Re: [PATCH 1/3] UefiCpuPkg/PiSmmCpuDxeSmm: update comments in IA32 SmmStartup()

[Laszlo Ersek <lersek@redhat.com>]

On 01/30/18 22:26, Kinney, Michael D wrote:
> Laszlo,
> 
> Maybe we can add a macro to help:
> 
> #define PATCH_X86_ASM(Label,Type,Value)  *((Type *)(&Label) - 1)  = (Type)(Value)
> 
>   PATCH_X86_ASM (gSmmCr0, UINT32, AsmReadCr0());

Before sending my previous email, I thought of something like this. Here
I slightly dislike:

- the unaligned access to a wider-than-byte object

- the larger potential for triggering undefined behavior in the C
  language implementation (due to type punning) -- doing the arithmetic
  in UINTN and using CopyMem for the data movement should mitigate that

- the requirement that the object to patch has to be followed by the
  same number of bytes (otherwise linking might fail, I think). For
  example if we'd like to patch 4 bytes at the very end of the assembly
  code, e.g. in a jmp instruction, we'd have to match that with a DD,
  just so the extern UINT32 global variable could be declared and
  linked. I think requiring just one byte trailing padding and making
  all such markers UINT8 is easier to handle

- the requirement that the declared type of "gSmmCr0" match the type
  passed to the macro as 2nd argument

That said, I'm fine using either of PATCH_X86_ASM() and PatchAssembly().

Thanks!
Laszlo

> 
> Mike
> 
>> -----Original Message-----
>> From: Kinney, Michael D
>> Sent: Tuesday, January 30, 2018 12:31 PM
>> To: Laszlo Ersek <lersek@redhat.com>; edk2-devel-01
>> <edk2-devel@lists.01.org>; Kinney, Michael D
>> <michael.d.kinney@intel.com>
>> Cc: Ni, Ruiyu <ruiyu.ni@intel.com>; Paolo Bonzini
>> <pbonzini@redhat.com>; Yao, Jiewen
>> <jiewen.yao@intel.com>; Dong, Eric <eric.dong@intel.com>
>> Subject: RE: [edk2] [PATCH 1/3]
>> UefiCpuPkg/PiSmmCpuDxeSmm: update comments in IA32
>> SmmStartup()
>>
>> Laszlo,
>>
>> We have already used this technique in other NASM files
>> to remove DBs.
>>
>> Let us know if you have suggestions on how to make the
>> C code that performs the patches easier to read and
>> maintain.
>>
>> Mike
>>
>>> -----Original Message-----
>>> From: edk2-devel [mailto:edk2-devel-
>> bounces@lists.01.org]
>>> On Behalf Of Laszlo Ersek
>>> Sent: Tuesday, January 30, 2018 10:17 AM
>>> To: Kinney, Michael D <michael.d.kinney@intel.com>;
>> edk2-
>>> devel-01 <edk2-devel@lists.01.org>
>>> Cc: Ni, Ruiyu <ruiyu.ni@intel.com>; Paolo Bonzini
>>> <pbonzini@redhat.com>; Yao, Jiewen
>>> <jiewen.yao@intel.com>; Dong, Eric
>> <eric.dong@intel.com>
>>> Subject: Re: [edk2] [PATCH 1/3]
>>> UefiCpuPkg/PiSmmCpuDxeSmm: update comments in IA32
>>> SmmStartup()
>>>
>>> On 01/30/18 18:22, Kinney, Michael D wrote:
>>>> Laszlo,
>>>>
>>>> The DBs can be removed if the label is moved after
>>>> the instruction and the patch is done to the label
>>>> minus the size of the patch value.
>>>
>>> Indeed I haven't thought of this.
>>>
>>> If I understand correctly, it means
>>>
>>>   extern UINT8 gSmmCr0;
>>>
>>>   *(UINT32*)(&gSmmCr0 - sizeof (UINT32)) =
>>> (UINT32)AsmReadCr0 ();
>>>
>>> TBH, the DB feels less ugly to me than this :)
>>>
>>> Still, if you think it would be an acceptable price to
>>> pay for removing
>>> the remaining DBs, I can respin.
>>>
>>> Thanks
>>> Laszlo
>>>
>>>>> -----Original Message-----
>>>>> From: edk2-devel [mailto:edk2-devel-
>>> bounces@lists.01.org]
>>>>> On Behalf Of Laszlo Ersek
>>>>> Sent: Tuesday, January 30, 2018 7:34 AM
>>>>> To: edk2-devel-01 <edk2-devel@lists.01.org>
>>>>> Cc: Ni, Ruiyu <ruiyu.ni@intel.com>; Yao, Jiewen
>>>>> <jiewen.yao@intel.com>; Dong, Eric
>>> <eric.dong@intel.com>;
>>>>> Paolo Bonzini <pbonzini@redhat.com>
>>>>> Subject: [edk2] [PATCH 1/3]
>> UefiCpuPkg/PiSmmCpuDxeSmm:
>>>>> update comments in IA32 SmmStartup()
>>>>>
>>>>> The gSmmCr3, gSmmCr4, gSmmCr0 and gSmmJmpAddr global
>>>>> variables  are used
>>>>> for patching assembly instructions, thus we can
>> never
>>>>> remove the DB
>>>>> encodings for those instructions. At least we should
>>> add
>>>>> the intended
>>>>> meanings in comments.
>>>>>
>>>>> This patch only changes comments.
>>>>>
>>>>> Cc: Eric Dong <eric.dong@intel.com>
>>>>> Cc: Jian J Wang <jian.j.wang@intel.com>
>>>>> Cc: Jiewen Yao <jiewen.yao@intel.com>
>>>>> Cc: Paolo Bonzini <pbonzini@redhat.com>
>>>>> Cc: Ruiyu Ni <ruiyu.ni@intel.com>
>>>>> Contributed-under: TianoCore Contribution Agreement
>>> 1.1
>>>>> Signed-off-by: Laszlo Ersek <lersek@redhat.com>
>>>>> ---
>>>>>  UefiCpuPkg/PiSmmCpuDxeSmm/Ia32/SmmInit.nasm | 8
>> ++++-
>>> ---
>>>>>  1 file changed, 4 insertions(+), 4 deletions(-)
>>>>>
>>>>> diff --git
>>> a/UefiCpuPkg/PiSmmCpuDxeSmm/Ia32/SmmInit.nasm
>>>>> b/UefiCpuPkg/PiSmmCpuDxeSmm/Ia32/SmmInit.nasm
>>>>> index e96dd8d2392a..08534dba64b7 100644
>>>>> --- a/UefiCpuPkg/PiSmmCpuDxeSmm/Ia32/SmmInit.nasm
>>>>> +++ b/UefiCpuPkg/PiSmmCpuDxeSmm/Ia32/SmmInit.nasm
>>>>> @@ -44,34 +44,34 @@ global ASM_PFX(SmmStartup)
>>>>>  ASM_PFX(SmmStartup):
>>>>>      DB      0x66
>>>>>      mov     eax, 0x80000001             ; read
>>>>> capability
>>>>>      cpuid
>>>>>      DB      0x66
>>>>>      mov     ebx, edx                    ; rdmsr
>> will
>>>>> change edx. keep it in ebx.
>>>>> -    DB      0x66, 0xb8
>>>>> +    DB      0x66, 0xb8                  ; mov eax,
>>> imm32
>>>>>  ASM_PFX(gSmmCr3): DD 0
>>>>>      mov     cr3, eax
>>>>>      DB      0x67, 0x66
>>>>>      lgdt    [cs:ebp + (ASM_PFX(gcSmiInitGdtr) -
>>>>> ASM_PFX(SmmStartup))]
>>>>> -    DB      0x66, 0xb8
>>>>> +    DB      0x66, 0xb8                  ; mov eax,
>>> imm32
>>>>>  ASM_PFX(gSmmCr4): DD 0
>>>>>      mov     cr4, eax
>>>>>      DB      0x66
>>>>>      mov     ecx, 0xc0000080             ; IA32_EFER
>>> MSR
>>>>>      rdmsr
>>>>>      DB      0x66
>>>>>      test    ebx, BIT20                  ; check NXE
>>>>> capability
>>>>>      jz      .1
>>>>>      or      ah, BIT3                    ; set NXE
>> bit
>>>>>      wrmsr
>>>>>  .1:
>>>>> -    DB      0x66, 0xb8
>>>>> +    DB      0x66, 0xb8                  ; mov eax,
>>> imm32
>>>>>  ASM_PFX(gSmmCr0): DD 0
>>>>>      DB      0xbf, PROTECT_MODE_DS, 0    ; mov di,
>>>>> PROTECT_MODE_DS
>>>>>      mov     cr0, eax
>>>>> -    DB      0x66, 0xea                   ; jmp far
>>>>> [ptr48]
>>>>> +    DB      0x66, 0xea                  ; jmp far
>>>>> [ptr48]
>>>>>  ASM_PFX(gSmmJmpAddr):
>>>>>      DD      @32bit
>>>>>      DW      PROTECT_MODE_CS
>>>>>  @32bit:
>>>>>      mov     ds, edi
>>>>>      mov     es, edi
>>>>> --
>>>>> 2.14.1.3.gb7cf6e02401b
>>>>>
>>>>>
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>>>
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