Re: [PATCH kvm-unit-tests v5 13/14] pci: add msi support for 32/64bit address

[Peter Xu <peterx@redhat.com>]

On Tue, Nov 22, 2016 at 01:30:41PM +0100, Andrew Jones wrote:
> On Tue, Nov 22, 2016 at 03:21:37PM +0800, Peter Xu wrote:
> > On Tue, Nov 22, 2016 at 08:03:06AM +0100, Alexander Gordeev wrote:
> > > On Tue, Nov 22, 2016 at 12:59:20PM +0800, Peter Xu wrote:
> > > > On Mon, Nov 21, 2016 at 08:27:52PM +0100, Alexander Gordeev wrote:
> > > > 
> > > > [...]
> > > > 
> > > > > > +void pci_cap_walk(struct pci_dev *dev)
> > > > > > +{
> > > > > > +	uint8_t cap_offset;
> > > > > > +	uint8_t cap_id;
> > > > > > +
> > > > > > +	cap_offset = pci_config_readb(dev->bdf, PCI_CAPABILITY_LIST);
> > > > > > +	while (cap_offset) {
> > > > > > +		cap_id = pci_config_readb(dev->bdf, cap_offset);
> > > > > > +		printf("PCI detected cap 0x%x\n", cap_id);
> > > > > > +		if (cap_handlers[cap_id])
> > > > > > +			cap_handlers[cap_id](dev, cap_offset);
> > > > > > +		cap_offset = pci_config_readb(dev->bdf, cap_offset + 1);
> > > > > > +	}
> > > > > > +}
> > > > > 
> > > > > Are you sure the function above is safe without range (sanity) checks?
> > > > 
> > > > No. :) But if something goes wrong, I guess that's possibly a QEMU PCI
> > > > bug. I can add some check if you think is necessary, like, make sure
> > > > the loop goes no more than a specific value?
> > > 
> > > Yes - I suppose PCI cap. list has a limit.
> > 
> > Will do. 0xff should suffice. Thanks,
> 
> Well, cap_offset has to be 0xff or smaller, since it comes from readb,
> which returns uint8_t. So there's no need to check cap_offset. As for
> dereferencing cap_handlers with cap_id, which is what I think Alex
> was asking about, then after its read we should simply add
> 
>  assert(cap_id <= PCI_CAP_ID_MAX);
> 
> Thanks,
> drew

IIUC here we need to count the loop in case we have a loop in the cap
list (which is possibly QEMU bug, but hardly happen, like cap A points
to cap B, while cap B points back to cap A). In that case, I'll do
this in the loop:

  assert(++count <= 255);

To make sure the loop ends somewhere.

IMHO we don't need to check cap_id since cap_id is only used in:

  if (cap_handlers[cap_id])
    cap_handlers[cap_id](dev, cap_offset);

As long as we provide correct handlers, we'll naturally skip all
unknown cap_ids.

Thanks,

-- peterx