Re: [RFC 09/13] x86/mm: Disable interrupts when flushing the TLB using CR3

From: Linus Torvalds
Date: Fri Jan 08 2016 - 18:41:16 EST


On Fri, Jan 8, 2016 at 3:15 PM, Andy Lutomirski <luto@xxxxxxxxxx> wrote:
> + /*
> + * We mustn't be preempted or handle an IPI while reading and
> + * writing CR3. Preemption could switch mms and switch back, and
> + * an IPI could call leave_mm. Either of those could cause our
> + * PCID to change asynchronously.
> + */
> + raw_local_irq_save(flags);
> native_write_cr3(native_read_cr3());
> + raw_local_irq_restore(flags);

This seems sad for two reasons:

- it adds unnecessary overhead on non-pcid setups (32-bit being an
example of that)

- on pcid setups, wouldn't invpcid_flush_single_context() be better?

So on the whole I hate it.

Why isn't this something like

if (static_cpu_has_safe(X86_FEATURE_INVPCID)) {
invpcid_flush_single_context();
return;
}
native_write_cr3(native_read_cr3());

*without* any flag saving crud?

And yes, that means that we'd require X86_FEATURE_INVPCID in order to
use X86_FEATURE_PCID, but that seems fine.

Or is there some reason you wanted the odd flags version? If so, that
should be documented.

Linus