From: Tom Lendacky <thomas.lenda...@amd.com>
When running as an SEV-ES guest in 32-bit mode, it is not possible to
perform a CPUID instruction because it will require communicating with
the hypervisor using the GHCB. However, writes to the GHCB when in
32-bit mode will be will be encrypted and thus not able to be read
by the hypervisor.
To get around this, add an IDT entry for the #VC exception. The #VC
handler will use the GHCB CPUID request/response protocol to obtain
the requested CPUID function values and provide these to the guest.
Signed-off-by: Tom Lendacky <thomas.lenda...@amd.com>
---
OvmfPkg/ResetVector/ResetVector.inf | 2 +
OvmfPkg/ResetVector/Ia32/PageTables64.asm | 173 ++++++++++++++++++++++
OvmfPkg/ResetVector/ResetVector.nasmb | 1 +
3 files changed, 176 insertions(+)
diff --git a/OvmfPkg/ResetVector/ResetVector.inf
b/OvmfPkg/ResetVector/ResetVector.inf
index b0ddfa5832a2..960b47cd0797 100644
--- a/OvmfPkg/ResetVector/ResetVector.inf
+++ b/OvmfPkg/ResetVector/ResetVector.inf
@@ -35,3 +35,5 @@ [BuildOptions]
[Pcd]
gUefiOvmfPkgTokenSpaceGuid.PcdOvmfSecPageTablesBase
gUefiOvmfPkgTokenSpaceGuid.PcdOvmfSecPageTablesSize
+ gUefiOvmfPkgTokenSpaceGuid.PcdOvmfSecPeiTempRamBase
+ gUefiOvmfPkgTokenSpaceGuid.PcdOvmfSecPeiTempRamSize
diff --git a/OvmfPkg/ResetVector/Ia32/PageTables64.asm
b/OvmfPkg/ResetVector/Ia32/PageTables64.asm
index abad009f20f5..c6071fe934de 100644
--- a/OvmfPkg/ResetVector/Ia32/PageTables64.asm
+++ b/OvmfPkg/ResetVector/Ia32/PageTables64.asm
@@ -37,6 +37,15 @@ BITS 32
; If SEV is disabled then EAX will be zero.
;
CheckSevFeature:
+ ;
+ ; Set up exception handlers to check for SEV-ES
+ ; Load temporary RAM stack based on PCDs
+ ; Establish exception handlers
+ ;
+ mov esp, SEV_TOP_OF_STACK
+ mov eax, ADDR_OF(idtr)
+ lidt [cs:eax]
+
; Check if we have a valid (0x8000_001F) CPUID leaf
mov eax, 0x80000000
cpuid
@@ -73,6 +82,15 @@ NoSev:
xor eax, eax
SevExit:
+ ;
+ ; Clear exception handlers and stack
+ ;
+ push eax
+ mov eax, ADDR_OF(idtr_clear)
+ lidt [cs:eax]
+ pop eax
+ mov esp, 0
+
OneTimeCallRet CheckSevFeature
;
@@ -146,3 +164,158 @@ pageTableEntriesLoop:
mov cr3, eax
OneTimeCallRet SetCr3ForPageTables64
+
+SevEsIdtCommon:
+ hlt
+ jmp SevEsIdtCommon
+ iret
+
+SevEsIdtVmmComm:
+ ;
+ ; If we're here, then we are an SEV-ES guest and this
+ ; was triggered by a CPUID instruction
+ ;
+ pop ecx ; Error code
+ cmp ecx, 0x72 ; Be sure it was CPUID
+ jne SevEsIdtCommon
+
+ ;
+ ; Set up local variable room on the stack
+ ; CPUID function : + 28
+ ; CPUID register : + 24
+ ; GHCB MSR (EAX) : + 20
+ ; GHCB MSR (EDX) : + 16
+ ; CPUID result (EDX) : + 12
+ ; CPUID result (ECX) : + 8
+ ; CPUID result (EBX) : + 4
+ ; CPUID result (EAX) : + 0
+ sub esp, 32
+
+ ; Save CPUID function and initial register request
+ mov [esp + 28], eax
+ xor eax, eax
+ mov [esp + 24], eax
+
+ ; Save current GHCB MSR value
+ mov ecx, 0xc0010130
+ rdmsr
+ mov [esp + 20], eax
+ mov [esp + 16], edx
+
+NextReg:
+ ;
+ ; Setup GHCB MSR
+ ; GHCB_MSR[63:32] = CPUID function
+ ; GHCB_MSR[31:30] = CPUID register
+ ; GHCB_MSR[11:0] = CPUID request protocol
+ ;
+ mov eax, [esp + 24]
+ cmp eax, 4
+ jge VmmDone
+
+ shl eax, 30
+ or eax, 0x004
+ mov edx, [esp + 28]
+ mov ecx, 0xc0010130
+ wrmsr
+
+ ; Issue VMGEXIT (rep; vmmcall)
+ db 0xf3
+ db 0x0f
+ db 0x01
+ db 0xd9
+
+ ;
+ ; Read GHCB MSR
+ ; GHCB_MSR[63:32] = CPUID register value
+ ; GHCB_MSR[31:30] = CPUID register
+ ; GHCB_MSR[11:0] = CPUID response protocol
+ ;
+ mov ecx, 0xc0010130
+ rdmsr
+ mov ecx, eax
+ and ecx, 0xfff
+ cmp ecx, 0x005
+ jne SevEsIdtCommon
+
+ ; Save returned value
+ shr eax, 30
+ and eax, 0x3
+ shl eax, 2
+ mov ecx, esp
+ add ecx, eax
+ mov [ecx], edx
+
+ ; Next register
+ inc word [esp + 24]
+
+ jmp NextReg
+
+VmmDone:
+ ;
+ ; At this point we have all CPUID register values. Restore the GHCB MSR,
+ ; set the return register values and return.
+ ;
+ mov eax, [esp + 20]
+ mov edx, [esp + 16]
+ mov ecx, 0xc0010130
+ wrmsr
+
+ mov eax, [esp + 0]
+ mov ebx, [esp + 4]
+ mov ecx, [esp + 8]
+ mov edx, [esp + 12]
+
+ add esp, 32
+ add word [esp], 2 ; Skip over the CPUID instruction
+ iret
+
+ALIGN 2
+
+idtr:
+ dw IDT_END - IDT_BASE - 1 ; Limit
+ dd ADDR_OF(IDT_BASE) ; Base
+
+idtr_clear:
+ dw 0 ; Limit
+ dd 0 ; Base
+
+ALIGN 16
+
+;
+; The Interrupt Descriptor Table (IDT)
+; This will be used to determine if SEV-ES is enabled. Upon execution
+; of the CPUID instruction, a VMM Communication Exception will occur.
+; This will tell us if SEV-ES is enabled. We can use the current value
+; of the GHCB MSR to determine the SEV attributes.
+;
+IDT_BASE:
+;
+; Vectors 0 - 28
+;
+%rep 29
+ dw (ADDR_OF(SevEsIdtCommon) & 0xffff) ; Offset low bits 15..0
+ dw 0x10 ; Selector
+ db 0 ; Reserved
+ db 0x8E ; Gate Type
(IA32_IDT_GATE_TYPE_INTERRUPT_32)
+ dw (ADDR_OF(SevEsIdtCommon) >> 16) ; Offset high bits 31..16
+%endrep
+;
+; Vector 29 (VMM Communication Exception)
+;
+ dw (ADDR_OF(SevEsIdtVmmComm) & 0xffff) ; Offset low bits 15..0
+ dw 0x10 ; Selector
+ db 0 ; Reserved
+ db 0x8E ; Gate Type
(IA32_IDT_GATE_TYPE_INTERRUPT_32)
+ dw (ADDR_OF(SevEsIdtVmmComm) >> 16) ; Offset high bits 31..16
+;
+; Vectors 30 - 31
+;
+%rep 2
+ dw (ADDR_OF(SevEsIdtCommon) & 0xffff) ; Offset low bits 15..0
+ dw 0x10 ; Selector
+ db 0 ; Reserved
+ db 0x8E ; Gate Type
(IA32_IDT_GATE_TYPE_INTERRUPT_32)
+ dw (ADDR_OF(SevEsIdtCommon) >> 16) ; Offset high bits 31..16
+%endrep
+IDT_END:
diff --git a/OvmfPkg/ResetVector/ResetVector.nasmb
b/OvmfPkg/ResetVector/ResetVector.nasmb
index 75cfe16654b1..3b213cd05ab2 100644
--- a/OvmfPkg/ResetVector/ResetVector.nasmb
+++ b/OvmfPkg/ResetVector/ResetVector.nasmb
@@ -55,6 +55,7 @@
%define PT_ADDR(Offset) (FixedPcdGet32 (PcdOvmfSecPageTablesBase) + (Offset))
%include "Ia32/Flat32ToFlat64.asm"
+ %define SEV_TOP_OF_STACK (FixedPcdGet32 (PcdOvmfSecPeiTempRamBase) +
FixedPcdGet32 (PcdOvmfSecPeiTempRamSize))
%include "Ia32/PageTables64.asm"
%endif
--
2.17.1
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