linux-kernel - Re: [PATCH] x86: Quark: Flush TLB via CR3 not CR4.PGE in setup

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Message-ID: <5423E1F7.5010000@nexus-software.ie>
Date:	Thu, 25 Sep 2014 10:35:51 +0100
From:	Bryan O'Donoghue <pure.logic@...us-software.ie>
To:	Ingo Molnar <mingo@...nel.org>
CC:	hpa@...or.com, mingo@...hat.com, tglx@...utronix.de,
	x86@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86: Quark: Flush TLB via CR3 not CR4.PGE in setup_arch()

>> diff --git a/arch/x86/kernel/setup.c b/arch/x86/kernel/setup.c
>> index 41ead8d..1d2396a 100644
>> --- a/arch/x86/kernel/setup.c
>> +++ b/arch/x86/kernel/setup.c
>> @@ -879,7 +879,10 @@ void __init setup_arch(char **cmdline_p)
>>   			KERNEL_PGD_PTRS);
>>
>>   	load_cr3(swapper_pg_dir);
>> -	__flush_tlb_all();
>> +	if (boot_cpu_data.x86 == 5 && boot_cpu_data.x86_model == 9)
>> +		__flush_tlb();
>> +	else
>> +		__flush_tlb_all();
>
> So why not make __flush_tlb_all() Quark-quirk-aware and be done
> with it, instead of having to validate every single
> __flush_tlb_all() user?
>
> Quark breaks the x86 'flush all TLBs' semantics - the way to fix
> it is to restore those semantics, not to sprinkle the breakage
> all around the code ...

Hi Ingo.

We have made __flush_tlb_all() Quark aware - because the previous patch 
we applied to arch/x86/kernel/cpu/intel.c

ee1b5b165c0a2f04d2107e634e51f05d0eb107de

+	if (c->x86 == 5 && c->x86_model == 9) {
+		pr_info("Disabling PGE capability bit\n");
+		setup_clear_cpu_cap(X86_FEATURE_PGE);
+	}

will cause cpu_has_pge() to be false and then the flush_tlb_all code 
will take the path we want __flush_tlb not __flush_tlb_global

static inline void __flush_tlb_all(void)
{
         if (cpu_has_pge)
                 __flush_tlb_global();
         else
                 __flush_tlb();
}

The code in setup_arch runs before the cpu_has_pge bit been clobbered.

The commit you did 02276a3a677d681f0cd227d7111c71fdbce23832 just adds a 
comment to setup_arch to indicate the behaviour we are relying on

setup_arch()
{

load_cr3(swapper_pg_dir);	/* this will flush the TLB on Quark */
__flush_tlb_all();		/*cpu_has_pge() is true at this point*/

......

/* this is where we latch the cpu cabability bits */
early_cpu_init();
}

--
BOD
--
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