With MSR_AMD64_SEV_DEBUG_SWAP enabled, the guest is not expected to
receive a #VC for reads or writes of DR7.

Update the SNP_FEATURES_PRESENT mask with MSR_AMD64_SNP_DEBUG_SWAP so
an SNP guest doesn't gracefully terminate during SNP feature negotiation
if MSR_AMD64_SEV_DEBUG_SWAP is enabled.

Since a guest is not expected to receive a #VC on DR7 accesses when
MSR_AMD64_SEV_DEBUG_SWAP is enabled, return an error from the #VC
handler in this situation.

Signed-off-by: Alexey Kardashevskiy <aik@amd.com>
Reviewed-by: Carlos Bilbao <carlos.bilbao@amd.com>
Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com>
Reviewed-by: Pankaj Gupta <pankaj.gupta@amd.com>
---
Changes:
v4:
* rebased on top of SNP feature negotiation

v2:
* use new bit definition
---
 arch/x86/boot/compressed/sev.c | 2 +-
 arch/x86/kernel/sev.c          | 6 ++++++
 2 files changed, 7 insertions(+), 1 deletion(-)

diff --git a/arch/x86/boot/compressed/sev.c b/arch/x86/boot/compressed/sev.c
index 014b89c89088..f6123808be42 100644
--- a/arch/x86/boot/compressed/sev.c
+++ b/arch/x86/boot/compressed/sev.c
@@ -313,7 +313,7 @@ static void enforce_vmpl0(void)
  * by the guest kernel. As and when a new feature is implemented in the
  * guest kernel, a corresponding bit should be added to the mask.
  */
-#define SNP_FEATURES_PRESENT (0)
+#define SNP_FEATURES_PRESENT	MSR_AMD64_SNP_DEBUG_SWAP
 
 void snp_check_features(void)
 {
diff --git a/arch/x86/kernel/sev.c b/arch/x86/kernel/sev.c
index b031244d6d2d..a515eb880970 100644
--- a/arch/x86/kernel/sev.c
+++ b/arch/x86/kernel/sev.c
@@ -1620,6 +1620,9 @@ static enum es_result vc_handle_dr7_write(struct ghcb *ghcb,
 	long val, *reg = vc_insn_get_rm(ctxt);
 	enum es_result ret;
 
+	if (sev_status & MSR_AMD64_SNP_DEBUG_SWAP)
+		return ES_VMM_ERROR;
+
 	if (!reg)
 		return ES_DECODE_FAILED;
 
@@ -1657,6 +1660,9 @@ static enum es_result vc_handle_dr7_read(struct ghcb *ghcb,
 	struct sev_es_runtime_data *data = this_cpu_read(runtime_data);
 	long *reg = vc_insn_get_rm(ctxt);
 
+	if (sev_status & MSR_AMD64_SNP_DEBUG_SWAP)
+		return ES_VMM_ERROR;
+
 	if (!reg)
 		return ES_DECODE_FAILED;
 
-- 
2.39.1

On Tue, Apr 11, 2023, Alexey Kardashevskiy wrote:
> With MSR_AMD64_SEV_DEBUG_SWAP enabled, the guest is not expected to
> receive a #VC for reads or writes of DR7.
> 
> Update the SNP_FEATURES_PRESENT mask with MSR_AMD64_SNP_DEBUG_SWAP so
> an SNP guest doesn't gracefully terminate during SNP feature negotiation
> if MSR_AMD64_SEV_DEBUG_SWAP is enabled.
> 
> Since a guest is not expected to receive a #VC on DR7 accesses when
> MSR_AMD64_SEV_DEBUG_SWAP is enabled, return an error from the #VC
> handler in this situation.
> 
> Signed-off-by: Alexey Kardashevskiy <aik@amd.com>
> Reviewed-by: Carlos Bilbao <carlos.bilbao@amd.com>
> Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com>
> Reviewed-by: Pankaj Gupta <pankaj.gupta@amd.com>
> ---
> Changes:
> v4:
> * rebased on top of SNP feature negotiation
> 
> v2:
> * use new bit definition
> ---
>  arch/x86/boot/compressed/sev.c | 2 +-
>  arch/x86/kernel/sev.c          | 6 ++++++
>  2 files changed, 7 insertions(+), 1 deletion(-)

Can you post this separately (or bribe Boris to grab it)?  IIUC, this has no
dependency on the KVM enabling, i.e. can/should go through the tip tree without
waiting for the KVM patches to be applied.

On 23/5/23 09:44, Sean Christopherson wrote:
> On Tue, Apr 11, 2023, Alexey Kardashevskiy wrote:
>> With MSR_AMD64_SEV_DEBUG_SWAP enabled, the guest is not expected to
>> receive a #VC for reads or writes of DR7.
>>
>> Update the SNP_FEATURES_PRESENT mask with MSR_AMD64_SNP_DEBUG_SWAP so
>> an SNP guest doesn't gracefully terminate during SNP feature negotiation
>> if MSR_AMD64_SEV_DEBUG_SWAP is enabled.
>>
>> Since a guest is not expected to receive a #VC on DR7 accesses when
>> MSR_AMD64_SEV_DEBUG_SWAP is enabled, return an error from the #VC
>> handler in this situation.
>>
>> Signed-off-by: Alexey Kardashevskiy <aik@amd.com>
>> Reviewed-by: Carlos Bilbao <carlos.bilbao@amd.com>
>> Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com>
>> Reviewed-by: Pankaj Gupta <pankaj.gupta@amd.com>
>> ---
>> Changes:
>> v4:
>> * rebased on top of SNP feature negotiation
>>
>> v2:
>> * use new bit definition
>> ---
>>   arch/x86/boot/compressed/sev.c | 2 +-
>>   arch/x86/kernel/sev.c          | 6 ++++++
>>   2 files changed, 7 insertions(+), 1 deletion(-)
> 
> Can you post this separately (or bribe Boris to grab it)?  IIUC, this has no
> dependency on the KVM enabling, i.e. can/should go through the tip tree without
> waiting for the KVM patches to be applied.

I definitely can, do you mind adding yours "rb"/"ab"? Thanks!


-- 
Alexey