Re: [Xen-devel] [PATCH 07 of 10] Transitive grant support

[Steven Smith <steven.smith@xxxxxxxxxx>]

> >>> Steven Smith <steven.smith@xxxxxxxxxx> 05.10.09 13:15 >>>
> >Would something like this be acceptable?
> >...
> >The __attribute__((packed))s are very much not a thing of beauty, but
> >it's at least fairly self-contained.
> 
> Hmm, this being a public header, I'm afraid using gcc extensions isn't
> appropriate.
Okay.  How about this one?

union grant_entry_v2 {
    grant_entry_header_t hdr;

    /*
     * The frame to which we are granting access.  This field has the
     * same meaning as the grant_entry_v1 field of the same name.
     */
    struct {
        grant_entry_header_t hdr;
        uint32_t pad0;
        uint64_t frame;
    } full_page;

    /*
     * If the grant type is GTF_grant_access and GTF_sub_page is set,
     * @domid is allowed to access bytes [@page_off,@page_off+@length)
     * in frame @frame.
     */
    struct {
        grant_entry_header_t hdr;
        uint16_t page_off;
        uint16_t length;
        uint64_t frame;
    } sub_page;

    /*
     * If the grant is GTF_transitive, @domid is allowed to use the
     * grant @gref in domain @trans_domid, as if it was the local
     * domain.  Obviously, the transitive access must be compatible
     * with the original grant.
     *
     * The current version of Xen does not allow transitive grants to
     * be mapped.
     */
    struct {
        grant_entry_header_t hdr;
        domid_t trans_domid;
        uint16_t pad0;
        grant_ref_t gref;
    } transitive;

    uint32_t __spacer[4]; /* Pad to a power of two */
};


Duplicating the header into every field is kind of icky, but I can't
see any other way of avoiding unfortunate padding without using gcc
extensions.

Steven.

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