Re: [edk2-devel] [PATCH 02/10] OvmfPkg/ResetVector: add ClearOvmfPageTables macro

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

On 2/28/24 09:22, Gerd Hoffmann wrote:
> On Wed, Feb 28, 2024 at 05:09:32AM +0100, Laszlo Ersek wrote:
>> On 2/22/24 12:54, Gerd Hoffmann wrote:
>>> Move code to clear the page tables to a nasm macro.
>>> No functional change.
>>>
>>> Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
>>> ---
>>>  OvmfPkg/ResetVector/Ia32/PageTables64.asm | 35 ++++++++++++-----------
>>>  1 file changed, 19 insertions(+), 16 deletions(-)
>>>
>>> diff --git a/OvmfPkg/ResetVector/Ia32/PageTables64.asm b/OvmfPkg/ResetVector/Ia32/PageTables64.asm
>>> index 6fec6f2beeea..378ba2feeb4f 100644
>>> --- a/OvmfPkg/ResetVector/Ia32/PageTables64.asm
>>> +++ b/OvmfPkg/ResetVector/Ia32/PageTables64.asm
>>> @@ -45,6 +45,24 @@ BITS    32
>>>  %define TDX_BSP         1
>>>  %define TDX_AP          2
>>>  
>>> +;
>>> +; For OVMF, build some initial page tables at
>>> +; PcdOvmfSecPageTablesBase - (PcdOvmfSecPageTablesBase + 0x6000).
>>> +;
>>> +; This range should match with PcdOvmfSecPageTablesSize which is
>>> +; declared in the FDF files.
>>> +;
>>> +; At the end of PEI, the pages tables will be rebuilt into a
>>> +; more permanent location by DxeIpl.
>>> +;
>>> +%macro ClearOvmfPageTables 0
>>> +    mov     ecx, 6 * 0x1000 / 4
>>> +    xor     eax, eax
>>> +.clearPageTablesMemoryLoop:
>>> +    mov     dword[ecx * 4 + PT_ADDR (0) - 4], eax
>>> +    loop    .clearPageTablesMemoryLoop
>>> +%endmacro
>>> +
>>>  ;
>>>  ; Modified:  EAX, EBX, ECX, EDX
>>>  ;
>>
>> Ah, this made me read up on local labels:
>>
>> https://www.nasm.us/xdoc/2.16.01/html/nasmdoc3.html#section-3.9
>>
>> Should we rather call the label
>>
>>   ..@clearPageTablesMemoryLoop
>>
>> ?
> 
> I've tried that and something (which I don't remember) didn't work as
> expected.  Given that each branch which uses that macro will have a jump
> label anyway (so the local label is expanded to something like
> TdxBspInit.clearPageTablesMemoryLoop) I've figured this is good enough

Sure, if you've tried it already, then the current method is fine!

Thanks!
Laszlo



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