Re: [PATCH] OvmfPkg/Sec: Clear the Cache Disable flag in the CR0 register

[Laszlo Ersek <lersek@redhat.com>]

On 02/19/19 20:51, Andrew Fish wrote:
> 
> 
>> On Feb 18, 2019, at 5:23 AM, Laszlo Ersek <lersek@redhat.com> wrote:
>>
>> generic comment (applies to all NASM usage I guess):
>>
>> On 02/18/19 11:10, Jordan Justen wrote:
>>
>>> +    mov     eax, cr0
>>> +    and     eax, ~(1 << 30)
>>> +    mov     cr0, eax
>>
>>> +    mov     rax, cr0
>>> +    and     eax, ~(1 << 30)
>>> +    mov     cr0, rax
>>
>> I've read up on the << and ~ operators in the NASM documentation, and I
>> think the above build-time calculations of the masks are well-defined
>> and correct.
>>
>> - bit shifts are always unsigned
>> - given bit position 30, ~(1 << 30) will be a value with 32 bits
>> - bit-neg simply flips bits (one's complement)
>>
>> On the other hand, I find these NASM specifics counter-intuitive. The
>> expression ~(1 << 30) looks like valid C, but in C, it means a quite
>> different thing.
>>
>> I think calculating the mask with "strict dword" somehow (not exactly
>> sure how) would make this more readable; or else the BTR instruction would.
>>
>> Opinions? (Again, pertaining to all NASM usage in edk2.)
>>
> 
> Lazlo,
> 
> I guess comments, or #defines, are other options?

Good point! They are.

Thanks,
Laszlo

> Thanks,
> 
> Andrew Fish
> 
>> Thanks
>> Laszlo
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>