Re: [PATCH 15/17] kvm: arm64: Get rid of fake page table levels

[Christoffer Dall]

On Wed, Apr 13, 2016 at 06:49:59PM +0100, Suzuki K Poulose wrote:
> On 12/04/16 13:14, Christoffer Dall wrote:
> >On Mon, Apr 11, 2016 at 03:33:45PM +0100, Suzuki K Poulose wrote:
> >>On 08/04/16 16:05, Christoffer Dall wrote:
> >>>On Mon, Apr 04, 2016 at 05:26:15PM +0100, Suzuki K Poulose wrote:
> >>
> >>>>diff --git a/arch/arm64/include/asm/stage2_pgtable.h b/arch/arm64/include/asm/stage2_pgtable.h
> >>>>index 751227d..139b4db 100644
> >>>>--- a/arch/arm64/include/asm/stage2_pgtable.h
> >>>>+++ b/arch/arm64/include/asm/stage2_pgtable.h
> >>>>@@ -22,32 +22,55 @@
> >>>>  #include <asm/pgtable.h>
> >>>>
> >>>>  /*
> >>>>- * In the case where PGDIR_SHIFT is larger than KVM_PHYS_SHIFT, we can address
> >>>>- * the entire IPA input range with a single pgd entry, and we would only need
> >>>>- * one pgd entry.  Note that in this case, the pgd is actually not used by
> >>>>- * the MMU for Stage-2 translations, but is merely a fake pgd used as a data
> >>>>- * structure for the kernel pgtable macros to work.
> >>>>+ * The hardware mandates concatenation of upto 16 tables at stage2 entry level.
> >>>
> >>>s/upto/up to/
> >>>
> >>>>+ * Now, the minimum number of bits resolved at any level is (PAGE_SHIFT - 3),
> >>>>+ * or in other words log2(PTRS_PER_PTE). On arm64, the smallest PAGE_SIZE
> >>>
> >>>not sure the log2 comment helps here.
> >>
> >>OK, will address both the above comments.
> >>
> >>>
> >>>>+ * 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 (KVM_PHYS_SHIFT - 4)
> >>>>+ * in normal translations(e.g, stage-1), since we cannot have another level in
> >>>>+ * the range (KVM_PHYS_SHIFT, KVM_PHYS_SHIFT - 4).
> >>>
> >>>Is it not a design decision to always choose the maximum number of
> >>>concatinated initial-level stage2 tables (with the constraint that
> >>>there's a minimum number required)?
> >>
> 
> I have changed the above comment to :
> 
> /*
>  * The hardware supports concatenation of up to 16 tables at stage2 entry level
>  * and we use the feature whenever possible.
>  *
>  * Now, the minimum number of bits resolved at any level is (PAGE_SHIFT - 3).
>  * 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 (KVM_PHYS_SHIFT - 4)
>  * in normal translations(e.g, stage1), since we cannot have another level in
>  * the range (KVM_PHYS_SHIFT, KVM_PHYS_SHIFT - 4).
>  */
> 
> 

Looks great.

> 
> >>>>+ * At the moment, we do not support a combination of guest IPA and host VA_BITS
> >>>>+ * where
> >>>>+ *       STAGE2_PGTABLE_LEVELS > CONFIG_PGTABLE_LEVELS
> >>>
> >>>can you change this comment to reverse the statement to avoid someone
> >>>seeing this as a constraint, when in fact it's a negative invariant?
> >>>
> >>>So the case we don't support is a sufficiently larger IPA space compared
> >>>to the host VA space such that the above happens?  (Since at the same
> >>>IPA space size as host VA space size, the stage-2 levels will always be
> >>>less than or equal to the host levels.)
> >>
> >>Correct.
> >>
> >>>
> >>>I don't see how that would ever work with userspace either so I think
> >>>this is a safe assumption and not something that ever needs fixing.  In
> >>
> >>For e.g, we can perfectly run a guest with 40bit IPA under a host with 16K+36bit
> >>VA. The moment we go above 40bit IPA, we could trigger the conditions above.
> >>I think it is perfectly fine for the guest to choose higher IPA width, and place
> >>its memory well above as long as the qemu/lkvm doesn't exhaust its VA. I just
> >>tried booting a VM with memory at 0x70_0000_0000 on a 16K+36bitVA host and it
> >>boots perfectly fine.
> >>
> >
> >Right, I was thinking about it as providing more than 36bits of *memory*
> >not address space in this case, so you're right, it is at least a
> >theoretically possible case.
> >
> 
> I have reworded the comment as follows:
> /*
>  * With all the supported VA_BITs and 40bit guest IPA, the following condition
>  * is always true:
>  *
>  *       CONFIG_PGTABLE_LEVELS >= STAGE2_PGTABLE_LEVELS
>  *
>  * We base our stage-2 page table walker helpers on this assumption and
>  * fall back to using the host version of the helper wherever possible.
>  * i.e, if a particular level is not folded (e.g, PUD) at stage2, we fall back
>  * to using the host version, since it is guaranteed it is not folded at host.
>  *
>  * If the condition breaks in the future, we can rearrange the host level
>  * definitions and reuse them for stage2. Till then...
>  */
> #if STAGE2_PGTABLE_LEVELS > CONFIG_PGTABLE_LEVELS
> #error "Unsupported combination of guest IPA and host VA_BITS."
> #endif
> 
> ---
> 
> Please let me know your comments.
> 
> 
Looks good, thanks for the cleanup!

-Christoffer