Re: [PATCH v3 2/3] vfio/pci: Support 8-byte PCI loads and stores

Jason Gunthorpe
2024-05-22T13:48:16Z

On Tue, May 21, 2024 at 06:40:13PM +0200, Gerd Bayer wrote:
> > > @@ -148,6 +155,15 @@ ssize_t vfio_pci_core_do_io_rw(struct
> > > vfio_pci_core_device *vdev, bool test_mem,
> > >    else
> > >    fillable = 0;
> > >
> > > +#if defined(ioread64) && defined(iowrite64)
> >
> > Can we check for #ifdef CONFIG_64BIT instead? In x86, ioread64 and
> > iowrite64 get declared as extern functions if CONFIG_GENERIC_IOMAP is
> > defined and this check always fails. In include/asm-generic/io.h,
> > asm-generic/iomap.h gets included which declares them as extern
> > functions.
>
> I thinks that should be possible - since ioread64/iowrite64 depend on
> CONFIG_64BIT themselves.
>
> > One more thing to consider io-64-nonatomic-hi-lo.h and
> > io-64-nonatomic-lo-hi.h, if included would define it as a macro that
> > calls a function that rw 32 bits back to back.

This might be a better way to go than trying to have vfio provide its
own emulation.

> Even today, vfio_pci_core_do_io_rw() makes no guarantees to its users
> that register accesses will be done in the granularity they've thought
> to use. The vfs layer may coalesce the accesses and this code will then
> read/write the largest naturally aligned chunks. I've witnessed in my
> testing that one device driver was doing 8-byte writes through the 8-
> byte capable vfio layer all of a sudden when run in a KVM guest.

Sure, KVM has emulation for various byte sizes, and does invoke vfio
with the raw size it got from guest, including larger than 8 sizes
from things like SSE instructions. This has nothing to do with the VFS
layer.

> So higher-level code needs to consider how to split register accesses
> appropriately to get the intended side-effects. Thus, I'd rather stay
> away from splitting 64bit accesses into two 32bit accesses - and decide
> if high or low order values should be written first.

The VFIO API is a byte for byte memcpy. VFIO should try to do the
largest single instruction accesses it knows how to do because some HW
is sensitive to that. Otherwise it does a memcpy loop.

Jason