linux-kernel - Re: [PATCH 15/24] x86/mm: Allow flushing for future ASID switches

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Date:   Thu, 30 Nov 2017 19:55:10 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Dave Hansen <dave.hansen@...ux.intel.com>
Cc:     Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org,
        Andy Lutomirski <luto@...capital.net>,
        Thomas Gleixner <tglx@...utronix.de>,
        "H . Peter Anvin" <hpa@...or.com>, Borislav Petkov <bp@...en8.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 15/24] x86/mm: Allow flushing for future ASID switches

On Thu, Nov 30, 2017 at 10:44:40AM -0800, Dave Hansen wrote:
> On 11/30/2017 08:18 AM, Peter Zijlstra wrote:
> > On Thu, Nov 30, 2017 at 07:51:17AM -0800, Dave Hansen wrote:
> >> On 11/30/2017 07:44 AM, Peter Zijlstra wrote:
> >>> On Mon, Nov 27, 2017 at 11:49:14AM +0100, Ingo Molnar wrote:
> >>>> @@ -338,24 +366,23 @@ static inline void __native_flush_tlb_single(unsigned long addr)
> >>>>  
> >>>>  static inline void __flush_tlb_all(void)
> >>>>  {
> >>>> +	if (boot_cpu_has(X86_FEATURE_PGE)) {
> >>>>  		__flush_tlb_global();
> >>>> +	} else {
> >>>>  		__flush_tlb();
> >>>> +		tlb_flush_shared_nonglobals();
> >>> I do however think this one is superfluous; if we do not have PGE we
> >>> also do not have PCID and every CR3 switch flushes everything.
> >>
> >> I tried to sprinkle these around at all the sites that did non-global
> >> kernel flushes.  In the case that it's superfluous !KAISER, it's a noop
> >> anyway.  In the (currently unsupported) case that we *do* need it, well,
> >> we need it.
> > 
> > I'm confused. When would we need it there?
> 
> __flush_tlb() does a flushing CR3 write that flushes the current PCID.
> If we need other PCIDs flushed, we have to do it via the
> tlb_flush_shared_nonglobals() mechanism.

But the thing is, you _cannot_ have PCID enabled in that branch.

> Does it matter today in practice?  Nope, we never have that situation.
> But, it also doesn't _hurt_ to have that line there in any way.

Well, it confused the heck out of me.