linux-kernel - Re: [PATCH v4 3/3] x86/efistub: Don't bother enabling SEV in the EFI stub

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Message-ID: <CAMj1kXHqhWpb2OZy+1N8-oPa2Kk0d56NS6tYH46Z92ui3h5wMQ@mail.gmail.com>
Date: Fri, 12 Sep 2025 10:26:54 +0200
From: Ard Biesheuvel <ardb@...nel.org>
To: Tom Lendacky <thomas.lendacky@....com>
Cc: Ard Biesheuvel <ardb+git@...gle.com>, linux-efi@...r.kernel.org, 
	linux-kernel@...r.kernel.org, x86@...nel.org, Borislav Petkov <bp@...en8.de>
Subject: Re: [PATCH v4 3/3] x86/efistub: Don't bother enabling SEV in the EFI stub

On Fri, 12 Sept 2025 at 09:29, Ard Biesheuvel <ardb@...nel.org> wrote:
>
> On Thu, 11 Sept 2025 at 23:53, Tom Lendacky <thomas.lendacky@....com> wrote:
> >
> > On 9/9/25 03:06, Ard Biesheuvel wrote:
> > > From: Ard Biesheuvel <ardb@...nel.org>
> > >
> > > One of the last things the EFI stub does before handing over to the core
> > > kernel when booting as a SEV guest is enabling SEV, even though this is
> > > mostly redundant: one of the first things the core kernel does is
> > > calling sme_enable(), after setting up the early GDT and IDT but before
> > > even setting up the kernel page tables. sme_enable() performs the same
> > > SEV-SNP initialization that the decompressor performs in sev_enable().
> > >
> > > So let's just drop this call to sev_enable(), and rely on the core
> > > kernel to initiaize SEV correctly.
> > >
> > > Signed-off-by: Ard Biesheuvel <ardb@...nel.org>
> > > ---
> > >  arch/x86/include/asm/sev.h              | 2 --
> > >  drivers/firmware/efi/libstub/x86-stub.c | 6 ------
> > >  2 files changed, 8 deletions(-)
> > >
> > > diff --git a/arch/x86/include/asm/sev.h b/arch/x86/include/asm/sev.h
> > > index d7be1ff3f7e0..b017e1dab705 100644
> > > --- a/arch/x86/include/asm/sev.h
> > > +++ b/arch/x86/include/asm/sev.h
> > > @@ -462,7 +462,6 @@ static __always_inline void sev_es_nmi_complete(void)
> > >               __sev_es_nmi_complete();
> > >  }
> > >  extern int __init sev_es_efi_map_ghcbs_cas(pgd_t *pgd);
> > > -extern void sev_enable(struct boot_params *bp);
> > >
> > >  /*
> > >   * RMPADJUST modifies the RMP permissions of a page of a lesser-
> > > @@ -588,7 +587,6 @@ static inline void sev_es_ist_exit(void) { }
> > >  static inline int sev_es_setup_ap_jump_table(struct real_mode_header *rmh) { return 0; }
> > >  static inline void sev_es_nmi_complete(void) { }
> > >  static inline int sev_es_efi_map_ghcbs_cas(pgd_t *pgd) { return 0; }
> > > -static inline void sev_enable(struct boot_params *bp) { }
> > >  static inline int pvalidate(unsigned long vaddr, bool rmp_psize, bool validate) { return 0; }
> > >  static inline int rmpadjust(unsigned long vaddr, bool rmp_psize, unsigned long attrs) { return 0; }
> > >  static inline void setup_ghcb(void) { }
> > > diff --git a/drivers/firmware/efi/libstub/x86-stub.c b/drivers/firmware/efi/libstub/x86-stub.c
> > > index c4ef645762ec..354bc3901193 100644
> > > --- a/drivers/firmware/efi/libstub/x86-stub.c
> > > +++ b/drivers/firmware/efi/libstub/x86-stub.c
> > > @@ -938,12 +938,6 @@ void __noreturn efi_stub_entry(efi_handle_t handle,
> > >               goto fail;
> > >       }
> > >
> > > -     /*
> > > -      * Call the SEV init code while still running with the firmware's
> > > -      * GDT/IDT, so #VC exceptions will be handled by EFI.
> > > -      */
> > > -     sev_enable(boot_params);
> >
> > I think we lose the check for GHCB_HV_FT_SNP_MULTI_VMPL by doing this. It
> > might need move into svsm_setup_ca() now.
> >
>
> Currently, this check only occurs inside sev_enable(), and so it
> happens too late to have an impact, given that the core kernel will
> set up all of this state from scratch right away.
>

Hmm, I only just spotted that this check only happens in the legacy
decompressor.

I think it makes sense for this check to live in svsm_setup_ca(), but
what is your take on the need to perform this check when accepting
memory from the stub using the CA address obtained from the firmware?
(i.e., way before sev_enable() is called)