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SubjectRe: [RFC 09/13] x86/mm: Disable interrupts when flushing the TLB using CR3
On Jan 8, 2016 3:41 PM, "Linus Torvalds" <torvalds@linux-foundation.org> wrote:
>
> On Fri, Jan 8, 2016 at 3:15 PM, Andy Lutomirski <luto@kernel.org> 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)

I can certainly skip the flag saving on !PCID.

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

I played with that and it was slower. I don't pretend that makes any sense.

> 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.

I have an SNB "Extreme" with PCID but not INVPCID, and there could be
a whole generation of servers like that. I think we should fully
support them.

We might be able to get away with just disabling preemption instead of
IRQs, at least if mm == active_mm.

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

What do you mean "odd"?

--Andy

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