Re: [RFC PATCH 2/2] x86/sev: Disable jump tables in SEV startup code

[Ard Biesheuvel]

On Mon, 27 Jan 2025 at 23:15, Ard Biesheuvel <ardb@xxxxxxxxxx> wrote:
>
> On Mon, 27 Jan 2025 at 18:10, Linus Torvalds
> <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> >
> > On Mon, 27 Jan 2025 at 03:43, Ard Biesheuvel <ardb+git@xxxxxxxxxx> wrote:
> > >
> > > +# jump tables are emitted using absolute references in non-PIC code
> > > +# so they cannot be used in the early SEV startup code
> > > +CFLAGS_core.o += -fno-jump-tables
> >
> > I can confirm that this looks like it fixes the problem for my
> > particular config.
> >
>
> Great.
>
> > Is the SEV code the only thing that needs this? And on the other hand,
> > isn't it just a _part_ of core.c that needs this? Maybe the "_head"
> > parts should be in a separate file?
> >
> > I'm looking at (for example) arch/x86/mm/mem_encrypt_identity.c and
> > arch/x86/kernel/head64.c, and it looks like it really should have that
> > -fno-jump-tables thing too.
> >
> > It just randomly may not have any switch tables or other things that
> > makes the compiler generate that code pattern.
> >
>
...
> The use case is also quite similar to the x86 one, as a matter of
> fact: the initial mapping of the arm64 kernel is also 1:1, and some
> non-trivial work is needed before the kernel's virtual mapping can be
> created, and doing all of that in C was becoming intractable.
>

Doing it in *assembler* was becoming intractable, in case that wasn't clear.