Re: [Qemu-devel] [PATCH RFC v2] s390/css: handle CCW_FLAG_SKIP

[Cornelia Huck <cohuck@redhat.com>]

On Tue, 7 May 2019 11:31:16 -0400
Eric Farman <farman@linux.ibm.com> wrote:

> On 5/7/19 5:08 AM, Pierre Morel wrote:
> > On 07/05/2019 10:12, Cornelia Huck wrote:  
> >> If a ccw has CCW_FLAG_SKIP set, and the command is of type
> >> read, read backwards, or sense, no data should be written
> >> to the guest for that command.
> >>
> >> Signed-off-by: Cornelia Huck <cohuck@redhat.com>
> >> ---
> >>
> >> v1 -> v2: fixed checks for command type [Eric]
> >>
> >> Still only lightly tested (it boots); I don't think I have a tool
> >> generating channel programs with the skip flag handy.  
> 
> FWIW, my test program hits this code once (read of a single page, with 
> SLI and SKIP flags set) before getting into the passthrough codepath.

Ah, good.

> 
> >>
> >> ---
> >>   hw/s390x/css.c         | 22 ++++++++++++++++++----
> >>   include/hw/s390x/css.h |  1 +
> >>   2 files changed, 19 insertions(+), 4 deletions(-)
> >>
> >> diff --git a/hw/s390x/css.c b/hw/s390x/css.c
> >> index 8fc9e35ba5d3..080ac7e5bc0b 100644
> >> --- a/hw/s390x/css.c
> >> +++ b/hw/s390x/css.c
> >> @@ -830,8 +830,12 @@ static int ccw_dstream_rw_noflags(CcwDataStream 
> >> *cds, void *buff, int len,
> >>       if (op == CDS_OP_A) {
> >>           goto incr;
> >>       }
> >> -    ret = address_space_rw(&address_space_memory, cds->cda,
> >> -                           MEMTXATTRS_UNSPECIFIED, buff, len, op);
> >> +    if (!cds->do_skip) {
> >> +        ret = address_space_rw(&address_space_memory, cds->cda,
> >> +                               MEMTXATTRS_UNSPECIFIED, buff, len, op);
> >> +    } else {
> >> +        ret = 0;
> >> +    }
> >>       if (ret != MEMTX_OK) {
> >>           cds->flags |= CDS_F_STREAM_BROKEN;
> >>           return -EINVAL;
> >> @@ -928,8 +932,13 @@ static int ccw_dstream_rw_ida(CcwDataStream *cds, 
> >> void *buff, int len,
> >>       do {
> >>           iter_len = MIN(len, cont_left);
> >>           if (op != CDS_OP_A) {
> >> -            ret = address_space_rw(&address_space_memory, cds->cda,
> >> -                                   MEMTXATTRS_UNSPECIFIED, buff, 
> >> iter_len, op);
> >> +            if (!cds->do_skip) {
> >> +                ret = address_space_rw(&address_space_memory, cds->cda,
> >> +                                       MEMTXATTRS_UNSPECIFIED, buff, 
> >> iter_len,
> >> +                                       op);
> >> +            } else {
> >> +                ret = 0;
> >> +            }
> >>               if (ret != MEMTX_OK) {
> >>                   /* assume inaccessible address */
> >>                   ret = -EINVAL; /* channel program check */
> >> @@ -968,6 +977,11 @@ void ccw_dstream_init(CcwDataStream *cds, CCW1 
> >> const *ccw, ORB const *orb)
> >>       cds->count = ccw->count;
> >>       cds->cda_orig = ccw->cda;
> >> +    /* skip is only effective for read, read backwards, or sense 
> >> commands */
> >> +    cds->do_skip = (ccw->flags & CCW_FLAG_SKIP) &&
> >> +        ((ccw->cmd_code & CCW_CMD_BASIC_SENSE) == CCW_CMD_BASIC_SENSE ||
> >> +         (ccw->cmd_code & 0x02) == 0x02 /* read */ ||
> >> +         (ccw->cmd_code & 0x0c) == 0x0c /* read backwards */);  
> > 
> > I think you should use masks like
> > ((code & 0x3) == 2) => READ
> > ((code & 0xf) == 0xc) => READ BACKWARD
> > ((code & 0xf) == 0x4) => SENSE  
> 
> I think Pierre is right.  In the v2 code, a control CCW (like NOP) would 
> still be flagged as a READ.

Sigh. I hope third time's the charm.