[RFC/RFT] x86/efistub: Don't bother enabling SEV in the EFI stub

From: Ard Biesheuvel <ardb@kernel.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.

So let's just drop this call to sev_enable(), and rely on the core
kernel to initiaize SEV correctly.

Cc: Tom Lendacky <thomas.lendacky@amd.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dionna Amalie Glaze <dionnaglaze@google.com>
Cc: Kevin Loughlin <kevinloughlin@google.com>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
---
 arch/x86/include/asm/sev.h              | 2 --
 drivers/firmware/efi/libstub/x86-stub.c | 6 ------
 2 files changed, 8 deletions(-)

I'm still waiting for my SEV-capable hardware to turn up so this was
build tested only.

On 4/14/25 08:04, Ard Biesheuvel wrote:
> From: Ard Biesheuvel <ardb@kernel.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.
> 
> So let's just drop this call to sev_enable(), and rely on the core
> kernel to initiaize SEV correctly.

SEV support does some things in sev_enable() that aren't done later in
the kernel proper. For example, we check certain hypervisor features and
save the CC blob into the boot parameters. And as I look closer, I see
that we don't call initialize_identity_maps() from the EFI stub so we
aren't calling snp_check_features() from the EFI stub, which we should
have been doing.

Just removing the call does cause an SNP boot to crash. My testing shows
just ensuring that the cc_blob_address in the boot_params structure is
set results in a successful boot under SNP. So some of sev_enable()
needs to remain and some things need to be moved into the kernel proper.

Thanks,
Tom

> 
> Cc: Tom Lendacky <thomas.lendacky@amd.com>
> Cc: Borislav Petkov <bp@alien8.de>
> Cc: Dionna Amalie Glaze <dionnaglaze@google.com>
> Cc: Kevin Loughlin <kevinloughlin@google.com>
> Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
> ---
>  arch/x86/include/asm/sev.h              | 2 --
>  drivers/firmware/efi/libstub/x86-stub.c | 6 ------
>  2 files changed, 8 deletions(-)
> 
> I'm still waiting for my SEV-capable hardware to turn up so this was
> build tested only.
> 
> diff --git a/arch/x86/include/asm/sev.h b/arch/x86/include/asm/sev.h
> index 7427eb233990..1db906d6924b 100644
> --- a/arch/x86/include/asm/sev.h
> +++ b/arch/x86/include/asm/sev.h
> @@ -442,7 +442,6 @@ static __always_inline void sev_es_nmi_complete(void)
>  		__sev_es_nmi_complete();
>  }
>  extern int __init sev_es_efi_map_ghcbs(pgd_t *pgd);
> -extern void sev_enable(struct boot_params *bp);
>  
>  /*
>   * RMPADJUST modifies the RMP permissions of a page of a lesser-
> @@ -533,7 +532,6 @@ 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(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 cafc90d4caaf..52ed13ec4fe9 100644
> --- a/drivers/firmware/efi/libstub/x86-stub.c
> +++ b/drivers/firmware/efi/libstub/x86-stub.c
> @@ -925,12 +925,6 @@ 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);
> -
>  	efi_5level_switch();
>  
>  	enter_kernel(kernel_entry, boot_params);

On 4/14/25 14:45, Ard Biesheuvel wrote:
> Hi Tom,
> 
> Thanks for taking a look.
> 
> 
> On Mon, 14 Apr 2025 at 21:10, Tom Lendacky <thomas.lendacky@amd.com> wrote:
>>
>> On 4/14/25 08:04, Ard Biesheuvel wrote:
>>> From: Ard Biesheuvel <ardb@kernel.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.
>>>
>>> So let's just drop this call to sev_enable(), and rely on the core
>>> kernel to initiaize SEV correctly.
>>
>> SEV support does some things in sev_enable() that aren't done later in
>> the kernel proper. For example, we check certain hypervisor features and
>> save the CC blob into the boot parameters.
> 
> I misread the last part: the kernel proper checks struct boot_params
> and setup_data, whereas the decompressor checks both of those as well
> as the EFI config table.
> 
> It would make sense for the EFI stub to populate the field in struct
> boot_params directly.
> 
>> And as I look closer, I see
>> that we don't call initialize_identity_maps() from the EFI stub so we
>> aren't calling snp_check_features() from the EFI stub, which we should
>> have been doing.
>>
> 
> The EFI stub has its own check for this, based on the same underlying
> logic (have_unsupported_snp_features()). But it checks this much
> earlier so it can fail gracefully and return to the firmware, rather
> than terminate.

Ah, whew! I forgot we have a slightly different call for that in the EFI
stub, thanks for reminding me.

Thanks,
Tom

> 
>> Just removing the call does cause an SNP boot to crash. My testing shows
>> just ensuring that the cc_blob_address in the boot_params structure is
>> set results in a successful boot under SNP. So some of sev_enable()
>> needs to remain and some things need to be moved into the kernel proper.
>>
> 
> Thanks, this is very helpful. I'll dig a bit deeper and try to address
> the points you raised.