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

[Peter Maydell <peter.maydell@linaro.org>]

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?

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)?

> note: I have renamed the KVM_CAP_ARM_VM_PHYS_SHIFT =>
> KVM_CAP_ARM_VM_IPA_SIZE
> above and can update the code, if the latter is better.

I think the new name is definitely clearer, thanks.

-- PMM