Re: [PATCH v2 2/2] x86/fred: Fix early boot failures on SEV-ES/SNP guests

[Tom Lendacky]

On 2/26/26 03:23, Nikunj A Dadhania wrote:
> FRED-enabled SEV-ES and SNP guests fail to boot due to the following issues
> in the early boot sequence:
> 
> * FRED does not have a #VC exception handler in the dispatch logic
> 
> * Early FRED #VC exceptions attempt to use uninitialized per-CPU GHCBs
>   instead of boot_ghcb
> 
> Add X86_TRAP_VC case to fred_hwexc() with a new exc_vmm_communication()
> function that provides the unified entry point FRED requires, dispatching
> to existing user/kernel handlers based on privilege level. The function is
> already declared via DECLARE_IDTENTRY_VC().
> 
> Fix early GHCB access by falling back to boot_ghcb in
> __sev_{get,put}_ghcb() when per-CPU GHCBs are not yet initialized.
> 
> Fixes: 14619d912b65 ("x86/fred: FRED entry/exit and dispatch code")
> Cc: stable@xxxxxxxxxxxxxxx # 6.9+
> Signed-off-by: Nikunj A Dadhania <nikunj@xxxxxxx>

Reviewed-by: Tom Lendacky <thomas.lendacky@xxxxxxx>

Looking at the code, I think there are a couple of call sights that can
be simplified now. Can you verify that? Then as a follow-on patch,
replace the checks in arch/x86/coco/sev/core.c to just call
__sev_{get,put}_ghcb() now (svsm_perform_call_protocol() and
__set_pages_state())?

Thanks,
Tom

> ---
>  arch/x86/coco/sev/noinstr.c |  6 ++++++
>  arch/x86/entry/entry_fred.c | 14 ++++++++++++++
>  2 files changed, 20 insertions(+)
> 
> diff --git a/arch/x86/coco/sev/noinstr.c b/arch/x86/coco/sev/noinstr.c
> index 9d94aca4a698..5afd663a1c21 100644
> --- a/arch/x86/coco/sev/noinstr.c
> +++ b/arch/x86/coco/sev/noinstr.c
> @@ -121,6 +121,9 @@ noinstr struct ghcb *__sev_get_ghcb(struct ghcb_state *state)
>  
>  	WARN_ON(!irqs_disabled());
>  
> +	if (!sev_cfg.ghcbs_initialized)
> +		return boot_ghcb;
> +
>  	data = this_cpu_read(runtime_data);
>  	ghcb = &data->ghcb_page;
>  
> @@ -164,6 +167,9 @@ noinstr void __sev_put_ghcb(struct ghcb_state *state)
>  
>  	WARN_ON(!irqs_disabled());
>  
> +	if (!sev_cfg.ghcbs_initialized)
> +		return;
> +
>  	data = this_cpu_read(runtime_data);
>  	ghcb = &data->ghcb_page;
>  
> diff --git a/arch/x86/entry/entry_fred.c b/arch/x86/entry/entry_fred.c
> index 88c757ac8ccd..fbe2d10dd737 100644
> --- a/arch/x86/entry/entry_fred.c
> +++ b/arch/x86/entry/entry_fred.c
> @@ -177,6 +177,16 @@ static noinstr void fred_extint(struct pt_regs *regs)
>  	}
>  }
>  
> +#ifdef CONFIG_AMD_MEM_ENCRYPT
> +noinstr void exc_vmm_communication(struct pt_regs *regs, unsigned long error_code)
> +{
> +	if (user_mode(regs))
> +		return user_exc_vmm_communication(regs, error_code);
> +	else
> +		return kernel_exc_vmm_communication(regs, error_code);
> +}
> +#endif
> +
>  static noinstr void fred_hwexc(struct pt_regs *regs, unsigned long error_code)
>  {
>  	/* Optimize for #PF. That's the only exception which matters performance wise */
> @@ -207,6 +217,10 @@ static noinstr void fred_hwexc(struct pt_regs *regs, unsigned long error_code)
>  #ifdef CONFIG_X86_CET
>  	case X86_TRAP_CP: return exc_control_protection(regs, error_code);
>  #endif
> +#ifdef CONFIG_AMD_MEM_ENCRYPT
> +	case X86_TRAP_VC: return exc_vmm_communication(regs, error_code);
> +#endif
> +
>  	default: return fred_bad_type(regs, error_code);
>  	}
>