Subject: Re: [PATCH 06/14] KVM: arm/arm64: Factor out VMID into struct kvm_vmid

From: Marc Zyngier <[email-masked]>
Date: Fri, 25 Jan 2019 10:09:05 +0000

Hi James,

Thanks for looking into this.

On Thu, 24 Jan 2019 19:01:57 +0000,
James Morse <[email-masked]> wrote:
> 
> Hi guys,
> 
> (CC: +Suzuki)
> 
> On 24/01/2019 14:00, Christoffer Dall wrote:
> > In preparation for nested virtualization where we are going to have more
> > than a single VMID per VM, let's factor out the VMID data into a
> > separate VMID data structure and change the VMID allocator to operate on
> > this new structure instead of using a struct kvm.
> > 
> > This also means that udate_vttbr now becomes update_vmid, and that the
> > vttbr itself is generated on the fly based on the stage 2 page table
> > base address and the vmid.
> > 
> > We cache the physical address of the pgd when allocating the pgd to
> > avoid doing the calculation on every entry to the guest and to avoid
> > calling into potentially non-hyp-mapped code from hyp/EL2.
> > 
> > If we wanted to merge the VMID allocator with the arm64 ASID allocator
> > at some point in the future, it should actually become easier to do that
> > after this patch.
> 
> > Note that to avoid mapping the kvm_vmid_bits variable into hyp, we
> > simply forego the masking of the vmid value in kvm_get_vttbr and rely on
> > update_vmid to always assign a valid vmid value (within the supported
> > range).
> 
> 
> > diff --git a/arch/arm64/include/asm/kvm_mmu.h b/arch/arm64/include/asm/kvm_mmu.h
> > index 8af4b1befa42..189d93461d33 100644
> > --- a/arch/arm64/include/asm/kvm_mmu.h
> > +++ b/arch/arm64/include/asm/kvm_mmu.h
> > @@ -596,5 +596,16 @@ static inline bool kvm_cpu_has_cnp(void)
> >  	return system_supports_cnp();
> >  }
> >  
> > +static __always_inline u64 kvm_get_vttbr(struct kvm *kvm)
> > +{
> > +	struct kvm_vmid *vmid = &kvm->arch.vmid;
> > +	u64 vmid_field, baddr;
> > +	u64 cnp = kvm_cpu_has_cnp() ? VTTBR_CNP_BIT : 0;
> > +
> > +	baddr = kvm->arch.pgd_phys;
> > +	vmid_field = (u64)vmid->vmid << VTTBR_VMID_SHIFT;
> > +	return kvm_phys_to_vttbr(baddr) | vmid_field | cnp;
> > +}
> 
> (32bits version is the same ... but I guess there is nowhere to put it!)

Yes, that's the usual conundrum.

> 
> 
> > diff --git a/virt/kvm/arm/arm.c b/virt/kvm/arm/arm.c
> > index 3dd240ea9e76..b77db673bb03 100644
> > --- a/virt/kvm/arm/arm.c
> > +++ b/virt/kvm/arm/arm.c
> > @@ -536,18 +529,12 @@ static void update_vttbr(struct kvm *kvm)
> >  		kvm_call_hyp(__kvm_flush_vm_context);
> >  	}
> >  
> > -	kvm->arch.vmid = kvm_next_vmid;
> > +	vmid->vmid = kvm_next_vmid;
> >  	kvm_next_vmid++;
> > -	kvm_next_vmid &= (1 << kvm_vmid_bits) - 1;
> > -
> > -	/* update vttbr to be used with the new vmid */
> > -	pgd_phys = virt_to_phys(kvm->arch.pgd);
> 
> > -	BUG_ON(pgd_phys & ~kvm_vttbr_baddr_mask(kvm));
> 
> Where did this go? (escaped during a turbulent rebase?)
> 
> This removes the only caller of kvm_vttbr_baddr_mask()... It looks
> like this is a safety check that the stage2-pgd is correctly aligned
> when the IPA size, and thus size of the top level entry can by set
> by user-space.
> 
> ... or is it unnecessary if alloc_pages_exact() can only return naturally
> aligned groups of pages? (which I haven't checked)

The rational is indeed that alloc_pages_exact allocates a power of 2
number of pages, and then frees the unrequested tail pages. This means
that we're always correctly aligned.

> Keeping it sounds like a good thing to have in case we accidentally merge
> stage2/host-stage1 pgd's somewhere down the line.

This seems hard to achieve for this very reason, as level-0
concatenation gets in the way of unifying the two allocators.

> (Suzuki suggested it would make more sense in kvm_alloc_stage2_pgd(), where we
> could fail vm creation, instead of BUG()ing).
> 
> (this was added by e55cac5bf2a9c ("kvm: arm/arm64: Prepare for VM specific
> stage2 translations"), the bulk of the logic is in 595583306434c ("kvm: arm64:
> Dynamic configuration of VTTBR mask"))

We could bring it back if people have a strong feeling about this, but
certainly not as a BUG_ON(). Failing it in gracefully in
kvm_alloc_stage2_pgd seems a more palatable solution, but I can't
convince myself that we need it.

Thanks,

	M.

-- 
Jazz is not dead, it just smell funny.

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