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

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Message-ID: <20140925145126.GA4434@gmail.com>
Date:	Thu, 25 Sep 2014 16:51:26 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Bryan O'Donoghue <pure.logic@...us-software.ie>
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()


* Bryan O'Donoghue <pure.logic@...us-software.ie> wrote:

> >>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.

So why is the comment in setup_arch() not talking about that?

> 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();
> }

I've zapped that commit for the time being, because I think that 
at minimum the comment is misleading - it says nothing about this 
being an early quirk and that normally __flush_tlb_all() will 
DTRT.

It talks about:

+       /*
+        * Locate the page directory and flush the TLB.
+        * On Quark X1000 rewriting CR3 flushes the TLB no if/else is required
+        * to choose between __flush_tlb() and __flush_tlb_all()
+        */
        load_cr3(swapper_pg_dir);
        __flush_tlb_all();

But it is completely silent on the real reason for why we don't 
need a Quark quirk here, which would be something like:

	/*
	 * On Quark CPUs we still have the PGE bit set so 
	 * __flush_tlb_all() is not yet doing what it says - but 
	 * accidentally we have a cr3 flush here which is what is 
	 * needed - so there's no need to add a Quark quirk here.
	 */

Right?

Thanks,

	Ingo
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