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

[Niklas Schnelle <schnelle@linux.ibm.com>]

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.

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.

Thanks,
Niklas