On Fri, 2024-04-19 at 10:47 -0600, Alex Williamson wrote:
> On Fri, 19 Apr 2024 13:11:35 -0300
> Jason Gunthorpe <jgg@xxxxxxxx> wrote:
>
> > 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@xxxxxxxxxx>
> > > > >
> > > > > 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@xxxxxxxxxx>
> > > > > Co-developed-by: Gerd Bayer <gbayer@xxxxxxxxxxxxx>
> > > > > Signed-off-by: Gerd Bayer <gbayer@xxxxxxxxxxxxx>
> > > > > ---
> > > > >
> > > > > 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
>
> Yes, I think so too, falling back to 4-byte accesses of course if
> 8-byte is not available. Thanks,
So I'll rework this to simply fall back to 32-bit if 64-bit is not
available in a v2. And we'll investigate the VFIO_PCI_MMAP case
separately.
Thank you,
Gerd
