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Date: Mon, 15 Aug 2016 12:56:58 +0200
From: Ard Biesheuvel <>
To: Catalin Marinas <>
Cc: Will Deacon <>, Mark Rutland <>, 
	Kees Cook <>,, 
	Julien Grall <>, James Morse <>, 
	"" <>
Subject: Re: [PATCH 0/7] arm64: Privileged Access Never
 using TTBR0_EL1 switching

On 15 August 2016 at 12:52, Catalin Marinas <> wrote:
> On Mon, Aug 15, 2016 at 12:43:31PM +0200, Ard Biesheuvel wrote:
>> But, how about we store the reserved ASID in TTBR1_EL1 instead, and
>> switch TCR_EL1.A1 and TCR_EL1.EPD0 in a single write? That way, we can
>> switch ASIDs and disable table walks atomically (I hope), and we
>> wouldn't need to change TTBR0_EL1 at all.
> I did this before for AArch32 + LPAE (patches on the list sometime last
> year I think). But the idea was nak'ed by the ARM architects. The
> TCR_EL1.A1 can be cached somewhere in the TLB state machine, so you need
> TLBI (IOW, toggling A1 does not guarantee an ASID switch).

But how is TTBR0_EL1 any different? The ARM ARM equally mentions that
any of its field can be cached in a TLB, so by that reasoning, setting
a new ASID in TTBR0_EL1 would also require TLB maintenance.

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