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

[Laszlo Ersek]

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.


>>> +};
>>> +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