Re: [kvm-unit-tests PATCH v4 04/12] pci: Rework pci_bar_addr()

[Andrew Jones <drjones@redhat.com>]

On Tue, Jun 07, 2016 at 08:38:55AM +0200, Alexander Gordeev wrote:
> On Mon, Jun 06, 2016 at 04:12:03PM +0200, Andrew Jones wrote:
> > > +phys_addr_t pci_bar_size(pcidevaddr_t dev, int bar_num)
> > >  {
> > >  	uint32_t bar = pci_config_readl(dev, PCI_BASE_ADDRESS_0 + bar_num * 4);
> > > +	phys_addr_t size = (int32_t)pci_bar_size32(dev, bar_num);
> > > +	phys_addr_t mask = (int32_t)pci_bar_mask(bar);
> > >  
> > > -	if (bar & PCI_BASE_ADDRESS_SPACE_IO)
> > > -		return bar & PCI_BASE_ADDRESS_IO_MASK;
> > > -	else
> > > -		return bar & PCI_BASE_ADDRESS_MEM_MASK;
> > > +	if (pci_bar_is64(dev, bar_num)) {
> > > +		uint32_t size_high = pci_bar_size32(dev, bar_num + 1);
> > > +		size = ((phys_addr_t)size_high << 32) | (uint32_t)size;
> > > +	}
> > > +
> > > +	return (~(size & mask)) + 1;
> > 
> > All this casting of size and mask is pointless. Please rework it
> > similar to what I did above.
> 
> This is the most compact and straight variant I was able to come up with:
> 
> 	uint32_t bar = pci_bar_get(dev, bar_num);
> 	phys_addr_t size = (int32_t)pci_bar_size32(dev, bar_num);

But this is wrong. If your 32-bit size was 0x80000000, then you now
say it's 0xffffffff80000000.

> 	phys_addr_t mask = (int32_t)pci_bar_mask(bar);

It might be OK to do that here, on a mask, but even if it is, then I
don't like it, because it's too subtle (like I said for the casting
in pci_bar_addr in my last reply). I don't like that it requires us to
know that masking bit 31 in a 32-bit mask means we also want to mask
63..32. That should at least be in a comment somewhere.

> 
> 	if (pci_bar_is64(dev, bar_num))
> 		size |= (phys_addr_t)pci_bar_size32(dev, bar_num + 1) << 32;
> 
> 	return (~(size & mask)) + 1;
> 
> The casting is needed to avoid putting explicitly all 1s into higher
> bits of size and/or mask. Otherwise (~(size & mask)) + 1 expression would
> not bring correct results. I really struggle to make something better
> readable.

It's not just about readability, it's about correctness. You shouldn't
use 32-bit sizes/masks with 64-bit data this way. Like I said in the
last reply, you should rework it like I showed you, operate on the
32-bit data with the 32-bit mask/size, and then eventually construct
64-bit data.

Thanks,
drew