On Wed, 2014-06-18 at 21:36 +1000, Alexey Kardashevskiy wrote:
> VFIO exposes BARs to user space as a byte stream so userspace can
> read it using pread()/pwrite(). Since this is a byte stream, VFIO should
> not do byte swapping and simply return values as it gets them from
> PCI device.
>
> Instead, the existing code assumes that byte stream in read/write is
> little-endian and it fixes endianness for values which it passes to
> ioreadXX/iowriteXX helpers. This works for little-endian as PCI is
> little endian and le32_to_cpu/... are stubs.
vfio read32:
val = cpu_to_le32(ioread32(io + off));
Where the typical x86 case, ioread32 is:
#define ioread32(addr) readl(addr)
and readl is:
__le32_to_cpu(__raw_readl(addr));
So we do canceling byte swaps, which are both nops on x86, and end up
returning device endian, which we assume is little endian.
vfio write32 is similar:
iowrite32(le32_to_cpu(val), io + off);
The implicit cpu_to_le32 of iowrite32() and our explicit swap cancel
out, so input data is device endian, which is assumed little.
> This also works for big endian but rather by an accident: it reads 4 bytes
> from the stream (@val is big endian), converts to CPU format (which should
> be big endian) as it was little endian (@val becomes actually little
> endian) and calls iowrite32() which does not do swapping on big endian
> system.
Really?
In arch/powerpc/kernel/iomap.c iowrite32() is just a wrapper around
writel(), which seems to use the generic implementation, which does
include a cpu_to_le32.
I also see other big endian archs like parisc doing cpu_to_le32 on
iowrite32, so I don't think this statement is true. I imagine it's
probably working for you because the swap cancel.
> This removes byte swapping and makes use ioread32be/iowrite32be
> (and 16bit versions) on big-endian systems. The "be" helpers take
> native endian values and do swapping at the moment of writing to a PCI
> register using one of "store byte-reversed" instructions.
So now you want iowrite32() on little endian and iowrite32be() on big
endian, the former does a cpu_to_le32 (which is a nop on little endian)
and the latter does a cpu_to_be32 (which is a nop on big endian)...
should we just be using __raw_writel() on both? There doesn't actually
seem to be any change in behavior here, it just eliminates back-to-back
byte swaps, which are a nop on x86, but not power, right?
> Suggested-by: Benjamin Herrenschmidt <b...@kernel.crashing.org>
> Signed-off-by: Alexey Kardashevskiy <a...@ozlabs.ru>
> ---
> drivers/vfio/pci/vfio_pci_rdwr.c | 20 ++++++++++++++++----
> 1 file changed, 16 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/vfio/pci/vfio_pci_rdwr.c
> b/drivers/vfio/pci/vfio_pci_rdwr.c
> index 210db24..f363b5a 100644
> --- a/drivers/vfio/pci/vfio_pci_rdwr.c
> +++ b/drivers/vfio/pci/vfio_pci_rdwr.c
> @@ -21,6 +21,18 @@
>
> #include "vfio_pci_private.h"
>
> +#ifdef __BIG_ENDIAN__
> +#define ioread16_native ioread16be
> +#define ioread32_native ioread32be
> +#define iowrite16_native iowrite16be
> +#define iowrite32_native iowrite32be
> +#else
> +#define ioread16_native ioread16
> +#define ioread32_native ioread32
> +#define iowrite16_native iowrite16
> +#define iowrite32_native iowrite32
> +#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
> @@ -50,9 +62,9 @@ static ssize_t do_io_rw(void __iomem *io, char __user *buf,
> if (copy_from_user(&val, buf, 4))
> return -EFAULT;
>
> - iowrite32(le32_to_cpu(val), io + off);
> + iowrite32_native(val, io + off);
> } else {
> - val = cpu_to_le32(ioread32(io + off));
> + val = ioread32_native(io + off);
>
> if (copy_to_user(buf, &val, 4))
> return -EFAULT;
> @@ -66,9 +78,9 @@ static ssize_t do_io_rw(void __iomem *io, char __user *buf,
> if (copy_from_user(&val, buf, 2))
> return -EFAULT;
>
> - iowrite16(le16_to_cpu(val), io + off);
> + iowrite16_native(val, io + off);
> } else {
> - val = cpu_to_le16(ioread16(io + off));
> + val = ioread16_native(io + off);
>
> if (copy_to_user(buf, &val, 2))
> return -EFAULT;
_______________________________________________
Linuxppc-dev mailing list
Linuxppc-dev@lists.ozlabs.org
https://lists.ozlabs.org/listinfo/linuxppc-dev
