OvmfPkg/ResetVector: cache the SEV status MSR value in workarea

BZ: https://bugzilla.tianocore.org/show_bug.cgi?id=3582

In order to probe the SEV feature the BaseMemEncryptLib and Reset vector
reads the SEV_STATUS MSR. Cache the value on the first read in the
workarea. In the next patches the value saved in the workarea will
be used by the BaseMemEncryptLib. This not only eliminates the extra
MSR reads it also helps cleaning up the code in BaseMemEncryptLib.

Cc: Min Xu <min.m.xu@intel.com>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Tom Lendacky <thomas.lendacky@amd.com>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Ard Biesheuvel <ardb+tianocore@kernel.org>
Cc: Erdem Aktas <erdemaktas@google.com>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>
Acked-by: Jiewen Yao <jiewen.yao@intel.com>

3 files changed, 28 insertions, 16 deletions

diff --git a/OvmfPkg/ResetVector/Ia32/AmdSev.asm b/OvmfPkg/ResetVector/Ia32/AmdSev.asm
index 1f827da3b9..864d683853 100644
--- a/OvmfPkg/ResetVector/Ia32/AmdSev.asm
+++ b/OvmfPkg/ResetVector/Ia32/AmdSev.asm
@@ -157,8 +157,9 @@ SevClearPageEncMaskForGhcbPage:
     jnz       SevClearPageEncMaskForGhcbPageExit

 

     ; Check if SEV-ES is enabled

-    cmp       byte[SEV_ES_WORK_AREA], 1

-    jnz       SevClearPageEncMaskForGhcbPageExit

+    mov       ecx, 1

+    bt        [SEV_ES_WORK_AREA_STATUS_MSR], ecx

+    jnc       SevClearPageEncMaskForGhcbPageExit

 

     ;

     ; The initial GHCB will live at GHCB_BASE and needs to be un-encrypted.

@@ -219,12 +220,16 @@ GetSevCBitMaskAbove31Exit:
 ; If SEV is disabled then EAX will be zero.

 ;

 CheckSevFeatures:

-    ; Set the first byte of the workarea to zero to communicate to the SEC

-    ; phase that SEV-ES is not enabled. If SEV-ES is enabled, the CPUID

-    ; instruction will trigger a #VC exception where the first byte of the

-    ; workarea will be set to one or, if CPUID is not being intercepted,

-    ; the MSR check below will set the first byte of the workarea to one.

-    mov     byte[SEV_ES_WORK_AREA], 0

+    ;

+    ; Clear the workarea, if SEV is enabled then later part of routine

+    ; will populate the workarea fields.

+    ;

+    mov    ecx, SEV_ES_WORK_AREA_SIZE

+    mov    eax, SEV_ES_WORK_AREA

+ClearSevEsWorkArea:

+    mov    byte [eax], 0

+    inc    eax

+    loop   ClearSevEsWorkArea

 

     ;

     ; Set up exception handlers to check for SEV-ES

@@ -265,6 +270,10 @@ CheckSevFeatures:
     ; Set the work area header to indicate that the SEV is enabled

     mov     byte[WORK_AREA_GUEST_TYPE], 1

 

+    ; Save the SevStatus MSR value in the workarea

+    mov     [SEV_ES_WORK_AREA_STATUS_MSR], eax

+    mov     [SEV_ES_WORK_AREA_STATUS_MSR + 4], edx

+

     ; Check for SEV-ES memory encryption feature:

     ; CPUID  Fn8000_001F[EAX] - Bit 3

     ;   CPUID raises a #VC exception if running as an SEV-ES guest

@@ -280,10 +289,6 @@ CheckSevFeatures:
     bt        eax, 1

     jnc       GetSevEncBit

 

-    ; Set the first byte of the workarea to one to communicate to the SEC

-    ; phase that SEV-ES is enabled.

-    mov       byte[SEV_ES_WORK_AREA], 1

-

 GetSevEncBit:

     ; Get pte bit position to enable memory encryption

     ; CPUID Fn8000_001F[EBX] - Bits 5:0

@@ -313,7 +318,10 @@ NoSev:
     ;

     ; Perform an SEV-ES sanity check by seeing if a #VC exception occurred.

     ;

-    cmp       byte[SEV_ES_WORK_AREA], 0

+    ; If SEV-ES is enabled, the CPUID instruction will trigger a #VC exception

+    ; where the RECEIVED_VC offset in the workarea will be set to one.

+    ;

+    cmp       byte[SEV_ES_WORK_AREA_RECEIVED_VC], 0

     jz        NoSevPass

 

     ;

@@ -407,9 +415,9 @@ SevEsIdtVmmComm:
     ; If we're here, then we are an SEV-ES guest and this

     ; was triggered by a CPUID instruction

     ;

-    ; Set the first byte of the workarea to one to communicate that

+    ; Set the recievedVc field in the workarea to communicate that

     ; a #VC was taken.

-    mov     byte[SEV_ES_WORK_AREA], 1

+    mov     byte[SEV_ES_WORK_AREA_RECEIVED_VC], 1

 

     pop     ecx                     ; Error code

     cmp     ecx, 0x72               ; Be sure it was CPUID

diff --git a/OvmfPkg/ResetVector/Ia32/Flat32ToFlat64.asm b/OvmfPkg/ResetVector/Ia32/Flat32ToFlat64.asm
index eb3546668e..c5c683ebed 100644
--- a/OvmfPkg/ResetVector/Ia32/Flat32ToFlat64.asm
+++ b/OvmfPkg/ResetVector/Ia32/Flat32ToFlat64.asm
@@ -42,7 +42,8 @@ Transition32FlatTo64Flat:
     ;

     xor     ebx, ebx

 

-    cmp     byte[SEV_ES_WORK_AREA], 0

+    mov     ecx, 1

+    bt      [SEV_ES_WORK_AREA_STATUS_MSR], ecx

     jz      EnablePaging

 

     ;

diff --git a/OvmfPkg/ResetVector/ResetVector.nasmb b/OvmfPkg/ResetVector/ResetVector.nasmb
index cc364748b5..9421f48189 100644
--- a/OvmfPkg/ResetVector/ResetVector.nasmb
+++ b/OvmfPkg/ResetVector/ResetVector.nasmb
@@ -100,8 +100,11 @@
   %define GHCB_BASE (FixedPcdGet32 (PcdOvmfSecGhcbBase))

   %define GHCB_SIZE (FixedPcdGet32 (PcdOvmfSecGhcbSize))

   %define SEV_ES_WORK_AREA (FixedPcdGet32 (PcdSevEsWorkAreaBase))

+  %define SEV_ES_WORK_AREA_SIZE 25

+  %define SEV_ES_WORK_AREA_STATUS_MSR (FixedPcdGet32 (PcdSevEsWorkAreaBase))

   %define SEV_ES_WORK_AREA_RDRAND (FixedPcdGet32 (PcdSevEsWorkAreaBase) + 8)

   %define SEV_ES_WORK_AREA_ENC_MASK (FixedPcdGet32 (PcdSevEsWorkAreaBase) + 16)

+  %define SEV_ES_WORK_AREA_RECEIVED_VC (FixedPcdGet32 (PcdSevEsWorkAreaBase) + 24)

   %define SEV_ES_VC_TOP_OF_STACK (FixedPcdGet32 (PcdOvmfSecPeiTempRamBase) + FixedPcdGet32 (PcdOvmfSecPeiTempRamSize))

   %define SEV_SNP_SECRETS_BASE  (FixedPcdGet32 (PcdOvmfSnpSecretsBase))

   %define SEV_SNP_SECRETS_SIZE  (FixedPcdGet32 (PcdOvmfSnpSecretsSize))