Re: [RFC PATCH 01/19] OvmfPkg: Reserve the Secrets and Cpuid page for the SEV-SNP guest

["Laszlo Ersek" <lersek@redhat.com>]

On 04/07/21 02:44, James Bottomley wrote:
> On Wed, 2021-04-07 at 00:21 +0000, Xu, Min M wrote:
>> Hi, Laszlo
>>
>> For Intel TDX supported guest, all processors start in 32-bit
>> protected
>> mode, while for Non-Td guest, it starts in 16-bit real mode. To make
>> the
>> ResetVector work on both Td-guest and Non-Td guest, ResetVector are
>> updated as below:
>> ------------------------------------------------------------------
>>   ALIGN   16
>>   resetVector:
>>   ;
>>   ; Reset Vector
>>   ;
>>   ; This is where the processor will begin execution
>>   ;
>>       nop
>>       nop
>>       smsw    ax
>>       test    al, 1
>>       jnz     EarlyBspPmEntry
>>       jmp     EarlyBspInitReal16
> 
> Well, then use the rel8 jump like the compiler would in this situation:
> 
>       smsw    ax
>       test    al, 1
>       jz      1f
>       jmp     EarlyBspPmEntry
> 1:
>       jmp     EarlyBspInitReal16
> 
> So now both entries can be 32k away.

The problem is that we need NASM to generate such *shared* entry code
that behaves correctly when executed in either 16-bit or 32-bit mode.

The rel8 near jumps ("short jumps") are like that -- for example, the
"74 cb" opcode decodes to the same "JZ rel8" in both modes.

But the rel16 ("non-short") near jumps turn into rel32 near jumps when
decoded in 32-bit mode. For example, "E9 cw" decodes to "JMP rel16" in
16-bit mode, but it gets parsed as "E9 cd" (= "JMP rel32") in 32-bit mode.

So the idea is to add more BITS directives, for covering the non-short
near jumps themselves:

> ; instructions up to and including the rel8 JZ decode identically
> ; between BITS 16 and BITS 32
> BITS 16
>       smsw    ax
>       test    al, 1
>       jz     Real
>
> ; the unconditional near jumps are mode-specific
> BITS 32
>       jmp     near EarlyBspPmEntry
> BITS 16
> Real:
>       jmp     near EarlyBspInitReal16
>
> ; --------------------
>
> BITS 16
> EarlyBspInitReal16:
>       nop
>
> BITS 32
> EarlyBspPmEntry:
>       nop

$ nasm -f bin jz.nasmb

Decoded (executed) in 16-bit mode:

$ ndisasm -b 16 -k 7,5 -k 0x10,1 jz
00000000  0F01E0            smsw ax
00000003  A801              test al,0x1
00000005  7405              jz 0xc         ; taken
00000007  skipping 0x5 bytes
0000000C  E90000            jmp word 0xf
0000000F  90                nop
00000010  skipping 0x1 bytes

Decoded (executed) in 32-bit mode:

$ ndisasm -b 32 -k 0xc,4 jz
00000000  0F01E0            smsw eax
00000003  A801              test al,0x1
00000005  7405              jz 0xc         ; not taken
00000007  E904000000        jmp dword 0x10
0000000C  skipping 0x4 bytes
00000010  90                nop


With the garbage *not* hidden:

$ ndisasm -b 16 -s 0xc jz

00000000  0F01E0            smsw ax
00000003  A801              test al,0x1
00000005  7405              jz 0xc          ; taken
00000007  E90400            jmp word 0xe    ; garbage
0000000A  0000              add [bx+si],al  ; garbage
0000000C  E90000            jmp word 0xf
0000000F  90                nop
00000010  90                nop             ; garbage

$ ndisasm -b 32 -s 0x10 jz

00000000  0F01E0            smsw eax
00000003  A801              test al,0x1
00000005  7405              jz 0xc          ; not taken
00000007  E904000000        jmp dword 0x10
0000000C  E9                db 0xe9         ; garbage
0000000D  0000              add [eax],al    ; garbage
0000000F  90                nop             ; garbage
00000010  90                nop

Thanks
Laszlo