linux-kernel - Re: [PATCH] arm64/mm: Fix __enable_mmu() for new TGRAN range values

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Date:   Mon, 8 Mar 2021 15:03:20 +0000
From:   Mark Rutland <mark.rutland@....com>
To:     Will Deacon <will@...nel.org>
Cc:     Anshuman Khandual <anshuman.khandual@....com>,
        linux-arm-kernel@...ts.infradead.org,
        James Morse <james.morse@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Marc Zyngier <maz@...nel.org>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Ard Biesheuvel <ardb@...nel.org>, kvmarm@...ts.cs.columbia.edu,
        linux-efi@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] arm64/mm: Fix __enable_mmu() for new TGRAN range values

On Mon, Mar 08, 2021 at 01:30:53PM +0000, Will Deacon wrote:
> On Sun, Mar 07, 2021 at 05:24:21PM +0530, Anshuman Khandual wrote:
> > On 3/5/21 8:21 PM, Mark Rutland wrote:
> > > On Fri, Mar 05, 2021 at 08:06:09PM +0530, Anshuman Khandual wrote:

> > >> +#define ID_AA64MMFR0_TGRAN_2_SUPPORTED_DEFAULT	0x0
> > >> +#define ID_AA64MMFR0_TGRAN_2_SUPPORTED_NONE	0x1
> > >> +#define ID_AA64MMFR0_TGRAN_2_SUPPORTED_MIN	0x2
> > >> +#define ID_AA64MMFR0_TGRAN_2_SUPPORTED_MAX	0x7
> > >
> > > The TGRAN2 fields doesn't quite follow the usual ID scheme rules, so how
> > > do we deteremine the max value? Does the ARM ARM say anything in
> > > particular about them, like we do for some of the PMU ID fields?
> > 
> > Did not find anything in ARM ARM, regarding what scheme TGRAN2 fields
> > actually follow. I had arrived at more restrictive 0x7 value, like the
> > usual signed fields as the TGRAN4 fields definitely do not follow the
> > unsigned ID scheme. Would restricting max value to 0x3 (i.e LPA2) be a
> > better option instead ?
> 
> I don't think it helps much, as TGRAN64_2 doesn't even define 0x3.
> 
> So I think this patch is probably the best we can do, but the Arm ARM could
> really do with describing the scheme here.

I agree, and I've filed a ticket internally to try to get this cleaned
up.

I suspect that the answer is that these are basically unsigned, with
0x2-0xf indicating presence, but I can't guarantee that.

Thanks,
Mark.