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

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Date:	Fri, 8 Jan 2016 16:18:08 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Oleg Nesterov <oleg@...hat.com>, X86 ML <x86@...nel.org>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Borislav Petkov <bp@...en8.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	Brian Gerst <brgerst@...il.com>
Subject: Re: [RFC 09/13] x86/mm: Disable interrupts when flushing the TLB
 using CR3

On Jan 8, 2016 3:41 PM, "Linus Torvalds" <torvalds@...ux-foundation.org> wrote:
>
> On Fri, Jan 8, 2016 at 3:15 PM, Andy Lutomirski <luto@...nel.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