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

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Date:   Thu, 30 Nov 2017 10:48:56 -0800
From:   Andy Lutomirski <luto@...capital.net>
To:     Dave Hansen <dave.hansen@...ux.intel.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        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 AM, Dave Hansen
<dave.hansen@...ux.intel.com> 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.
>
> 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.

Should it be tlb_flush_shared_nonglobals_if_kernel_and_user_pagetables_are_separate()?

The whole idea that we can get away with ambiguous functions like
__flush_tlb() seems to be much less true with KAISER.  I think we
should maybe start getting rid of overly vague functions like this.