Re: [PATCH v5 18/18] kvm: arm64: Allow tuning the physical address size for VM

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

Hi Peter,

On 18/09/18 16:36, Peter Maydell wrote:
> On 18 September 2018 at 16:16, Suzuki K Poulose <Suzuki.Poulose@arm.com> wrote:
>> Hi Peter,
>>
>> On 18/09/2018 02:55, Peter Maydell wrote:
>>>
>>> On 17 September 2018 at 11:41, Suzuki K Poulose <suzuki.poulose@arm.com>
>>> wrote:
>>>>
>>>> --- a/Documentation/virtual/kvm/api.txt
>>>> +++ b/Documentation/virtual/kvm/api.txt
>>>> @@ -122,6 +122,14 @@ the default trap & emulate implementation (which
>>>> changes the virtual
>>>>    memory layout to fit in user mode), check KVM_CAP_MIPS_VZ and use the
>>>>    flag KVM_VM_MIPS_VZ.
>>>>
>>>> +To configure the physical address space size for a VM (IPA size) on
>>>> arm64,
>>>> +check KVM_CAP_ARM_VM_PHYS_SHIFT (which returns the maximum limit for the
>>>> +IPA shift) and use KVM_VM_TYPE_ARM_PHYS_SHIFT(PHYS_SHIFT). Bits[7-0] of
>>>> the
>>>> +machine type has been reserved for specifying the PHYS_SHIFT.
>>>> +The supported range is [32...IPA_LIMIT], where IPA_LIMIT could be
>>>> +identified by checking KVM_CAP_ARM_VM_PHYS_SHIFT. For backward
>>>> compatibility
>>>> +a value of 0 selects 40bits.
>>>> +
>>>
>>>
>>> Given this as the API documentation, I don't think I could figure out
>>> what I as a userspace user of it need to do without looking at the
>>> kernel code. Could I ask you to expand it so that it is a bit less
>>> terse and a bit more detailed? (For instance, what is a PHYS_SHIFT
>>> and why do I have to specify it rather than just telling the kernel
>>> I want a 48 bit guest address space?)
>>
>>
>> Thanks for the feedback.  I acknowledge that the documentation is not
>> quite clear for a userspace user. How about:
>>
>> "To configure the physical address space size for a VM (IPA size) on arm64,
>> check KVM_CAP_ARM_VM_IPA_SIZE and use KVM_VM_TYPE_ARM_IPA_SIZE(IPA_Bits)
>> as the argument to KVM_CREATE_VM, where IPA_Bits is the maximum width of
>> any physical address used by the VM. The IPA_Bits is encoded in Bits[7-0]
>> of the machine type, and must be one of { 0, 32, ... , Host_IPA_Limit },
>> where :
>> 1) IPA_Bits = 0 implies 40bits IPA (for backward compatibility)
>> 2) Host_IPA_Limit is the maximum limit for IPA_Bits on the host, which is
>>     dependent on the CPU capability and the host kernel configuration.
>>     This can be detected by checking the extension KVM_CAP_ARM_VM_IPA_SIZE
>> "
> 
> I think this is still somewhat confusing. In particular, you're
> describing both the "ask the kernel what it supports" API and
> the "tell the kernel what we want" API in a single sentence
> ("...check KVM_CAP_ARM_VM_IPA_SIZE and use KVM_VM_TYPE_ARM_IPA_SIZE...").
> There isn't a length limit on documentation, so why not describe
> them both clearly in separate sentences?

Sure, will do. I didn't want to hijack the KVM_CREATE_VM ioctl command
section to explain a kvm CAP.

> 
> Also, can I use any IPA value between 32 and Host_IPA_Limit, or
> are only certain values in that range supported (if so, which)?

Any value in the range is supported. I should probably add that
"The configured IPA size is different from what is observed by guest in
ID_AA64MMFR0_EL1[PARange] to avoid the confusion." So here it goes :

---

"On arm64, the physical address size for a VM (IPA Size limit) is limited
to 40bits by default. The limit can be configured if the host supports the
extension KVM_CAP_ARM_VM_IPA_SIZE. When supported, use
KVM_VM_TYPE_ARM_IPA_SIZE(IPA_Bits) to set the size in the machine type
identifier, where IPA_Bits is the maximum width of any physical
address used by the VM. The IPA_Bits is encoded in bits[7-0] of the
machine type identifier.

e.g, to configure a guest to use 48bit physical address size :

	vm_fd = ioctl(dev_fd, KVM_CREATE_VM, KVM_VM_TYPE_ARM_IPA_SIZE(48));

The requested size (IPA_Bits) must be :
   0 - Implies default 40bits (for backward compatibility)

   or

   N - Implies N bits, where N is a positive integer such that 32 <= N <= Host_IPA_Limit

Host_IPA_Limit is the maximum possible value for IPA_Bits on the host and
is dependent on the CPU capability and the kernel configuration. The limit can
be retrieved using KVM_CAP_ARM_VM_IPA_SIZE of the KVM_CHECK_EXTENSION
ioctl() at run-time.

Please note that configuring the IPA size does not affect the capability
exposed by the guest CPUs in ID_AA64MMFR0_EL1[PARange]. It only affects
the guest to host physical address (stage2) translations setup by the host.
"

Suzuki