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Subject: Re: [PATCH 15/24] x86/mm: Allow flushing for future ASID switches
To: Peter Zijlstra <peterz@infradead.org>
References: <20171127104923.14378-1-mingo@kernel.org>
 <20171127104923.14378-16-mingo@kernel.org>
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Cc: Ingo Molnar <mingo@kernel.org>, linux-kernel@vger.kernel.org,
        Andy Lutomirski <luto@amacapital.net>,
        Thomas Gleixner <tglx@linutronix.de>,
        "H . Peter Anvin" <hpa@zytor.com>, Borislav Petkov <bp@alien8.de>,
        Linus Torvalds <torvalds@linux-foundation.org>
From: Dave Hansen <dave.hansen@linux.intel.com>
Message-ID: <fd5bc34b-2bd2-13e2-b7e0-cf14e1c899f4@linux.intel.com>
Date: Thu, 30 Nov 2017 10:44:40 -0800
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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.