Re: [Xen-devel] [PATCH] amd-iommu: get rid of pointless IOMMU_PAGING_MODE_LEVEL_X definitions

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----Original Message-----
> From: Woods, Brian [mailto:Brian.Woods@xxxxxxx]
> Sent: 12 October 2018 18:14
> To: Paul Durrant <Paul.Durrant@xxxxxxxxxx>
> Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx; Suthikulpanit, Suravee
> <Suravee.Suthikulpanit@xxxxxxx>; Woods, Brian <Brian.Woods@xxxxxxx>
> Subject: Re: [PATCH] amd-iommu: get rid of pointless
> IOMMU_PAGING_MODE_LEVEL_X definitions
> 
> On Thu, Sep 27, 2018 at 01:46:01PM +0100, Paul Durrant wrote:
> > The levels are absolute numbers such that IOMMU_PAGING_MODE_LEVEL_X
> > evaluates to X (for the valid range of 0 - 7) so simply use numbers in
> > the code.
> >
> > No functional change.
> >
> > NOTE: This patch also adds emacs boilerplate to amd-iommu-defs.h
> >
> > Signed-off-by: Paul Durrant <paul.durrant@xxxxxxxxxx>
> 
> Is there a strong reason to get rid of these?  Some of examples below
> create seemingly magic numbers in the code.  While if you're familiar
> with the functions this isn't a big deal, otherwise you have to dig
> further to tell.
> 

The numbers aren't magic though. The spec refers to levels by number rather 
than any sort of name. If the levels were named then it would be absolutely 
right to #define <level name> <level number>, but that is not the case. Thus 
IMO getting rid of the definitions actually makes the code clearer for those 
(like myself) reading the spec.

> > +    pte = table + pfn_to_pde_idx(gfn, 1);
> 
> > +    need_flush = set_iommu_pde_present(pde, next_mfn, 0, iw, ir);
> 
> If there's a general consensus that getting rid of these is better, I
> don't mind and will agree to it.
> 

Anyone else care to comment?

  Paul

> --
> Brian Woods

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