Re: [PATCH v3 3/3] x86: Add Kconfig option to require NX bit support

[Alejandro Vallejo <alejandro.vallejo@cloud.com>]

On Wed, Jul 19, 2023 at 08:13:27AM +0200, Jan Beulich wrote:
> On 18.07.2023 15:19, Jan Beulich wrote:
> > On 29.06.2023 14:17, Alejandro Vallejo wrote:
> >> --- a/xen/arch/x86/boot/head.S
> >> +++ b/xen/arch/x86/boot/head.S
> >> @@ -123,6 +123,7 @@ multiboot2_header:
> >>  .Lbad_ldr_nih: .asciz "ERR: EFI ImageHandle is not provided by bootloader!"
> >>  .Lbad_efi_msg: .asciz "ERR: EFI IA-32 platforms are not supported!"
> >>  .Lbag_alg_msg: .asciz "ERR: Xen must be loaded at a 2Mb boundary!"
> >> +.Lno_nx_msg:   .asciz "ERR: Not an NX-capable CPU!"
> >>  
> >>          .section .init.data, "aw", @progbits
> >>          .align 4
> >> @@ -153,6 +154,11 @@ early_error: /* Here to improve the disassembly. */
> >>  .Lnot_aligned:
> >>          add     $sym_offs(.Lbag_alg_msg), %esi
> >>          jmp     .Lget_vtb
> >> +#ifdef CONFIG_REQUIRE_NX
> >> +.Lno_nx:
> >> +        add     $sym_offs(.Lno_nx_msg), %esi
> >> +        jmp     .Lget_vtb
> >> +#endif
> > 
> > Since I'm in the process of introducing more such paths (for the x86-64-v<N>
> > series), I'm curious: Have you actually had success with getting any output
> > from this code path? I see unreadable output come through serial (provided
> > it's the normal com1 I/O port location where the serial port is), which
> > likely is because baud rate wasn't configured yet, and hence I might have
> > success by changing the config of the receiving side. And I see nothing at
> > all on the screen. While kind of expected when in graphics mode, I wonder
> > whether this ever worked, or whether this has simply bitrotted because of
> > never actually coming into play.
I hacked the code to exercise the XD_DISABLE code path, but didn't try to
exercise the failure paths, I'm afraid. Sorry.
> 
> Pretty clearly this was broken in the course of adding MB2 support, by
> b28044226e1c using %esi as the "relocation base" after already having
> clobbered it. I'm working on a fix.
> 
> Jan
Uh-oh. Good catch.

Alejandro