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

Gerd Bayer
2024-05-21T16:40:24Z

On Fri, 2024-05-17 at 03:29 -0700, Ramesh Thomas wrote:
> On 4/25/2024 9:56 AM, 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.
>
> This is indeed necessary as back to back 32 bit may not be optimal in
> some devices.
>
> >
> > 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>
> > ---
> >   drivers/vfio/pci/vfio_pci_rdwr.c | 16 ++++++++++++++++
> >   include/linux/vfio_pci_core.h    |  3 +++
> >   2 files changed, 19 insertions(+)
> >
> > diff --git a/drivers/vfio/pci/vfio_pci_rdwr.c
> > b/drivers/vfio/pci/vfio_pci_rdwr.c
> > index 3335f1b868b1..8ed06edaee23 100644
> > --- a/drivers/vfio/pci/vfio_pci_rdwr.c
> > +++ b/drivers/vfio/pci/vfio_pci_rdwr.c
> > @@ -89,6 +89,9 @@ EXPORT_SYMBOL_GPL(vfio_pci_core_ioread##size);
> >   VFIO_IOREAD(8)
> >   VFIO_IOREAD(16)
> >   VFIO_IOREAD(32)
> > +#ifdef ioread64
> > +VFIO_IOREAD(64)
> > +#endif
> >
> >   #define
> > VFIO_IORDWR(size) \
> >   static int vfio_pci_core_iordwr##size(struct vfio_pci_core_device
> > *vdev,\
> > @@ -124,6 +127,10 @@ static int vfio_pci_core_iordwr##size(struct
> > vfio_pci_core_device *vdev,\
> >   VFIO_IORDWR(8)
> >   VFIO_IORDWR(16)
> >   VFIO_IORDWR(32)
> > +#if defined(ioread64) && defined(iowrite64)
> > +VFIO_IORDWR(64)
> > +#endif
> > +
> >   /*
> >    * Read or write from an __iomem region (MMIO or I/O port) with
> > an excluded
> >    * range which is inaccessible.  The excluded range drops writes
> > and fills
> > @@ -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.

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.

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.

> > + if (fillable >= 8 && !(off % 8)) {
> > + ret = vfio_pci_core_iordwr64(vdev,
> > iswrite, test_mem,
> > +      io, buf, off,
> > &filled);
> > + if (ret)
> > + return ret;
> > +
> > + } else
> > +#endif /* defined(ioread64) && defined(iowrite64) */
> >    if (fillable >= 4 && !(off % 4)) {
> >    ret = vfio_pci_core_iordwr32(vdev,
> > iswrite, test_mem,
> >         io, buf, off,
> > &filled);
> > diff --git a/include/linux/vfio_pci_core.h
> > b/include/linux/vfio_pci_core.h
> > index a2c8b8bba711..f4cf5fd2350c 100644
> > --- a/include/linux/vfio_pci_core.h
> > +++ b/include/linux/vfio_pci_core.h
> > @@ -157,5 +157,8 @@ int vfio_pci_core_ioread##size(struct
> > vfio_pci_core_device *vdev, \
> >   VFIO_IOREAD_DECLATION(8)
> >   VFIO_IOREAD_DECLATION(16)
> >   VFIO_IOREAD_DECLATION(32)
> > +#ifdef ioread64
> > +VFIO_IOREAD_DECLATION(64)
> nit: This macro is referenced only in this file. Can the typo be
> corrected (_DECLARATION)?

Sure thanks for pointing this out!
I'll single this editorial change out into a separate patch of the
series, though.

>
> > +#endif
> >
> >   #endif /* VFIO_PCI_CORE_H */
>
>

Thanks, Gerd