Re: [PATCH v5 16/18] kvm: arm64: Set a limit on the IPA size

[Suzuki K Poulose]

On 09/25/2018 10:59 AM, Auger Eric wrote:
> Hi Suzuki,
>
> On 9/17/18 12:41 PM, Suzuki K Poulose wrote:
> > So far we have restricted the IPA size of the VM to the default
> > value (40bits). Now that we can manage the IPA size per VM and
> > support dynamic stage2 page tables, we can allow VMs to have
> > larger IPA. This patch introduces a the maximum IPA size
> > supported on the host.
> to be reworded
>   This is decided by the following factors :

Sure


> > diff --git a/arch/arm64/kvm/reset.c b/arch/arm64/kvm/reset.c
> > index 51ecf0f7c912..76972b19bdd7 100644
> > --- a/arch/arm64/kvm/reset.c
> > +++ b/arch/arm64/kvm/reset.c
> > @@ -34,6 +34,9 @@
> >   #include <asm/kvm_coproc.h>
> >   #include <asm/kvm_mmu.h>
> >   
> > +/* Maximum phys_shift supported for any VM on this host */
> > +static u32 kvm_ipa_limit;
> > +
> >   /*
> >    * ARMv8 Reset Values
> >    */
> > @@ -135,6 +138,46 @@ int kvm_reset_vcpu(struct kvm_vcpu *vcpu)
> >   	return kvm_timer_vcpu_reset(vcpu);
> >   }
> >   
> > +void kvm_set_ipa_limit(void)
> > +{
> > +	unsigned int ipa_max, va_max, parange;
> > +
> > +	parange = read_sanitised_ftr_reg(SYS_ID_AA64MMFR0_EL1) & 0x7;
> > +	ipa_max = id_aa64mmfr0_parange_to_phys_shift(parange);
> > +
> > +	/* Raise the limit to the default size for backward compatibility */
> > +	if (ipa_max < KVM_PHYS_SHIFT) {
> > +		WARN_ONCE(1,
> > +			  "PARange is %d bits, unsupported configuration!",
> > +			  ipa_max);
> > +		ipa_max = KVM_PHYS_SHIFT;
> I don't really get what does happen in this case. The CPU cannot handle
> PA up to ipa_max so can the VM run properly? In case it is a
> showstopper, kvm_set_ipa_limit should return an error, cascaded by
> init_common_resources. Otherwise the warning message may be reworded.

I think this was a warning added to warn against the older
Foundation model which had a 36bit PA size. So the VTCR was progammed
with a 36bit limit, while the KVM guest was allowed to create 40bit
IPA space, though it wouldn't fly well if someone tried to.

With this series, I think we may expose the real IPA_MAX (which could
be < 40bit) and warn the user if someone tried to create a VM with
40bit IPA (vm_type == 0) and let the call succeed (for the sake of ABI).

Marc, Christoffer, Eric

Thoughts ?

> > +	}
> > +
> > +	/* Clamp it to the PA size supported by the kernel */
> > +	ipa_max = (ipa_max > PHYS_MASK_SHIFT) ? PHYS_MASK_SHIFT : ipa_max;
> > +	/*
> > +	 * Since our stage2 table is dependent on the stage1 page table code,
> > +	 * we must always honor the following condition:
> > +	 *
> > +	 *  Number of levels in Stage1 >= Number of levels in Stage2.
> > +	 *
> > +	 * So clamp the ipa limit further down to limit the number of levels.
> > +	 * Since we can concatenate upto 16 tables at entry level, we could
> > +	 * go upto 4bits above the maximum VA addressible with the current
> addressable?

Sure

> > +	 * number of levels.
> > +	 */
> > +	va_max = PGDIR_SHIFT + PAGE_SHIFT - 3;
> > +	va_max += 4;
> > +
> > +	if (va_max < ipa_max) {
> > +		kvm_info("Limiting IPA limit to %dbytes due to host VA bits limitation\n",
> > +			 va_max);
> > +		ipa_max = va_max;
> you have a trace for this limitation but none for the comparison against
> PHYS_MASK_SHIFT.

May be I could add a message which only mentions what is the limiting
factor kernel VA vs kernel PA support

> > +	}
> > +
> > +	kvm_ipa_limit = ipa_max;
> > +}
> > +
> >   /*
> >    * Configure the VTCR_EL2 for this VM. The VTCR value is common
> >    * across all the physical CPUs on the system. We use system wide
> > diff --git a/virt/kvm/arm/arm.c b/virt/kvm/arm/arm.c
> > index 43e716bc3f08..631f9a3ad99a 100644
> > --- a/virt/kvm/arm/arm.c
> > +++ b/virt/kvm/arm/arm.c
> > @@ -1413,6 +1413,8 @@ static int init_common_resources(void)
> >   	kvm_vmid_bits = kvm_get_vmid_bits();
> >   	kvm_info("%d-bit VMID\n", kvm_vmid_bits);
> >   
> > +	kvm_set_ipa_limit();
> As we have a kvm_info for the supported vmid_bits, may be good to output
> the max IPA size supported by the host whatever the applied clamps?

Sure, will do that.

Thanks
Suzuki