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

[Marc Zyngier]

On 06/07/18 14:49, Suzuki K Poulose wrote:
> On 04/07/18 23:03, Suzuki K Poulose wrote:
>> 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 <address@hidden>
>>>> Cc: Christoffer Dall <address@hidden>
>>>> Cc: Peter Maydel <address@hidden>
>>>> Cc: Paolo Bonzini <address@hidden>
>>>> Cc: Radim Krčmář <address@hidden>
>>>> Signed-off-by: Suzuki K Poulose <address@hidden>
>>>> ---
>>>>   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.
> 
> Here is one option. We could re-use the {V}TCR_ELx.{I}PS field format, which
> occupies 3 bits and has the following definitions. (ID_AA64MMFR0_EL1:PARange
> also has the field definitions, except that the field is 4bits wide, but
> only 3bits are used)
> 
> 000 32 bits, 4GB.
> 001 36 bits, 64GB.
> 010 40 bits, 1TB.
> 011 42 bits, 4TB.
> 100 44 bits, 16TB.
> 101 48 bits, 256TB.
> 110 52 bits, 4PB
> 
> But we need to map 0 => 40bits IPA to make our ABI backward compatible. So
> we could use the additional one bit to indicate that IPA size is requested
> in the 3 bits.
> 
> i.e,
> 
> machine_type:
> 
> Bit [2:0]     - Requested IPA size. Values follow VTCR_EL2.PS format.
> 
> Bit [3]               - 1 => IPA Size bits (Bits[2:0]) requested.
>               0 => Not requested
> 
> The only minor down side is restricting to the predefined values above,
> which is not a real issue for a VM.
> 
> Thoughts ?

I'd be very wary of using that 4th bit to do something that is not in
the architecture. We have only a single value left to be used (0b111),
and then your scheme clashes with the architecture definition.

I'd rather encode things in a way that is independent from the
architecture, and be done with it. You can map 0 to 40bits, and we have
the ability to express all values the architecture has (just in a
different order).

Thanks,

        M.
-- 
Jazz is not dead. It just smells funny...