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

[Thomas Huth]

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 ;-)

> >> +        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_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);

?

 Thomas