Messages in this thread | | | Date | Mon, 4 Oct 2021 18:23:04 +0100 | From | Will Deacon <> | Subject | Re: [PATCH V2] arm64/mm: Fix idmap on [16K|36VA|48PA] |
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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@arm.com> > > Cc: Will Deacon <will@kernel.org> > > Cc: James Morse <james.morse@arm.com> > > Cc: Marc Zyngier <maz@kernel.org> > > Cc: Mark Rutland <mark.rutland@arm.com> > > Cc: linux-arm-kernel@lists.infradead.org > > Cc: linux-kernel@vger.kernel.org > > Fixes: 215399392fe4 ("arm64: 36 bit VA") > > Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.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
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