Re: [PATCH v6 18/18] kvm: arm64: Allow tuning the physical address size for VM

[Christoffer Dall <christoffer.dall@arm.com>]

On Wed, Oct 31, 2018 at 05:55:13PM +0000, Suzuki K Poulose wrote:
> Hi Christoffer,
> 
> On 31/10/18 14:22, Christoffer Dall wrote:
> >On Wed, Sep 26, 2018 at 05:32:54PM +0100, Suzuki K Poulose wrote:
> >>Allow specifying the physical address size limit for a new
> >>VM via the kvm_type argument for the KVM_CREATE_VM ioctl. This
> >>allows us to finalise the stage2 page table as early as possible
> >>and hence perform the right checks on the memory slots
> >>without complication. The size is encoded as Log2(PA_Size) in
> >>bits[7:0] of the type field. For backward compatibility the
> >>value 0 is reserved and implies 40bits. Also, lift the limit
> >>of the IPA to host limit and allow lower IPA sizes (e.g, 32).
> >>
> >>The userspace could check the extension KVM_CAP_ARM_VM_IPA_SIZE
> >>for the availability of this feature. The cap check returns the
> >>maximum limit for the physical address shift supported by the host.
> >>
> >>Cc: Marc Zyngier <marc.zyngier@arm.com>
> >>Cc: Christoffer Dall <cdall@kernel.org>
> >>Cc: Peter Maydell <peter.maydell@linaro.org>
> >>Cc: Paolo Bonzini <pbonzini@redhat.com>
> >>Cc: Radim Krčmář <rkrcmar@redhat.com>
> >>Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
> >>---
> 
> >>@@ -192,17 +195,23 @@ int kvm_arm_config_vm(struct kvm *kvm, unsigned long type)
> >>  	u32 parange, phys_shift;
> >>  	u8 lvls;
> >>-	if (type)
> >>+	if (type & ~KVM_VM_TYPE_ARM_IPA_SIZE_MASK)
> >>  		return -EINVAL;
> >>+	phys_shift = KVM_VM_TYPE_ARM_IPA_SIZE(type);
> >>+	if (phys_shift) {
> >>+		if (phys_shift > kvm_ipa_limit ||
> >>+		    phys_shift < 32)
> >>+			return -EINVAL;
> >
> >I am concerned here that if we allow the user to set the phys_size to 32
> >bits, then we end up with 2 levels of stage2 page tables, which means
> >that the size of a stage2 pmd mapping becomes the size of a stage2 pgd
> >mapping, yet we can still decide in user_mem_abort() that a stage2 fault
> >is backed by PMD size mappings on the host, and attempt a huge mapping
> >at stage2, which then becomes a PGD level block map, I think.
> 
> Yes, you're right. We will have a pgd-level block map in that case.
> This should work transparently as PMD at stage2 is folded into PGD and
> we endup marking the PGD entry as huge and the stage2 accessors deal
> with it appropriately. This is similar to having a PMD huge page with
> 64K + 42bit VA (2 level page table) on stage1.
> 
> >
> >Is this handled somehow?  If so, how?
> 
> We don't prevent this. We have a guaranteed minimum number of levels
> at 2, which implies you can map a stage1 PMD huge page at stage2.
> I acknowledge that the Linux naming convention does cause some confusion
> for a "level" at stage1 and stage2 levels. But if you think of it
> from the hardware level (and like the ARM ARM defines it , Level 0-3),
> it is much simpler. i.e, you can map a huge page at level N in stage1
> into stage2 if you have that level N. It doesn't matter if stage2 has
> more or less number of levels than stage1, as long as stage2 table can
> deal with it.
> 

That is indeed a good way to reason about it.

> >
> >I can't see user_mem_abort() being modified to explicitly handle this
> >in your code, but perhaps the stage2_set_pmd_huge() call ends up
> >actually mapping at the stage2 pte level, but I can't tell that it does.
> 
> The stage2_set_pmd_huge() installs it at the PGD (level 2, which would
> have been PMD if we had levels > 2) slot.
> 
> pmd = stage2_get_pmd(addr)
>        \-> pud = stage2_get_pud(addr)
>              \-> pgd = kvm->arch.pgd + stage2_pgd_index(addr);
>              \-> (we have stage2_pgd_none(x) = 0 and
>              \-> stage2_pud_offset(pgd, addr) = pgd
>              \->returns (kvm->arch.pgd + stage2_pgd_index(addr);
>        \->  stage_pud_none(x) = 0 & stage2_pmd_offset(pud, addr) = pud
>        \->  returns pud (kvm->arch.pgd + stage2_pgd_index(addr))
> 
> and we install the PMD huge mapping at the location.
> 
> >In any case, I think user_mem_abort() should give up on pmd/pud huge
> >mappings if the size mapped by the stage2/stage1 pmd/pud levels don't
> >line up.  What do you think?
> 
> Does it matter ? Personally I don't think it matters much as long as we
> are able to map the "huge" page at stage1 in the stage2 as huge, even if
> the stage2 has lesser levels and manage it well. Given that PMD huge
> pages are quite common, it would be good to exploit it when we can.

What I couldn't convince myself of was whether having 2 levels at stage2
implied the entry level block mapping being of the same size as the
stage1 block mapping, but given your explanation above, I think that's
fine.

> 
> On the other hand, for stage2 PUD we are checking if the stage2 has a
> PUD level (kvm_has_stage2_pud()). May be we should relax it just like
> we do for PMD to check (kvm_stage2_levels > 2).
> 

Depends on how the code ends up looking like I suppose, but the more
symmetry we can have between the approach for host PMD and host PUD and
host PTE mappings, the better.


Thanks,

    Christoffer