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

[Laszlo Ersek <lersek@redhat.com>]

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.)

Thanks
Laszlo