Re: [Xen-devel] [PATCH v4 03/11] public: xen.h: add definitions for UUID handling

[Volodymyr Babchuk <volodymyr_babchuk@xxxxxxxx>]

Hi Jan,

On 23.08.17 14:29, Jan Beulich wrote:
> > > > On 23.08.17 at 13:08, <volodymyr_babchuk@xxxxxxxx> wrote:
> > On 23.08.17 11:10, Jan Beulich wrote:
> > > > > > On 22.08.17 at 16:37, <volodymyr_babchuk@xxxxxxxx> wrote:
> > > > I can't see why you want to map UUID to a certain structure.
> > > This is so that the type cannot mistakenly be passed to a function
> > > taking unsigned char *, or be assigned to a variable of that type.
> > Right, I see the point there.
> >
> > > Please see our TYPE_SAFE() macro which we use to specifically
> > > enclose certain scalar types in a structure to that they won't be
> > > compatible with other types deriving from the same scalar base type.
> > I see. So what about
> >
> > struct xen_uuid_t
> > {
> >        uint8_t a[16];
> > };
> >
> > then?
> Yes, that's what I had asked for as the minimal solution. That
> would be in line with (but better than) xen_domain_handle_t,
> which I've just realized we also have.
>
> > One can convert it to union with different representations (array,
> > RFC4122 struct, etc) later if there will be need for this.
> Well, why don't you make it a union but stick to just the array
> for now if you dislike making it similar to the EFI one? That way
> we can add further representations if needed/desired without
> breaking existing consumers.
My first intention was to declare union with all possible 
representations, so it would be possible to access the same UUID as an 
array of bytes or, for example, as Microsoft GUID. Like this:
typedef union {
    /* UUID represented as a 128-bit object */
    uint8_t obj[16];

    /* Representation according to RFC 4122 */
    struct {
        __be32  time_low;
        __be16  time_mid;
        __be16  time_hi_and_version;
        __u8    clock_seq_hi_and_reserved;
        __u8    clock_seq_low;
        __u8    node[6];
    } rfc4122;

    /* Microsoft/Intel style GUID representation */
    struct {
        __le32  Data1;
        __le16  Data2;
        __le16  Data3;
        __u8    Data4[8];
    } guid;

    /* SMCCC compatible format */
    struct {
        __le32 r0;
        __le32 r1;
        __le32 r2;
        __le32 r3;
    } smccc;
} xen_uuid_t;


But looks like we can't use something like __packed or 
__attribute__((__packed__)) in the public header. This means that we 
can't rely on right overlapping and users of this union should take care 
to read and write only to one chosen substructure. I think, this is 
error-prone, so it is better to stick to
typedef struct
{
       uint8_t a[16];
} xen_uuid_t;

BTW, I'm very interested how it can be guaranteed that structures 
defined in xen.h will have the same size and alignment on both sides of 
communication channel, taking into account, then we rely only on C89 
standard.
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