Re: [PATCH v3 3/3] vfio/pci: Continue to refactor vfio_pci_core_do_io_rw

Alex Williamson
2024-04-29T16:32:06Z

On Thu, 25 Apr 2024 18:56:04 +0200
Gerd Bayer <gbayer@linux.ibm.com> wrote:

> Convert if-elseif-chain into switch-case.
> Separate out and generalize the code from the if-clauses so the result
> can be used in the switch statement.
>
> Signed-off-by: Gerd Bayer <gbayer@linux.ibm.com>
> ---
> drivers/vfio/pci/vfio_pci_rdwr.c | 30 ++++++++++++++++++++++++------
> 1 file changed, 24 insertions(+), 6 deletions(-)
>
> diff --git a/drivers/vfio/pci/vfio_pci_rdwr.c b/drivers/vfio/pci/vfio_pci_rdwr.c
> index 8ed06edaee23..634c00b03c71 100644
> --- a/drivers/vfio/pci/vfio_pci_rdwr.c
> +++ b/drivers/vfio/pci/vfio_pci_rdwr.c
> @@ -131,6 +131,20 @@ VFIO_IORDWR(32)
> VFIO_IORDWR(64)

#define MAX_FILL_SIZE 8
#else
#define MAX_FILL_SIZE 4

> #endif
>
> +static int fill_size(size_t fillable, loff_t off)
> +{
> + unsigned int fill_size;

unsigned int fill_size = MAX_FILL_SIZE;

> +#if defined(ioread64) && defined(iowrite64)
> + for (fill_size = 8; fill_size >= 0; fill_size /= 2) {
> +#else
> + for (fill_size = 4; fill_size >= 0; fill_size /= 2) {
> +#endif /* defined(ioread64) && defined(iowrite64) */
> + if (fillable >= fill_size && !(off % fill_size))
> + return fill_size;
> + }
> + return -1;
> +}
> +
> /*
> * 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
> @@ -155,34 +169,38 @@ ssize_t vfio_pci_core_do_io_rw(struct vfio_pci_core_device *vdev, bool test_mem,
> else
> fillable = 0;
>
> + switch (fill_size(fillable, off)) {
> #if defined(ioread64) && defined(iowrite64)
> - if (fillable >= 8 && !(off % 8)) {
> + case 8:
> ret = vfio_pci_core_iordwr64(vdev, iswrite, test_mem,
> io, buf, off, &filled);
> if (ret)
> return ret;
> + break;
>
> - } else

AFAICT, avoiding this dangling else within the #ifdef is really the
only tangible advantage of conversion to a switch statement. Getting
rid of that alone while keeping, and actually increasing, the inline
ifdefs in the code doesn't seem worthwhile to me. I'd probably only go
this route if we could make vfio_pci_iordwr64() stubbed as a BUG_ON
when we don't have the ioread64 and iowrite64 accessors, in which case
the switch helper would never return 8 and the function would be
unreachable.

> #endif /* defined(ioread64) && defined(iowrite64) */
> - if (fillable >= 4 && !(off % 4)) {
> + case 4:
> ret = vfio_pci_core_iordwr32(vdev, iswrite, test_mem,
> io, buf, off, &filled);
> if (ret)
> return ret;
> + break;
>
> - } else if (fillable >= 2 && !(off % 2)) {
> + case 2:
> ret = vfio_pci_core_iordwr16(vdev, iswrite, test_mem,
> io, buf, off, &filled);
> if (ret)
> return ret;
> + break;
>
> - } else if (fillable) {
> + case 1:
> ret = vfio_pci_core_iordwr8(vdev, iswrite, test_mem,
> io, buf, off, &filled);
> if (ret)
> return ret;
> + break;
>
> - } else {
> + default:

This condition also seems a little more obfuscated without being
preceded by the 'if (fillable)' test, which might warrant handling
separate from the switch if we continue to pursue the switch
conversion. Thanks,

Alex

> /* Fill reads with -1, drop writes */
> filled = min(count, (size_t)(x_end - off));
> if (!iswrite) {