Re: [Qemu-devel] [PATCH] Teach block/vdi about "discarded" (no longer allocated) blocks

[Eric Sunshine]

On Oct 27, 2011, at 12:12 PM, Stefan Weil wrote:

> Am 27.10.2011 10:53, schrieb Kevin Wolf:
> > Am 26.10.2011 21:51, schrieb Eric Sunshine:
> > > An entry in the VDI block map will hold an offset to the actual  
> > > block if
> > > the block is allocated, or one of two specially-interpreted values  
> > > if
> > > not allocated. Using VirtualBox terminology, value  
> > > VDI_IMAGE_BLOCK_FREE
> > > (0xffffffff) represents a never-allocated block (semantically  
> > > arbitrary
> > > content). VDI_IMAGE_BLOCK_ZERO (0xfffffffe) represents a "discarded"
> > > block (semantically zero-filled). block/vdi knows only about
> > > VDI_IMAGE_BLOCK_FREE. Teach it about VDI_IMAGE_BLOCK_ZERO.
> > >
> > > Signed-off-by: Eric Sunshine <address@hidden>
> >
> > Thanks, applied to the block branch.
> >
> > Kevin
>
>
> Kevin, I don't want to block improvements. Nevertheless
> I'd like to see a small modification in this patch:
> both #defines should be implemented without a type cast.
> Please change them or wait until Eric sends an update.
>
> My favorite is this:
>
> #define VDI_UNALLOCATED UINT32_MAX
> #define VDI_DISCARD     (VDI_UNALLOCATED - 1)
>
> This would also be ok:
>
> #define VDI_UNALLOCATED 0xffffffffU
> #define VDI_DISCARD     0xfffffffeU
>
> Using the macro names and the definitions (with type cast)
> from the original VirtualBox code would also be ok.

I originally implemented the change using the macro names, comments,  
and definitions from the VirtualBox code, but found that it made the  
diff so noisy that it obscured the simpler underlying change of  
teaching block/vdi about "discarded" blocks. Sticking with the  
original VDI_UNALLOCATED macro name kept the diff noise level down.

At any rate, if Kevin can amend the commit with one of your above  
suggestions, that would be simplest. Otherwise, I can re-roll. Let me  
know which is preferred.

-- ES