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

[Stefan Weil]

Am 28.10.2011 11:22, schrieb Kevin Wolf:
> Am 28.10.2011 10:15, schrieb Eric Sunshine:
> > On Oct 28, 2011, at 4:00 AM, Kevin Wolf wrote:
> >
> > > Am 27.10.2011 18:12, schrieb Stefan Weil:
> > > > 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 did see your comments, and I waited for the endianness thing to be
> > > answered. However, how the definition of these constants is written is
> > > really not a functional defect, but simply a matter of taste. It's an
> > > old rule that whoever does the work also decides on the details.
> > >
> > > I really think it's wasting our time if we need to discuss if a type
> > > cast in the constant definition is only allowed after typedefing
> > > uint32_t to something else like in VBox.
> > >
> > > So my preferred way is to leave the patch as it is. The code is simple
> > > and clear and objectively seen it won't get any better with your taste
> > > applied. If Eric prefers, I can update it to use 0xffffffffU, though.
> > The 0xffffffffU notation has the benefit of being explicit, whereas
> > the ((uint32_t)~0) notation, taken from the VirtualBox source, is
> > somewhat magical for a reader who does not perform an automatic
> > ((uint32_t)~0) == 0xffffffffU conversion in his head. Consequently,
> > the 0xffffffffU notation might a better choice, if it's not too much
> > bother for you to amend the patch.
> I'll amend it with this change:
>
> diff --git a/block/vdi.c b/block/vdi.c
> index 25790c4..523a640 100644
> --- a/block/vdi.c
> +++ b/block/vdi.c
> @@ -115,10 +115,10 @@ void uuid_unparse(const uuid_t uu, char *out);
>   #define VDI_TEXT "<<<  QEMU VM Virtual Disk Image>>>\n"
>
>   /* A never-allocated block; semantically arbitrary content. */
> -#define VDI_UNALLOCATED ((uint32_t)~0)
> +#define VDI_UNALLOCATED 0xffffffffU
>
>   /* A discarded (no longer allocated) block; semantically zero-filled. */
> -#define VDI_DISCARDED ((uint32_t)~1)
> +#define VDI_DISCARDED   0xfffffffeU
>
>   #define VDI_IS_ALLOCATED(X) ((X)<  VDI_DISCARDED)

Thanks for this update. With your patch added to Eric's, I can add

Acked-by: Stefan Weil <address@hidden>