On Tue, Feb 19, 2013 at 10:02:51AM +0100, Stefan Hajnoczi wrote:
> On Mon, Feb 18, 2013 at 06:03:30PM -0500, Jeff Cody wrote:
> > +/* the guid is a 16 byte unique ID - the definition for this used by
> > + * Microsoft is not just 16 bytes though - it is a structure that is
> > defined,
> > + * so we need to follow it here so that endianness does not trip us up */
> > +
> > +typedef struct ms_guid {
> > + uint32_t data1;
> > + uint16_t data2;
> > + uint16_t data3;
> > + uint8_t data4[8];
> > +} ms_guid;
> > +
> > +#define guid_cmp(a, b) \
> > + (memcmp(&(a), &(b), sizeof(ms_guid)) == 0)
>
> Inline memcmp() is simple enough, no need for a macro. "cmp" is a bad
> name since cmp functions usually return -1, 0, 1 for sort(3) usage.
> "eq" would be a better name, but simply using memcmp() is clearest IMO.
>
Thanks for the reviews, Stefan, I appreciate it.
Yeah, I went back and forth on that in my head, and thought guid_cmp
(or guid_eq) was more readable. But I am fine just using memcmp
instead, that may be better.
> > +/* Individual region table entry. There may be a maximum of 2047 of these
> > + *
> > + * There are two known region table properties. Both are required.
> > + * BAT (block allocation table): 2DC27766F62342009D64115E9BFD4A08
> > + * Metadata: 8B7CA20647904B9AB8FE575F050F886E
> > + */
> > +typedef struct vhdx_region_table_entry {
> > + ms_guid guid; /* 128-bit unique identifier */
> > + uint64_t file_offset; /* offset of the object in the
> > file.
> > + Must be multiple of 1MB */
> > + uint32_t length; /* length, in bytes, of the object
> > */
> > + union vhdx_rt_bitfield {
> > + struct {
> > + uint32_t required:1; /* 1 if this region must be
> > recognized
> > + in order to load the file */
> > + uint32_t reserved:31;
> > + } bits;
> > + uint32_t data;
> > + } bitfield;
>
> Bitfield in a file format structure, yikes. Endianness, layout, etc.
> Better to use uint32_t flags with a VHDX_RT_FLAG_REQUIRED bitmask
> constant?
Yeah, pretty ugly - it is also how it is present in the VHDX spec,
which is why I left the structure definition the same. The endianness
of it has to be dealt with either way during the parsing and writing,
so I didn't see any compelling reason to abstract the struct away from
a bitfield.
>
> > +/* This is a packed struct that generally should not have alignment issues,
> > + * as it is just uint64_t at heart */
> > +typedef struct QEMU_PACKED vhdx_bat_entry {
> > + union vhdx_bat_bitfield {
> > + struct {
> > + uint64_t state:3; /* state of the block (see
> > above) */
> > + uint64_t reserved:17;
> > + uint64_t file_offset_mb:44; /* offset within file in 1MB
> > units */
> > + } bits;
> > + uint64_t data;
> > + } bitfield;
>
> More bitfields...
And uint64_t ones, at that! This one makes more sense, spec-wise, in
that the BAT is likely over a MB as it is. This is also the one
struct that I did use packed, since it is essentially a uint64_t; I
wasn't too worried about it.
