linux-kernel - Re: [PATCH V2] arm64/mm: Fix idmap on [16K|36VA|48PA]

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Message-ID: <20211004172303.GA28223@willie-the-truck>
Date:   Mon, 4 Oct 2021 18:23:04 +0100
From:   Will Deacon <will@...nel.org>
To:     Mark Rutland <mark.rutland@....com>
Cc:     Anshuman Khandual <anshuman.khandual@....com>,
        linux-arm-kernel@...ts.infradead.org,
        Catalin Marinas <catalin.marinas@....com>,
        James Morse <james.morse@....com>,
        Marc Zyngier <maz@...nel.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH V2] arm64/mm: Fix idmap on [16K|36VA|48PA]

On Mon, Oct 04, 2021 at 11:49:58AM +0100, Mark Rutland wrote:
> Hi Anshuman,
> 
> On Tue, Sep 28, 2021 at 11:03:45AM +0530, Anshuman Khandual wrote:
> > When creating the idmap, the kernel may add one extra level to idmap memory
> > outside the VA range. But for [16K|36VA|48PA], we need two levels to reach
> > 48 bits. If the bootloader places the kernel in memory above (1 << 46), the
> > kernel will fail to enable the MMU. Although we are not aware of a platform
> > where this happens, it is worth to accommodate such scenarios and prevent a
> > possible kernel crash.
> 
> I think it's worth noting here that ARM64_VA_BITS_36 depends on EXPERT,
> so very few people are likely to be using this configuration.
> 
> > Lets fix this problem by carefully analyzing existing VA_BITS with respect
> > to maximum possible mapping with the existing PGDIR level i.e (PGDIR_SHIFT
> > + PAGE_SHIFT - 3) and then evaluating how many extra page table levels are
> > required to accommodate the reduced idmap_t0sz to map __idmap_text_end.
> > 
> > Cc: Catalin Marinas <catalin.marinas@....com>
> > Cc: Will Deacon <will@...nel.org>
> > Cc: James Morse <james.morse@....com>
> > Cc: Marc Zyngier <maz@...nel.org>
> > Cc: Mark Rutland <mark.rutland@....com>
> > Cc: linux-arm-kernel@...ts.infradead.org
> > Cc: linux-kernel@...r.kernel.org
> > Fixes: 215399392fe4 ("arm64: 36 bit VA")
> > Signed-off-by: Anshuman Khandual <anshuman.khandual@....com>
> > ---
> > This applies on v5.15-rc3.
> > 
> > This is a different approach as compared to V1 which still applies on the
> > latest mainline. Besides this enables all upcoming FEAT_LPA2 combinations
> > as well. Please do suggest which approach would be preferred.
> > - Anshuman
> > 
> >  V1:  https://lore.kernel.org/all/1627879359-30303-1-git-send-email-anshuman.khandual@arm.com/
> > RFC: https://lore.kernel.org/lkml/1627019894-14819-1-git-send-email-anshuman.khandual@arm.com/
> 
> If we need something to backport, I'm not opposed to taking one of these
> patches (and as v1 is simpler, I'd prefer that), but I think either
> approach is further bodging around the `map_memory` macro not being a
> great fit for the idmap creation, and it would be better to rework the
> structure of the pagetable creation code to do the right thing from the
> outset.
> 
> Catalin, Will, do you have any preference as to having a backportable
> fix for this?

Given that I don't think this has ever worked (i.e. this isn't a
regression), I don't see the need to backport the fix unless somebody is
running into real problems.

Will