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

[Peter Zijlstra <peterz@infradead.org>]

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?

> +
> +	on_each_cpu_mask(mm_cpumask(mm), enable_lam_func, mm, true);
> +out:
> +	mutex_unlock(&mm->context.lock);
> +	return ret;
> +}