Re: [PATCH v7 10/22] x86/decompressor: Use standard calling convention for trampoline

[Ard Biesheuvel]

On Mon, 31 Jul 2023 at 13:29, Borislav Petkov <bp@xxxxxxxxx> wrote:
>
> On Fri, Jul 28, 2023 at 11:09:04AM +0200, Ard Biesheuvel wrote:
> > Update the trampoline code so its arguments are passed via RDI and RSI,
> > which matches the ordinary SysV calling convention for x86_64. This will
> > allow this code to be called directly from C.
> >
> > Acked-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> > Signed-off-by: Ard Biesheuvel <ardb@xxxxxxxxxx>
> > ---
> >  arch/x86/boot/compressed/head_64.S | 26 +++++++++-----------
> >  arch/x86/boot/compressed/pgtable.h |  2 +-
> >  2 files changed, 12 insertions(+), 16 deletions(-)
> >
> > diff --git a/arch/x86/boot/compressed/head_64.S b/arch/x86/boot/compressed/head_64.S
> > index 1892679be2b6897a..491d985be75fd5b0 100644
> > --- a/arch/x86/boot/compressed/head_64.S
> > +++ b/arch/x86/boot/compressed/head_64.S
> > @@ -443,9 +443,9 @@ SYM_CODE_START(startup_64)
> >       movq    %r15, %rdi              /* pass struct boot_params pointer */
> >       call    paging_prepare
> >
> > -     /* Save the trampoline address in RCX */
> > -     movq    %rax, %rcx
> > -
> > +     /* Pass the trampoline address and boolean flag as args #1 and #2 */
> > +     movq    %rax, %rdi
> > +     movq    %rdx, %rsi
> >       leaq    TRAMPOLINE_32BIT_CODE_OFFSET(%rax), %rax
> >       call    *%rax
> >
> > @@ -534,15 +534,15 @@ SYM_FUNC_END(.Lrelocated)
> >  /*
> >   * This is the 32-bit trampoline that will be copied over to low memory.
> >   *
> > - * ECX contains the base address of the trampoline memory.
> > - * Non zero RDX means trampoline needs to enable 5-level paging.
> > + * EDI contains the base address of the trampoline memory.
> > + * Non-zero ESI means trampoline needs to enable 5-level paging.
> >   */
>
> This is confusing - this talks about 32-bit and 32-bit registers but
> uses the 64-bit calling convention because it gets called by 64-bit
> code. Please add a short sentence clarifying that.
>

Ok