Re: [edk2-devel] [RFC PATCH 01/19] OvmfPkg: Reserve the Secrets and Cpuid page for the SEV-SNP guest

["Min Xu" <min.m.xu@intel.com>]

On Wednesday, April 7, 2021 11:03 PM, Laszlo wrote:
> 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:

Yes this is the root cause. TDX requires the startup mode to be 32-bit
protected mode while the legacy VM startup mode is 16-bit real mode.
Add more BITS directives can work round this. I have tried it and it works.

So my initial solution is to use *jmp rel8* because it works in both 16-bit
and 32-bit mode. But *jmp rel8* depends on the distance which should
be less than 128 bytes. If more metadata is added in the ResetVector.asm
then we have to use the BITS solution. 
 
> 
> > ; 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
> 
> 
> 
> 
>