Re: [Qemu-devel] [PATCH v2 repost 5/9] i386: add bios linker/loader

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, Jul 15, 2013 at 09:33:25AM +0200, Laszlo Ersek wrote:
> On 07/14/13 13:41, Michael S. Tsirkin wrote:
> > On Fri, Jul 12, 2013 at 04:17:28PM +0200, Laszlo Ersek wrote:
> >> On 07/10/13 15:51, Michael S. Tsirkin wrote:
> 
> >>> +struct BiosLinkerLoaderEntry {
> >>> +    uint32_t command;
> >>> +    union {
> >>> +        /*
> >>> +         * COMMAND_ALLOCATE - allocate a table from @alloc_file
> >>> +         * subject to @alloc_align alignment (must be power of 2)
> >>> +         * and @alloc_zone (can be HIGH or FSEG) requirements.
> >>> +         *
> >>> +         * Must appear exactly once for each file, and before
> >>> +         * this file is referenced by any other command.
> >>> +         */
> >>> +        struct {
> >>> +            char alloc_file[BIOS_LINKER_LOADER_FILESZ];
> >>> +            uint32_t alloc_align;
> >>> +            uint8_t alloc_zone;
> >>> +        };
> >>
> >> I think in OVMF we won't rely on the alloc_zone / alloc_align members,
> >> but that's OVMF's private business.
> > 
> > RSDP must be in FSEG though
> 
> I didn't express myself clearly, sorry. The default edk2 ACPI table
> protocol that OVMF uses should allocate RSDP and the like automatically
> in correct regions. (Allocating reserved memory for
> External(XXXX,OpRegionObj) needs a different call though.)
> 
> >>> +
> >>> +        /* padding */
> >>> +        char pad[124];
> >>> +    };
> >>
> >> The unnamed union member is a gcc-ism. I'd give it a short name (like
> >> "u"), but feel free to ignore this.
> >>
> > 
> > This isn't a gcc-ism. It's in C1x:
> > 
> > An unnamed member whose type specifier is a structure specifier with no
> > tag is called an anonymous structure; an unnamed member whose type
> > specifier is a union specifier with no tag is called an anonymous union.
> > The members of an anonymous structure or union are considered to be
> > members of the containing structure or union. This applies recursively
> > if the containing structure or union is also anonymous.
> 
> This part of the discussion is academic, but the unnamed union member is
> a gcc-ism in the qemu source, because AFAIK qemu uses the gnu89 dialect
> by default, and gnu99 on Solaris.
> 

The important thing is that it's part of the standard now,
so won't go away or interfere with porting to a new compiler
if we ever try to do it.

> >>> +};
> >>> +typedef struct BiosLinkerLoaderEntry BiosLinkerLoaderEntry;
> >>
> >> Probably not needed in practice, but for documentation purposes I
> >> suggest QEMU_PACKED from "include/qemu/compiler.h".
> > 
> > It's not required in practice but I can add this though I'm not sure -
> > what does this document?
> 
> To me it documents that we rely on the absence of inter-member padding.
> 
> Thanks
> Laszlo