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

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Message-ID: <CALCETrWmPoLWPeVqEPf5_s1PRL_RgOQ_AS6HHWdWkmTC7nQ3jQ@mail.gmail.com>
Date:	Wed, 13 Jan 2016 16:34:37 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	Dave Hansen <dave.hansen@...ux.intel.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Oleg Nesterov <oleg@...hat.com>, X86 ML <x86@...nel.org>,
	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 Wed, Jan 13, 2016 at 3:56 PM, Dave Hansen
<dave.hansen@...ux.intel.com> wrote:
> On 01/13/2016 03:51 PM, Andy Lutomirski wrote:
>> On Wed, Jan 13, 2016 at 3:43 PM, Dave Hansen
>> <dave.hansen@...ux.intel.com> wrote:
>>> On 01/13/2016 03:35 PM, Andy Lutomirski wrote:
>>>> Can anyone here ask a hardware or microcode person what's going on
>>>> with CR3 writes possibly being faster than INVPCID?  Is there some
>>>> trick to it?
>>>
>>> I just went and measured it myself this morning.  "INVPCID Type 3" (all
>>> contexts no global) on a Skylake system was 15% slower than a CR3 write.
>>>
>>> Is that in the same ballpark from what you've observed?
>>
>> It's similar, except that I was comparing "INVPCID Type 1" (single
>> context no globals) to a CR3 write.
>
> Ahh, because you're using PCID...  That one I saw as being ~1.85x the
> number of cycles that a CR3 write was.
>

I think that settles it, then:

if (static_cpu_has_safe(X86_FEATURE_PCID)) {
  raw_local_irqsave();
  native_write_cr3(native_read_cr3());
  raw_local_irqrestore();
} else {
  native_write_cr3(native_read_cr3());
}

I don't think it's worth hacking more complexity into switch_mm to
make that annoyance go away.

--Andy