Re: [Qemu-devel] [PATCH v2 4/4] s390x/css: support ccw IDA

[Cornelia Huck <cohuck@redhat.com>]

On Tue, 19 Sep 2017 13:50:05 +0800
Dong Jia Shi <bjsdjshi@linux.vnet.ibm.com> wrote:

> * Halil Pasic <pasic@linux.vnet.ibm.com> [2017-09-13 13:50:29 +0200]:
> 
> > Let's add indirect data addressing support for our virtual channel
> > subsystem. This implementation does no bother with any kind of
> > prefetching. We simply step trough the IDAL on demand.
> > 
> > Signed-off-by: Halil Pasic <pasic@linux.vnet.ibm.com>
> > ---
> >  hw/s390x/css.c | 109 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++-
> >  1 file changed, 108 insertions(+), 1 deletion(-)
> > 
> > diff --git a/hw/s390x/css.c b/hw/s390x/css.c
> > index 6b0cd8861b..e34b2af4eb 100644
> > --- a/hw/s390x/css.c
> > +++ b/hw/s390x/css.c
> > @@ -819,6 +819,113 @@ incr:
> >      return 0;
> >  }
> > 
> > +/* returns values between 1 and bsz, where bs is a power of 2 */
> > +static inline uint16_t ida_continuous_left(hwaddr cda, uint64_t bsz)
> > +{
> > +    return bsz - (cda & (bsz - 1));
> > +}
> > +
> > +static inline uint64_t ccw_ida_block_size(uint8_t flags)
> > +{
> > +    return 1ULL << (((flags ^ CDS_F_C64) & (CDS_F_C64 | CDS_F_I2K)) ? 11 : 12);  
> If CDS_F_C64 is set, (flags ^ CDS_F_C64) will be 0, so (1ULL << 11) will
> be the result regardless the I2K flag? The logic seems wrong.

I've stared at that condition now for a bit, but all it managed was to
get me more confused... probably just need a break.

> 
> I2K is meaningful only when C64 is 1, otherwise it is ignored. The logic
> here should be:
> if ((flags & CDS_F_C64) && !(flags & CDS_F_I2K)) {
>     return 1ULL << 12;
> }
>     return 1ULL << 11;

But I do think your version is more readable...

> 
> > +}
> > +
> > +static inline int ida_read_next_idaw(CcwDataStream *cds)
> > +{
> > +    union {uint64_t fmt2; uint32_t fmt1; } idaw;  
>                                            ^
> Nit.
> 
> > +    bool is_fmt2 = cds->flags & CDS_F_C64;
> > +    int ret;
> > +    hwaddr idaw_addr;
> > +
> > +    if (is_fmt2) {
> > +        idaw_addr = cds->cda_orig + sizeof(idaw.fmt2) * cds->at_idaw;
> > +        if (idaw_addr & 0x07) {
> > +            return -EINVAL; /* channel program check */
> > +        }
> > +        ret = address_space_rw(&address_space_memory, idaw_addr,
> > +                               MEMTXATTRS_UNSPECIFIED, (void *) &idaw.fmt2,
> > +                               sizeof(idaw.fmt2), false);
> > +        cds->cda = be64_to_cpu(idaw.fmt2);
> > +    } else {
> > +        idaw_addr = cds->cda_orig + sizeof(idaw.fmt1) * cds->at_idaw;
> > +        if (idaw_addr & 0x03) {  
> ?:
> (idaw_addr & 0x80000003)

Yes.

> 
> > +            return -EINVAL; /* channel program check */
> > +
> > +        }
> > +        ret = address_space_rw(&address_space_memory, idaw_addr,
> > +                               MEMTXATTRS_UNSPECIFIED, (void *) &idaw.fmt1,
> > +                               sizeof(idaw.fmt1), false);
> > +        cds->cda = be64_to_cpu(idaw.fmt1);
> > +    }
> > +    ++(cds->at_idaw);
> > +    if (ret != MEMTX_OK) {
> > +        /* assume inaccessible address */
> > +        return -EINVAL; /* channel program check */
> > +
> > +    }
> > +    return 0;
> > +}
> > +
> > +static int ccw_dstream_rw_ida(CcwDataStream *cds, void *buff, int len,
> > +                              CcwDataStreamOp op)
> > +{
> > +    uint64_t bsz = ccw_ida_block_size(cds->flags);
> > +    int ret = 0;
> > +    uint16_t cont_left, iter_len;
> > +
> > +    ret = cds_check_len(cds, len);
> > +    if (ret <= 0) {
> > +        return ret;
> > +    }
> > +    if (!cds->at_idaw) {
> > +        /* read first idaw */
> > +        ret = ida_read_next_idaw(cds);
> > +        if (ret) {
> > +            goto err;
> > +        }
> > +        cont_left = ida_continuous_left(cds->cda, bsz);
> > +    } else {
> > +        cont_left = ida_continuous_left(cds->cda, bsz);
> > +        if (cont_left == bsz) {
> > +            ret = ida_read_next_idaw(cds);
> > +            if (ret) {
> > +                goto err;
> > +            }
> > +            if (cds->cda & (bsz - 1)) {  
> Could move this check into ida_read_next_idaw?

I'd like to avoid further code movement...

> 
> > +                ret = -EINVAL; /* channel program check */
> > +                goto err;
> > +            }
> > +        }
> > +    }
> > +    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);  
> Ahh, now I recall that explictly defining CDS_OP_R to 0 and CDS_OP_W to
> 1 in 'struct CcwDataStreamOp' do have a meaning. Does it make sense to
> make it more obvious by adding some comment there?

Would you have a good text for that?

> 
> > +            if (ret != MEMTX_OK) {
> > +                /* assume inaccessible address */
> > +                ret = -EINVAL; /* channel program check */
> > +                goto err;
> > +            }
> > +        }
> > +        cds->at_byte += iter_len;
> > +        cds->cda += iter_len;
> > +        len -= iter_len;
> > +        if (!len) {
> > +            break;
> > +        }
> > +        ret = ida_read_next_idaw(cds);
> > +        if (ret) {
> > +            goto err;
> > +        }
> > +        cont_left = bsz;
> > +    } while (true);
> > +    return ret;
> > +err:
> > +    cds->flags |= CDS_F_STREAM_BROKEN;
> > +    return ret;
> > +}
> > +
> >  void ccw_dstream_init(CcwDataStream *cds, CCW1 const *ccw, ORB const *orb)
> >  {
> >      /*
> > @@ -835,7 +942,7 @@ void ccw_dstream_init(CcwDataStream *cds, CCW1 const *ccw, ORB const *orb)
> >      if (!(cds->flags & CDS_F_IDA)) {
> >          cds->op_handler = ccw_dstream_rw_noflags;
> >      } else {
> > -        assert(false);
> > +        cds->op_handler = ccw_dstream_rw_ida;
> >      }
> >  }
> > 
> > -- 
> > 2.13.5
> >   
> 
> Generally, the logic looks fine to me.
> 

It did pass Halil's test; but that can only test fmt-2 + 4k blocks, as
this is what the kernel infrastructure provides.

Halil, do you have some more comments?