Re: PCI: disable I/O or mem before probing BAR size

[Michael <macallan%netbsd.org@localhost>]

On Mon, 4 May 2020 16:09:41 -0400 (EDT)
Mouse <mouse%Rodents-Montreal.ORG@localhost> wrote:

> > while trying to boot a Xen PVH kernel as dom0, I found that Xen
> > doesn't allow changing memory-mapped PCI BARs if memory decode is
> > enabled in the command register.  
> 
> Is this permitted behaviour for a PCI device according to the PCI
> specs?  I'm just thinking that, if not, it's a bug in Xen and, while
> NetBSD might want to work around it, the code to do so should make it a
> lot clearer to code readers that it _is_ a bug workaround.  (If it _is_
> permitted, it's pretty clearly a bug in NetBSD and needs fixing, of
> course - and in that case there probably is a bunch more code that
> needs fixing too; I've seen enough things that don't mess with decoding
> enables before checking for decode width....)

I think disabling access to whatever the BAR maps while fudging with it
is a pretty good idea on its own. Who knows what else lives in that
area.

have fun
Michael