Re: [PATCH] x86/boot: skip realmode init code when running as Xen PV guest

[Juergen Gross]

On 21.11.22 20:08, Borislav Petkov wrote:
> On Mon, Nov 21, 2022 at 05:24:33PM +0100, Juergen Gross wrote:
> > When running as a Xen PV guest there is no need for setting up the
> > realmode trampoline, as realmode isn't supported in this environment.
> >
> > Trying to setup the trampoline has been proven to be problematic in
> > some cases, especially when trying to debug early boot problems with
> > Xen requiring to keep the EFI boot-services memory mapped (some
> > firmware variants seem to claim basically all memory below 1M for boot
> > services).
> >
> > Skip the trampoline setup code for Xen PV guests.
> >
> > Fixes: 084ee1c641a0 ("x86, realmode: Relocator for realmode code")
> > Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
> > ---
> >   arch/x86/include/asm/realmode.h | 4 ++--
> >   arch/x86/realmode/init.c        | 3 +++
> >   2 files changed, 5 insertions(+), 2 deletions(-)
>   
> > diff --git a/arch/x86/include/asm/realmode.h b/arch/x86/include/asm/realmode.h
> > index fd6f6e5b755a..5bfce58f1bab 100644
> > --- a/arch/x86/include/asm/realmode.h
> > +++ b/arch/x86/include/asm/realmode.h
> > @@ -78,8 +78,8 @@ extern unsigned char secondary_startup_64_no_verify[];
> >   
> >   static inline size_t real_mode_size_needed(void)
> >   {
> > -	if (real_mode_header)
> > -		return 0;	/* already allocated. */
> > +	if (real_mode_header || cpu_feature_enabled(X86_FEATURE_XENPV))
> > +		return 0;	/* already allocated or not needed. */
> >   
> >   	return ALIGN(real_mode_blob_end - real_mode_blob, PAGE_SIZE);
> >   }
> > diff --git a/arch/x86/realmode/init.c b/arch/x86/realmode/init.c
> > index 41d7669a97ad..1826700b156e 100644
> > --- a/arch/x86/realmode/init.c
> > +++ b/arch/x86/realmode/init.c
> > @@ -202,6 +202,9 @@ static void __init set_real_mode_permissions(void)
> >   
> >   static int __init init_real_mode(void)
> >   {
> > +	if (cpu_feature_enabled(X86_FEATURE_XENPV))a
>
> This reminds me of the notorious if (xen) sprinkling from years ago.
> Please don't do that.

Okay, what about plan B:

- rework realmode/rm to:
  + replace header.S with main.c making it possible to initialize
    struct real_mode_header using the struct definition
  + optional: merge stack.S into main.c
- include realmode/rm addresses needed outside of it in struct
  real_mode_header
- setup a dummy struct real_mode_header in Xen PV code removing the
  need to skip init_real_mode(), but making it basically a nop

Would you be fine with that?


Juergen

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