTL);
> >       schib->scsw.ctrl |= SCSW_FCTL_CLEAR_FUNC | SCSW_ACTL_CLEAR_PEND;
> >   
> > -    return do_subchannel_work(sch);
> > +    ccode = do_subchannel_work(sch);
> > +
> > +    if (ccode != IOINST_CC_EXPECTED) {
> > +        schib->scsw.ctrl = old_scsw_ctrl;
> > +    }
> > +
> > +    return ccode;
> >   }
> >   
> >   IOInstEnding css_do_hsch(SubchDev *sch)
> >   {
> >       SCHIB *schib = &sch->curr_status;
> > +    uint16_t old_scsw_ctrl;
> > +    IOInstEnding ccode;
> >   
> >       if (~(schib->pmcw.flags) & (PMCW_FLAGS_MASK_DNV |
> > PMCW_FLAGS_MASK_ENA)) {
> >           return IOINST_CC_NOT_OPERATIONAL;
> > @@ -1553,6 +1569,12 @@ IOInstEnding css_do_hsch(SubchDev *sch)
> >           return IOINST_CC_BUSY;
> >       }
> >   
> > +    /*
> > +     * Save the current scsw.ctrl in case HSCH fails and we need
> > +     * to revert the scsw to the status quo ante.
> > +     */
> > +    old_scsw_ctrl = schib->scsw.ctrl;
> > +
> >       /* Trigger the halt function. */
> >       schib->scsw.ctrl |= SCSW_FCTL_HALT_FUNC;
> >       schib->scsw.ctrl &= ~SCSW_FCTL_START_FUNC;
> > @@ -1564,7 +1586,13 @@ IOInstEnding css_do_hsch(SubchDev *sch)
> >       }
> >       schib->scsw.ctrl |= SCSW_ACTL_HALT_PEND;
> >   
> > -    return do_subchannel_work(sch);
> > +    ccode = do_subchannel_work(sch);
> > +
> > +    if (ccode != IOINST_CC_EXPECTED) {
> > +        schib->scsw.ctrl = old_scsw_ctrl;
> > +    }
> > +
> > +    return ccode;
> >   }
> >   
> >   static void css_update_chnmon(SubchDev *sch)
> > @@ -1605,6 +1633,8 @@ static void css_update_chnmon(SubchDev *sch)
> >   IOInstEnding css_do_ssch(SubchDev *sch, ORB *orb)
> >   {
> >       SCHIB *schib = &sch->curr_status;
> > +    uint16_t old_scsw_ctrl, old_scsw_flags;
> > +    IOInstEnding ccode;
> >   
> >       if (~(schib->pmcw.flags) & (PMCW_FLAGS_MASK_DNV |
> > PMCW_FLAGS_MASK_ENA)) {
> >           return IOINST_CC_NOT_OPERATIONAL;
> > @@ -1626,11 +1656,26 @@ IOInstEnding css_do_ssch(SubchDev *sch, ORB
> > *orb)
> >       }
> >       sch->orb = *orb;
> >       sch->channel_prog = orb->cpa;
> > +
> > +    /*
> > +     * Save the current scsw.ctrl and scsw.flags in case SSCH fails
> > and we need
> > +     * to revert the scsw to the status quo ante.
> > +     */
> > +    old_scsw_ctrl = schib->scsw.ctrl;
> > +    old_scsw_flags = schib->scsw.flags;
> > +
> >       /* Trigger the start function. */
> >       schib->scsw.ctrl |= (SCSW_FCTL_START_FUNC |
> > SCSW_ACTL_START_PEND);
> >       schib->scsw.flags &= ~SCSW_FLAGS_MASK_PNO;
> >   
> > -    return do_subchannel_work(sch);
> > +    ccode = do_subchannel_work(sch);
> > +
> > +    if (ccode != IOINST_CC_EXPECTED) {
> > +        schib->scsw.ctrl = old_scsw_ctrl;
> > +        schib->scsw.flags = old_scsw_flags;
> > +    }
> > +
> > +    return ccode;
> >   }
> >   
> >   static void copy_irb_to_guest(IRB *dest, const IRB *src, const PMCW
> > *pmcw,