Re: [RFC PATCH v1 1/5] OvmfPkg/ResetVector: Set memory encryption when SEV is active

[Brijesh Singh <brijesh.ksingh@gmail.com>]

On Mar 7, 2017 10:25 AM, "Tom Lendacky" <thomas.lendacky@amd.com> wrote:

On 3/6/2017 5:27 PM, Brijesh Singh wrote:

> SEV guest VMs have the concept of private and shared memory. Private
> memory is encrypted with the guest-specific key, while shared memory
> may be encrypted with hypervisor key. The C-bit (encryption attribute)
> in PTE indicates whether the page is private or shared.
>
> If SEV is active, set the memory encryption attribute while building
> the page table.
>
> Contributed-under: TianoCore Contribution Agreement 1.0
> Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>
> ---
>  OvmfPkg/ResetVector/Ia32/PageTables64.asm |   52
> +++++++++++++++++++++++++++++
>  1 file changed, 52 insertions(+)
>
> diff --git a/OvmfPkg/ResetVector/Ia32/PageTables64.asm
> b/OvmfPkg/ResetVector/Ia32/PageTables64.asm
> index 6201cad..eaf9732 100644
> --- a/OvmfPkg/ResetVector/Ia32/PageTables64.asm
> +++ b/OvmfPkg/ResetVector/Ia32/PageTables64.asm
> @@ -26,6 +26,7 @@ BITS    32
>  %define PAGE_GLOBAL           0x0100
>  %define PAGE_2M_MBO            0x080
>  %define PAGE_2M_PAT          0x01000
> +%define KVM_FEATURE_SEV         0x08
>
>  %define PAGE_2M_PDE_ATTR (PAGE_2M_MBO + \
>                            PAGE_ACCESSED + \
> @@ -37,6 +38,33 @@ BITS    32
>                         PAGE_READ_WRITE + \
>                         PAGE_PRESENT)
>
> +; Check if Secure Encrypted Virtualization (SEV) feature
> +; is enabled in KVM
> +;
> +;  If SEV is enabled, then EAX will contain Memory encryption bit position
> +;
> +CheckKVMSEVFeature:
> +    ; Check for SEV feature
> +    ;  CPUID KVM_FEATURE - Bit 8
> +    mov       eax, 0x40000001
> +    cpuid
> +    bt        eax, KVM_FEATURE_SEV
> +    jnc       NoSev
> +
> +    ; Get memory encryption information
> +    ; CPUID Fn8000_001F[EBX] - Bits 5:0
> +    ;
> +    mov       eax,  0x8000001f
> +    cpuid
> +    mov       eax, ebx
> +    and       eax, 0x3f
> +    jmp       SevExit
> +
> +NoSev:
> +    xor       eax, eax
> +
> +SevExit:
> +    OneTimeCallRet CheckKVMSEVFeature
>
>  ;
>  ; Modified:  EAX, ECX
> @@ -60,18 +88,41 @@ clearPageTablesMemoryLoop:
>      mov     dword[ecx * 4 + PT_ADDR (0) - 4], eax
>      loop    clearPageTablesMemoryLoop
>
> +    ; Check if it SEV-enabled Guest
> +    ;
> +    OneTimeCall   CheckKVMSEVFeature
> +    xor     edx, edx
> +    test    eax, eax
> +    jz      SevNotActive
> +
> +    ; If SEV is enabled, Memory encryption bit is always above 31
> +    mov     ebx, 32
> +    sub     ebx, eax
> +    bts     edx, eax
> +
> +SevNotActive:
> +
> +    ;
>      ;
>      ; Top level Page Directory Pointers (1 * 512GB entry)
>      ;
> +    ; edx contain the memory encryption bit mask, must be applied
> +    ; to upper 31 bit on 64-bit address
> +    ;
>      mov     dword[PT_ADDR (0)], PT_ADDR (0x1000) + PAGE_PDP_ATTR
> +    mov     dword[PT_ADDR (4)], edx
>
>      ;
>      ; Next level Page Directory Pointers (4 * 1GB entries => 4GB)
>      ;
>      mov     dword[PT_ADDR (0x1000)], PT_ADDR (0x2000) + PAGE_PDP_ATTR
> +    mov     dword[PT_ADDR (0x1004)], edx
>      mov     dword[PT_ADDR (0x1008)], PT_ADDR (0x3000) + PAGE_PDP_ATTR
> +    mov     dword[PT_ADDR (0x100C)], edx
>      mov     dword[PT_ADDR (0x1010)], PT_ADDR (0x4000) + PAGE_PDP_ATTR
> +    mov     dword[PT_ADDR (0x1004)], edx
>

Shouldn't this be 0x1014?


Agreed will fix it. Seems like my copy/paste error.



     mov     dword[PT_ADDR (0x1018)], PT_ADDR (0x5000) + PAGE_PDP_ATTR
> +    mov     dword[PT_ADDR (0x100C)], edx
>

Same here, shouldn't this be 0x101C?


Agreed will fix it.


Thanks,
Tom



>      ;
>      ; Page Table Entries (2048 * 2MB entries => 4GB)
> @@ -83,6 +134,7 @@ pageTableEntriesLoop:
>      shl     eax, 21
>      add     eax, PAGE_2M_PDE_ATTR
>      mov     [ecx * 8 + PT_ADDR (0x2000 - 8)], eax
> +    mov     [(ecx * 8 + PT_ADDR (0x2000 - 8)) + 4], edx
>      loop    pageTableEntriesLoop
>
>      ;
>
>