Re: [PATCH] vfio/pci: Support 8-byte PCI loads and stores

[Jason Gunthorpe <jgg@ziepe.ca>]

On Fri, Apr 19, 2024 at 05:57:52PM +0200, Niklas Schnelle wrote:
> On Fri, 2024-04-19 at 10:58 -0300, Jason Gunthorpe wrote:
> > On Fri, Apr 19, 2024 at 03:53:23PM +0200, Gerd Bayer wrote:
> > > From: Ben Segal <bpsegal@us.ibm.com>
> > > 
> > > Many PCI adapters can benefit or even require full 64bit read
> > > and write access to their registers. In order to enable work on
> > > user-space drivers for these devices add two new variations
> > > vfio_pci_core_io{read|write}64 of the existing access methods
> > > when the architecture supports 64-bit ioreads and iowrites.
> > > 
> > > Signed-off-by: Ben Segal <bpsegal@us.ibm.com>
> > > Co-developed-by: Gerd Bayer <gbayer@linux.ibm.com>
> > > Signed-off-by: Gerd Bayer <gbayer@linux.ibm.com>
> > > ---
> > > 
> > > Hi all,
> > > 
> > > we've successfully used this patch with a user-mode driver for a PCI
> > > device that requires 64bit register read/writes on s390.
> > 
> > But why? S390 already has a system call for userspace to do the 64 bit
> > write, and newer S390 has a userspace instruction to do it.
> > 
> > Why would you want to use a VFIO system call on the mmio emulation
> > path?
> > 
> > mmap the registers and access them normally?
> 
> It's a very good point and digging into why this wasn't used by
> Benjamin. It turns out VFIO_PCI_MMAP is disabled for S390 which it
> really shouldn't be especially now that we have the user-space
> instructions. Before that though Benjamin turned to this interface
> which then lead him to this limitation. So yeah we'll definitely verify
> that it also works via VFIO_PCI_MMAP and send a patch to enable that.

Make sense to me!

> That said I still think it's odd not to have the 8 byte case working
> here even if it isn't the right approach. Could still be useful for
> debug/testing without having to add the MIO instructions or the our
> special syscall.

Yes, this also makes sense, but this patch needs some adjusting

Jason