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

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Message-ID: <802166ae-9420-41f6-95ed-6570ef660369@amd.com>
Date: Thu, 11 Sep 2025 16:53:08 -0500
From: Tom Lendacky <thomas.lendacky@....com>
To: Ard Biesheuvel <ardb+git@...gle.com>, linux-efi@...r.kernel.org
Cc: linux-kernel@...r.kernel.org, x86@...nel.org,
 Ard Biesheuvel <ardb@...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 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.

Thanks,
Tom

> -
>  	efi_5level_switch();
>  
>  	enter_kernel(kernel_entry, boot_params);