Re: [Qemu-ppc] [Qemu-devel] [PATCH qemu v10 14/14] spapr_pci/spapr_pci_vfio: Support Dynamic DMA Windows (DDW)

[Alexey Kardashevskiy]

On 07/07/2015 09:35 PM, Thomas Huth wrote:
> On Tue, 7 Jul 2015 20:43:44 +1000
> Alexey Kardashevskiy <address@hidden> wrote:
>
> > On 07/07/2015 07:33 PM, Thomas Huth wrote:
> > > On Mon,  6 Jul 2015 12:11:10 +1000
> > > Alexey Kardashevskiy <address@hidden> wrote:
> ...
> > > > +static void rtas_ibm_create_pe_dma_window(PowerPCCPU *cpu,
> > > > +                                          sPAPRMachineState *spapr,
> > > > +                                          uint32_t token, uint32_t nargs,
> > > > +                                          target_ulong args,
> > > > +                                          uint32_t nret, target_ulong rets)
> > > > +{
> > > > +    sPAPRPHBState *sphb;
> > > > +    sPAPRTCETable *tcet = NULL;
> > > > +    uint32_t addr, page_shift, window_shift, liobn;
> > > > +    uint64_t buid;
> > > > +    long ret;
> > > > +
> > > > +    if ((nargs != 5) || (nret != 4)) {
> > >
> > > Pascal bracket style again :-(
> >
> >
> > Am I breaking any code design guideline here?
>
> No, but my Pascal allergy causes me to sneeze here ;-)

I feel cold when I do not see braces in cases like this ;)


> > > > +        goto param_error_exit;
> > > > +    }
> > > > +
> > > > +    buid = ((uint64_t)rtas_ld(args, 1) << 32) | rtas_ld(args, 2);
> >
> > But here braces are ok? :-/
>
> You could remove them, too. But I did not need to sneeze here.

:)


> > > > +    addr = rtas_ld(args, 0);
> > > > +    sphb = spapr_pci_find_phb(spapr, buid);
> > > > +    if (!sphb || !sphb->ddw_enabled) {
> > > > +        goto param_error_exit;
> > > > +    }
> > > > +
> > > > +    page_shift = rtas_ld(args, 3);
> > > > +    window_shift = rtas_ld(args, 4);
> > > > +    liobn = spapr_phb_get_free_liobn(sphb);
> > > > +
> > > > +    if (!liobn || !(sphb->page_size_mask & (1ULL << page_shift))) {
> > > > +        goto hw_error_exit;
> > > > +    }
> > > > +
> > > > +    ret = spapr_phb_dma_init_window(sphb, liobn, page_shift,
> > > > +                                    1ULL << window_shift);
> > >
> > > As already mentioned in a comment to another patch in this series, I
> > > think it maybe might be better to do some sanity checks on the
> > > window_shift value, too?
> >
> >
> > Well, as you suggested, I added a check to spapr_phb_dma_init_window()
> > which makes this code return RTAS_OUT_HW_ERROR. Or I can add this here:
> >
> > if (window_shift < page_shift) {
> >       goto param_error_exit;
> > }
> >
> > and RTAS handler will return RTAS_OUT_PARAM_ERROR.
> > SPAPR does not say what is the correct reponse in this case...
>
> Both error codes sound ok for me here, so do whatever you think is best.


RTAS_OUT_PARAM_ERROR it is then.


> > > > +
> > > > +    rtas_st(rets, 0, RTAS_OUT_SUCCESS);
> > > > +    rtas_st(rets, 1, liobn);
> > > > +    rtas_st(rets, 2, tcet->bus_offset >> 32);
> > > > +    rtas_st(rets, 3, tcet->bus_offset & ((uint32_t) -1));
> > >
> > > Why don't you simply use 0xffffffff instead of ((uint32_t) -1) ?
> > > That's shorter and much easier to understand at a first glance than
> > > calulating the type-cast in your brain ;-)
> >
> >
> > At a first glance I cannot tell if there are 7 or 8 or 9 "f"s in
> > 0xffffffff. I may accidentally add/remove one "f" and nobody will notice.
> > Such typecast of (-1) is quite typical.
>
> But IMHO it's ugly to use it to mask a value to the lower 32 bits this
> way. At least I had to read this twice to understand what you're
> trying to achieve here. So if you don't like the 0xffffffff, what about
> simply using:
>
>      rtas_st(rets, 3, (uint32_t)tcet->bus_offset);
>
> ?

I believe there are compilers which will warn me than I am loosing upper 
32bits.



--
Alexey