Subject: Re: [RFC PATCH 1/2] kvm/arm: add capability to forward hypercall to user space

From: James Morse <[email-masked]>
Date: Mon, 21 Oct 2019 17:42:53 +0100

Hi Heyi,

On 09/10/2019 13:33, Guoheyi wrote:
> On 2019/10/2 1:19, James Morse wrote:
>> On 24/09/2019 16:20, Heyi Guo wrote:
>>> As more SMC/HVC usages emerge on arm64 platforms, like SDEI, it makes
>>> sense for kvm to have the capability of forwarding such calls to user
>>> space for further emulation.
>> (what do you mean by further? Doesn't user-space have to do all of it?)
> For kvm will always handle hvc/smc guest exit for the first step, even if it is only a
> simple forwarding, I called the user-space processing as "further emulation".
> 
>>> We reuse the existing term "hypercall" for SMC/HVC, as well as the
>>> hypercall structure in kvm_run to exchange arguments and return
>>> values. The definition on arm64 is as below:
>>>
>>> exit_reason: KVM_EXIT_HYPERCALL
>>>
>>> Input:
>>>    nr: the immediate value of SMC/HVC calls; not really used today.
>>>    args[6]: x0..x5 (This is not fully conform with SMCCC which requires
>>>             x6 as argument as well, but use space can use GET_ONE_REG
>>>             ioctl for such rare case).

>> If this structure isn't right for us, we could define a different one for arm/arm64.
>> (we did this for kvm_vcpu_events)

> Do you mean that we can move the hypercall struct definition to arch specific kvm_host.h?
> For it is in the common kvm_run structure, we'll need to change every kvm supported
> architectures, including x86, mips, powerpc, s390. Is it acceptable?

Ah! Sorry, I'd missed this was in the kvm_run structure. The get-events example doesn't
apply here as that was a separate ioctl().

>>> Return:
>>>    args[0..3]: x0..x3 as defined in SMCCC. We need to extract
>>>                args[0..3] and write them to x0..x3 when hypercall exit
>>>                returns.

>> Are we saying that KVM_EXIT_HYPERCALL expects to be used with SMC-CC?
>> (if so, we should state that).

> Yes I followed SMC-CC when writing this.

>>
>> I'm not certain we should tie this to SMC-CC.
>>
>> If we don't tie it to SMC-CC this selection of in/out registers looks odd, there is
>> nothing about HVC/SMC that uses these registers, its just the SMC convention.

> Maybe we don't need to tie it to SMC-CC, and simply load all values in args[6] to GP
> registers...
> And then there is either no strong reason to extend hypercall structure for ARM.

>>> Flag hypercall_forward is added to turn on/off hypercall forwarding
>>> and the default is false. Another flag hypercall_excl_psci is to
>>> exclude PSCI from forwarding for backward compatible, and it only
>>> makes sense to check its value when hypercall_forward is enabled.

>> Calling out PSCI like this is something we shouldn't do. There will be, (are!) other
>> SMC-CC calls that the kernel provides emulation for, we can't easily add to this list.

> Yes; I didn't figure out good way to keep compatibility and future extension...

I think the best trick is not to interpret the SMC/HVC calls from the guest. The kernel
obviously does, but the API shouldn't force us to.

>> I think the best way to avoid this, is to say the hypercall mechanism forwards 'unhandled
>> SMC/HVC' to user-space. Which things the kernel chooses to handle can change.
>>
>> We need a way for user-space to know which SMC/HVC calls the kernel will handle, and will
>> not forward. A suggestion is to add a co-processor that lists these by #imm and r0/x0
>> value. User-space can then query any call to find out if it would be exported if the guest
>> made that call. Something like kvm_arm_get_fw_reg().

> Do you mean we add only one co-processor to list all SMC/HVC calls kernel will handle?

Yes, some way of listing them.
e.g. user-space wants to handle HVC's with #imm==0 and w0==0x84000000, this co-processor
would list that as one of the things that the kernel will handle.

If we can find a way of describing 64bit register values that would save them from being a
problem in the future, but it may be too complicated to describe a 64bit register space
and 16 bits of immediate.

I think its okay for this co-processor to be SMC-CC specific, as its describing what the
kernel supports. The KVM-api in contrast should be flexible enough to describe anything
any guest may wish to do.

> So
> the reg size should be large enough to hold the list, each entry of which contains a #imm
> and r0/x0 pair? Is the reg size fixed by definition or it can be queried by user-space? If
> it is fixed, what's the size should we choose?

(fixed/not-fixed - its a trade-off for complexity now, but no-one may ever use the full
flexibility).

I think we can assume the kernel will only offer things that look like SMC-CC to the
guest. If the guest does something outside this space, its up to user-space to handle. (so
the KVM-API must support non-SMC-CC stuff). I think we should define a co-processor for
SMC/HVC where the #imm is 0. This then gives us 32bits of space we can map directly onto
the w0 values.

> Does it make sense to extend the entry to hold the function ID base and limit, so that it
> can describe the whole range for each function group, like PSCI, SDEI, etc?

This may be over-complex, user-space would always need to enumerate the whole thing. I
think commonly user-space would only want to know about one entry: For cases where we know
the structure, user-space can just query the '_VERSION' call. If that isn't supported,
user-space can assume the rest of that space is unimplemented. (the kernel shouldn't
provide an incomplete emulation of these APIs)

>> For 32bit, are we going to export SMC/HVC calls that failed their condition-code checks?

> I'm not familiar with 32bit, either we don't have 32bit platforms to test the code. So my
> preference is not to make many changes to 32bit...

I'm not that familiar with it either ... You don't have anything with aarch32 support at
EL1? I don't think we should add an API that only works with Aarch64 guests.

For 32bit, we either need to expose these condition-code bits, and say user-space should
work out if it needs to do anything. Or, handle this in the kernel, in which case we don't
need to expose the condition-code bits, but we should document that the kernel will do the
check.

Nested-virt may cause some 'fun' here. If user-space starts an aarch64 guest at EL2, it
may start its own aarch32 guest at EL1. If the aarch32 guest makes an SMC, who handles it?
If user-space's aarch64 guest didn't set the traps for SMC, I think this should be
delivered to user-space, which may be surprised by the request from an aarch32 guest.

(its also possible nested-virt has me confused, it is pretty mind bending!)

>> The hypercall structure should probably indicate whether the SMC/HVC call came from
>> aarch32 or aarch64, as the behaviour may be different.

> How about to use the longmode field in hypercall structure? Standard service calls will
> indicate this in function ID, but we may need to know before parsing the function ID,
> isn't it?

Sure, as its a __u32, we could dump the guest PSTATE from SPSR in there.

I think the last thing is 'ret', and whether we should provide a way of passing 'x0' back
to the guest, or expect user-space to use set-one-reg. Most of the time user-space will
only want to set x0, and doing this would let us initialise it to all-ones in the kernel,
which means the guest gets the unknown-smc value back if user-space ignores the exit.

Thanks,

James

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