Re: [PATCH] arm64: Enable PCI write-combine resources under sysfs

Jason Gunthorpe
2020-09-14T22:57:53Z

On Tue, Sep 15, 2020 at 08:00:27AM +1000, Benjamin Herrenschmidt wrote:
> On Tue, 2020-09-15 at 07:42 +1000, Benjamin Herrenschmidt wrote:
> >
> > > which is back to my original question, how do you do DMA using
> > > /sys/xx/resources? Why not use VFIO like everything else?
> >
> > Note: All this doesnt' change the fact that sys/xx/resources_wc
> > exists
> > for other archs and I see no reasons so far not to have it on ARM...
>
> Also... it looks like VFIO also doesn't provide a way to do WC yet
> unfortunately :-(

Yes, but if the driving reason for this patch is because a VFIO user
like EFA DPDK is trying to work around VFIO limitations, then I'd say
the VFIO mmap should be amended, and not so much worring about sysfs.

While there is no reason for ARM to not show the sysfs, it really
should never be used. Modern kernels in secure boot don't even show
it, for instance.

Jason