Re: [PATCH v3 15/20] kvm: arm/arm64: Allow tuning the physical address size for VM

[Suzuki K Poulose <suzuki.poulose@arm.com>]

On 07/04/2018 04:51 PM, Will Deacon wrote:
> Hi Suzuki,
> 
> On Fri, Jun 29, 2018 at 12:15:35PM +0100, Suzuki K Poulose wrote:
>> Allow specifying the physical address size for a new VM via
>> the kvm_type argument for KVM_CREATE_VM ioctl. This allows
>> us to finalise the stage2 page table format 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 the bits[7:0]
>> of the type field and can encode more information in the future if
>> required. The IPA size is still capped at 40bits.
>>
>> Cc: Marc Zyngier <marc.zyngier@arm.com>
>> Cc: Christoffer Dall <cdall@kernel.org>
>> Cc: Peter Maydel <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>
>> ---
>>   arch/arm/include/asm/kvm_mmu.h   |  2 ++
>>   arch/arm64/include/asm/kvm_arm.h | 10 +++-------
>>   arch/arm64/include/asm/kvm_mmu.h |  2 ++
>>   include/uapi/linux/kvm.h         | 10 ++++++++++
>>   virt/kvm/arm/arm.c               | 24 ++++++++++++++++++++++--
>>   5 files changed, 39 insertions(+), 9 deletions(-)
> 
> [...]
> 
>> diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
>> index 4df9bb6..fa4cab0 100644
>> --- a/include/uapi/linux/kvm.h
>> +++ b/include/uapi/linux/kvm.h
>> @@ -751,6 +751,16 @@ struct kvm_ppc_resize_hpt {
>>   #define KVM_S390_SIE_PAGE_OFFSET 1
>>   
>>   /*
>> + * On arm/arm64, machine type can be used to request the physical
>> + * address size for the VM. Bits [7-0] have been reserved for the
>> + * PA size shift (i.e, log2(PA_Size)). For backward compatibility,
>> + * value 0 implies the default IPA size, which is 40bits.
>> + */
>> +#define KVM_VM_TYPE_ARM_PHYS_SHIFT_MASK	0xff
>> +#define KVM_VM_TYPE_ARM_PHYS_SHIFT(x)		\
>> +	((x) & KVM_VM_TYPE_ARM_PHYS_SHIFT_MASK)
> 
> This seems like you're allocating quite a lot of bits in a non-extensible
> interface to a fairly esoteric parameter. Would it be better to add another
> ioctl, or condense the number of sizes you support instead?

As I explained in the other thread, we need the size as soon as the VM
is created. The major challenge is keeping the backward compatibility by
mapping 0 to 40bits. I will give it a thought.

Suzuki