Re: [RFCv2 04/10] x86/mm: Introduce X86_THREAD_LAM_U48 and X86_THREAD_LAM_U57

Peter Zijlstra
2022-05-12T14:37:28Z

On Thu, May 12, 2022 at 02:24:22PM +0200, Thomas Gleixner wrote:
> On Wed, May 11 2022 at 09:02, Peter Zijlstra wrote:
> > On Wed, May 11, 2022 at 05:27:45AM +0300, Kirill A. Shutemov wrote:
> >
> >> +#define LAM_NONE 0
> >> +#define LAM_U57 1
> >> +#define LAM_U48 2
> >
> >> +#define X86_THREAD_LAM_U48 0x1
> >> +#define X86_THREAD_LAM_U57 0x2
> >
> > Seriously pick an order and stick with it. I would suggest keeping the
> > hardware order and then you can do:
> >
> >> +static inline unsigned long lam_to_cr3(u8 lam)
> >> +{
> >
> > return (lam & 0x3) << X86_CR3_LAM_U57;
>
> This "works" because the hardware ignores LAM_U48 if LAM_U57 is set, but
> I'd rather make that exclusive in the prctl() as setting both does not
> make any sense.

Yes, I was very much assuming the interface would not allow setting
both, that would be daft.