linux-kernel - Re: [PATCH 04/23] x86, tlb: make CR4-based TLB flushes more robust

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Date:   Wed, 1 Nov 2017 22:25:26 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Dave Hansen <dave.hansen@...ux.intel.com>
cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        moritz.lipp@...k.tugraz.at, daniel.gruss@...k.tugraz.at,
        michael.schwarz@...k.tugraz.at, luto@...nel.org,
        torvalds@...ux-foundation.org, keescook@...gle.com,
        hughd@...gle.com, x86@...nel.org
Subject: Re: [PATCH 04/23] x86, tlb: make CR4-based TLB flushes more robust

On Tue, 31 Oct 2017, Dave Hansen wrote:
> Our CR4-based TLB flush currently requries global pages to be
> supported *and* enabled.  But, we really only need for them to be
> supported.  Make the code more robust by alllowing X86_CR4_PGE to
> clear as well as set.

That's not what the patch is actually doing.

>  	cr4 = this_cpu_read(cpu_tlbstate.cr4);
> -	/* clear PGE */
> -	native_write_cr4(cr4 & ~X86_CR4_PGE);
> -	/* write old PGE again and flush TLBs */
> +	/*
> +	 * This function is only called on systems that support X86_CR4_PGE
> +	 * and where always set X86_CR4_PGE.  Warn if we are called without
> +	 * PGE set.
> +	 */
> +	WARN_ON_ONCE(!(cr4 & X86_CR4_PGE));

Because if CR4_PGE is not set, this warning triggers. So this defeats the
toggle mode you are implementing.

> +	/*
> +	 * Architecturally, any _change_ to X86_CR4_PGE will fully flush the
> +	 * TLB of all entries including all entries in all PCIDs and all
> +	 * global pages.  Make sure that we _change_ the bit, regardless of
> +	 * whether we had X86_CR4_PGE set in the first place.
> +	 */
> +	native_write_cr4(cr4 ^ X86_CR4_PGE);
> +	/* Put original CR3 value back: */

That want's to be CR4. Restoring CR3 to CR4 might be suboptimal.

>  	native_write_cr4(cr4);

Thanks,

	tglx