[v2] x86/fred: Fix SEV-ES/SNP guest boot failures

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

Posted by Nikunj A Dadhania 1 month, 1 week ago

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@vger.kernel.org # 6.9+
Signed-off-by: Nikunj A Dadhania <nikunj@amd.com>
---
 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);
 	}
 
-- 
2.48.1

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

Posted by Tom Lendacky 1 month, 1 week ago

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@vger.kernel.org # 6.9+
> Signed-off-by: Nikunj A Dadhania <nikunj@amd.com>

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

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);
>  	}
>

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

Posted by Nikunj A. Dadhania 1 month, 1 week ago


On 2/26/2026 7:44 PM, Tom Lendacky wrote:
> 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@vger.kernel.org # 6.9+
>> Signed-off-by: Nikunj A Dadhania <nikunj@amd.com>
> 
> Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com>

Thank you!

> 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())?

Sure, will send a follow-on patch.

Regards,
Nikunj