On Tue, Nov 06, 2018 at 12:13:18PM +0000, Suzuki K Poulose wrote:
>
>
> On 06/11/2018 11:45, Christoffer Dall wrote:
> >On Tue, Nov 06, 2018 at 09:52:59AM +0000, Suzuki K Poulose wrote:
> >>
> >>
> >>On 06/11/2018 08:42, Christoffer Dall wrote:
> >>>On Mon, Nov 05, 2018 at 03:00:34PM +0000, Suzuki K Poulose wrote:
> >>>>
> >>>>
> >>>>On 02/11/18 14:25, Christoffer Dall wrote:
> >>>>>On Fri, Nov 02, 2018 at 11:02:38AM +0000, Suzuki K Poulose wrote:
> >>>>>>Hi
> >>>>>>
> >>>>>>On 02/11/18 07:53, Christoffer Dall wrote:
> >>>>>>>In attempting to re-construct the logic for our stage 2 page table
> >>>>>>>layout I found the reaoning in the comment explaining how we calculate
> >>>>>>>the number of levels used for stage 2 page tables a bit backwards.
> >>>>>>>
> >>>>>>>This commit attempts to clarify the comment, to make it slightly easier
> >>>>>>>to read without having the Arm ARM open on the right page.
> >>>>>>>
> >>>>>>>While we're at it, fixup a typo in a comment that was recently changed.
> >>>>>>>
> >>>>>>>Signed-off-by: Christoffer Dall <christoffer.dall@xxxxxxx>
> >>>>>>>---
> >>>>>>> arch/arm64/include/asm/stage2_pgtable.h | 17 ++++++++++-------
> >>>>>>> virt/kvm/arm/mmu.c | 2 +-
> >>>>>>> 2 files changed, 11 insertions(+), 8 deletions(-)
> >>>>>>>
> >>>>>>>diff --git a/arch/arm64/include/asm/stage2_pgtable.h b/arch/arm64/include/asm/stage2_pgtable.h
> >>>>>>>index d352f6df8d2c..9c387320b28c 100644
> >>>>>>>--- a/arch/arm64/include/asm/stage2_pgtable.h
> >>>>>>>+++ b/arch/arm64/include/asm/stage2_pgtable.h
> >>>>>>>@@ -31,15 +31,18 @@
> >>>>>>> /*
> >>>>>>> * The hardware supports concatenation of up to 16 tables at stage2 entry level
> >>>>>>>- * and we use the feature whenever possible.
> >>>>>>>+ * and we use the feature whenever possible, which means we resolve 4 bits of
> >>>>>>
> >>>>>>s/we resolve 4 bits/we resolve 4 additional bits/ ?
> >>>>>>
> >>>>>
> >>>>>yes
> >>>>>
> >>>>>>>+ * address at the entry level.
> >>>>>>> *
> >>>>>>>- * Now, the minimum number of bits resolved at any level is (PAGE_SHIFT - 3).
> >>>>>>>+ * This implies, the total number of page table levels required for
> >>>>>>>+ * IPA_SHIFT at stage2 expected by the hardware can be calculated using
> >>>>>>>+ * the same logic used for the (non-collapsable) stage1 page tables but for
> >>>>>>>+ * (IPA_SHIFT - 4).
> >>>>>>>+ *
> >>>>>>>+ * Note, the minimum number of bits resolved at any level is (PAGE_SHIFT - 3).
> >>>>>>
> >>>>>>May be we could improve it further by :
> >>>>>>
> >>>>>>s/resolved at any level/resolved at any *non-entry* level/
> >>>>>>
> >>>>>>as, we could resolve as small as 1 bit at the entry level.
> >>>>>>
> >>>>>>
> >>>>>
> >>>>>yes
> >>>>>
> >>>>>>> * On arm64, the smallest PAGE_SIZE supported is 4k, which means
> >>>>>>>- * (PAGE_SHIFT - 3) > 4 holds for all page sizes.
> >>>>>>>- * This implies, the total number of page table levels at stage2 expected
> >>>>>>>- * by the hardware is actually the number of levels required for (IPA_SHIFT - 4)
> >>>>>>>- * in normal translations(e.g, stage1), since we cannot have another level in
> >>>>>>>- * the range (IPA_SHIFT, IPA_SHIFT - 4).
> >>>>>>>+ * (PAGE_SHIFT - 3) > 4 holds for all page sizes
> >>>>>>>+ * and therefore we will need a minimum of two levels for stage2 in all cases.
> >>>>>>
> >>>>>>I think the above statement is misleading. The minimum number of
> >>>>>>levels has nothing to do with the concatenation.
> >>>>>
> >>>>>Architecturally surely it does? (The point of concatenation is to
> >>>>>reduce the minimal number of levels required.)
> >>>>>
> >>>>>Maybe you mean the minimum number of levels imposed by KVM here?
> >>>>>
> >>>>>>For e.g, we could
> >>>>>>still create a stage2 with 1 level (32bit IPA on 64K, 29bit + 3bit
> >>>>>>concatenated), going by the same rules above. The only reason why
> >>>>>>we limit the number of levels to 2, is to prevent splitting stage1 PMD
> >>>>>>huge mappings (which are quite common) at stage2.
> >>>>>>
> >>>>>
> >>>>>So I wasn't entirely clear what the comment was trying to say with the
> >>>>>"(PAGE_SHIFT - 3) > 4 holds for all page size" statement, so I though
> >>>>>that was there to show that we'll need a minimum of two levels, but
> >>>>>maybe that was written under the assumption of the limitations of
> >>>>>IPA_SHIFT (was KVM_PHYS_SIZE).
> >>>>
> >>>>See below.
> >>>>
> >>>>>
> >>>>>Since you wrote the original comment, and I couldn't correctly parse
> >>>>>that, and I apparently still didn't fully understand, can you suggest an
> >>>>>alternative wording?
> >>>>
> >>>>I think trying to over explain the concept has created more confusion.
> >>>>The whole paragraph is trying to prove that we only need :
> >>>>
> >>>> ARM64_HW_PGTABLE_LEVELS(IPA_SHIFT - 4)
> >>>>to map IPA_SHIFT bits at stage2 with maximum utilization of the
> >>>>concatenation at entry level.
> >>>
> >>>Yes, I think that comes across more clearly in my rewording up to the
> >>>"Note, ...".
> >>
> >>Yes it does and I prefer your version than mine.
> >>
> >>>
> >>>>
> >>>>Right now the comment tries to establish it via the hard route, by
> >>>>proving that there cannot be an intermediate level in the range
> >>>>[IPA_SHIFT, IPA_SHIFT - 4]. Or in other words :
> >>>>
> >>>>(ARM64_HW_PGTABLE_LEVELS(IPA_SHIFT - 4) + 1) >=
> >>>> ARM64_HW_PGTABLE_LEVELS(IPA_SHIFT)
> >>>>
> >>>>I don't know if it is worth the explanation and then causing further
> >>>>confusion.
> >>>
> >>>I think you're then actually trying to explain two things.
> >>>
> >>>First, we can get the number of levels by using the stage 1 calculation
> >>>and adjust for concatenation by subtracting 4 from the number of bits we
> >>>need to translate.
> >>>
> >>>Second, some further reasoning about *why* that is true.
> >>>
> >>>It remains unclear to me exactly what your point about
> >>> '(PAGE_SHIFT - 3) > 4'
> >>>is and how that supports the second point. Also I'm not entirely sure
> >>>why we need that.
> >>
> >>So, we have to prove that :
> >> x + 1 >= y, where :
> >
> >Why do we have to prove that?
>
> To make sure that we don't do concatenation at a level which is not
> architecturally correct and end up concatenating more tables than
> the architecture supports. i.e, if we were to go down two levels
> below, we would be concatenating more than 16 tables at the entry
> level and thus doing something not supported by the architecture.
>
> We need to be sure about the correctness of the logic, as we don't
> explicitly "check" how many tables are concatenated for any input
> IPA_SHIFT+Page_Size combination in the stage2 page table code.
>
> I guess, it is simpler to think of it the "non-formal" way. i.e,
> we could at the maximum "not-resolve" 4 bits using a normal table
> with Levels(IPA_SHIFT - 4).
>
Yes :)
v2 on its way.
Thanks,
Christoffer
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