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

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