Re: [PATCHv6 06/11] x86/mm: Provide arch_prctl() interface for LAM

[Kirill A. Shutemov]

On Mon, Aug 15, 2022 at 03:37:16PM +0200, Peter Zijlstra wrote:
> On Mon, Aug 15, 2022 at 07:17:58AM +0300, Kirill A. Shutemov wrote:
> 
> > +static void enable_lam_func(void *mm)
> > +{
> > +	struct mm_struct *loaded_mm = this_cpu_read(cpu_tlbstate.loaded_mm);
> > +	unsigned long lam_mask;
> > +	unsigned long cr3;
> > +
> > +	if (loaded_mm != mm)
> > +		return;
> > +
> > +	lam_mask = READ_ONCE(loaded_mm->context.lam_cr3_mask);
> > +
> > +	/* Update CR3 to get LAM active on the CPU */
> > +	cr3 = __read_cr3();
> > +	cr3 &= ~(X86_CR3_LAM_U48 | X86_CR3_LAM_U57);
> > +	cr3 |= lam_mask;
> > +	write_cr3(cr3);
> > +	set_tlbstate_cr3_lam_mask(lam_mask);
> > +}
> > +
> > +static int prctl_enable_tagged_addr(struct mm_struct *mm, unsigned long nr_bits)
> > +{
> > +	int ret = 0;
> > +
> > +	if (!cpu_feature_enabled(X86_FEATURE_LAM))
> > +		return -ENODEV;
> > +
> > +	mutex_lock(&mm->context.lock);
> > +
> > +	/* Already enabled? */
> > +	if (mm->context.lam_cr3_mask) {
> > +		ret = -EBUSY;
> > +		goto out;
> > +	}
> > +
> > +	if (!nr_bits) {
> > +		ret = -EINVAL;
> > +		goto out;
> > +	} else if (nr_bits <= 6) {
> > +		mm->context.lam_cr3_mask = X86_CR3_LAM_U57;
> > +		mm->context.untag_mask =  ~GENMASK(62, 57);
> > +	} else {
> > +		ret = -EINVAL;
> > +		goto out;
> > +	}
> > +
> > +	/* Make lam_cr3_mask and untag_mask visible on other CPUs */
> > +	smp_mb();
> 
> smp_mb() doesn't make visible -- it is about ordering, what does it
> order against that the below on_each_cpu_mask() doesn't already take
> care of?

You are right it is redundant. I will drop it.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov