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

[Gerd Bayer <gbayer@linux.ibm.com>]

On Mon, 2024-04-29 at 10:32 -0600, Alex Williamson wrote:
> 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) {
> 
> 

Well, overall this sounds like it creates more headaches than it tries
to solve - and that is a strong hint to not do it.

I'll drop this further refactoring in the next version.

Thanks, Gerd