Re: [PATCH v8 04/15] x86: Secure Launch Resource Table header file

[ross . philipson]

On 2/21/24 6:03 PM, 'Andrew Cooper' via trenchboot-devel wrote:
> On 15/02/2024 8:08 am, Ard Biesheuvel wrote:
> > On Wed, 14 Feb 2024 at 23:31, Ross Philipson <ross.philipson@xxxxxxxxxx> wrote:
> > > +/*
> > > + * Primary SLR Table Header
>
> I know it's just a comment, but SLR ought to be written in longhand here.

Will do, thanks. Ross.

> > > + */
> > > +struct slr_table {
> > > +       u32 magic;
> > > +       u16 revision;
> > > +       u16 architecture;
> > > +       u32 size;
> > > +       u32 max_size;
> > > +       /* entries[] */
> > > +} __packed;
> > Packing this struct has no effect on the layout so better drop the
> > __packed here. If this table is part of a structure that can appear
> > misaligned in memory, better to pack the outer struct or deal with it
> > there in another way.
>
> As you note, __packed does two things not one.
>
> The consumer of the random integer that is expected to be a pointer to a
> struct lsr_table doesn't know whether it was invoked by a 16bit
> bootloader or a 32bit bootloader, and this really does make a difference
> for an ABI described only in C.
>
> Then again, we're holding off on setting the spec in stone until there's
> an agreement in principle, so we could retrofit a statement about the
> expected alignment of this structure in memory.
>
> The sane choices are either 8b alignment (there are uint64_t's in
> entires[], but I also see there are some misaligned uint64_t's too,
> which is dull), or using the good old x86 fallback or paragraph
> alignment just in case we really want to extend it with a uint128_t in
> future.
>
> Thoughts?
>
> ~Andrew