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

[Andrew Fish <afish@apple.com>]

> 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?

Thanks,

Andrew Fish

> Thanks
> Laszlo
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