Message-ID: <c753c2a8-024d-5bef-8987-96582084991e@arm.com>
Date:   Thu, 16 Mar 2023 14:58:39 +0000
From:   Robin Murphy <robin.murphy@....com>
To:     Nicolin Chen <nicolinc@...dia.com>
Cc:     jgg@...dia.com, will@...nel.org, eric.auger@...hat.com,
        kevin.tian@...el.com, baolu.lu@...ux.intel.com, joro@...tes.org,
        shameerali.kolothum.thodi@...wei.com, jean-philippe@...aro.org,
        linux-arm-kernel@...ts.infradead.org, iommu@...ts.linux.dev,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1 14/14] iommu/arm-smmu-v3: Add
 arm_smmu_cache_invalidate_user

On 2023-03-16 00:01, Nicolin Chen wrote:
> On Mon, Mar 13, 2023 at 01:07:42PM +0000, Robin Murphy wrote:
>> External email: Use caution opening links or attachments
>>
>>
>> On 2023-03-11 12:38, Nicolin Chen wrote:
>>> On Fri, Mar 10, 2023 at 05:53:46PM +0000, Robin Murphy wrote:
>>>
>>>>>>> +     case CMDQ_OP_TLBI_NH_VA:
>>>>>>> +             cmd.tlbi.asid = inv_info->asid;
>>>>>>> +             fallthrough;
>>>>>>> +     case CMDQ_OP_TLBI_NH_VAA:
>>>>>>> +             if (!granule_size || !(granule_size & smmu->pgsize_bitmap) ||
>>>>>>
>>>>>> Non-range invalidations with TG=0 are perfectly legal, and should not be
>>>>>> ignored.
>>>>>
>>>>> I assume that you are talking about the pgsize_bitmap check.
>>>>>
>>>>> QEMU embeds a !tg case into the granule_size [1]. So it might
>>>>> not be straightforward to cover that case. Let me see how to
>>>>> untangle different cases and handle them accordingly.
>>>>
>>>> Oh, double-checking patch #2, that might be me misunderstanding the
>>>> interface. I hadn't realised that the UAPI was apparently modelled on
>>>> arm_smmu_tlb_inv_range_asid() rather than actual SMMU commands :)
>>>
>>> Yea. In fact, most of the invalidation info in QEMU was packed
>>> for the previously defined general cache invalidation structure,
>>> and the range invalidation part is still not quite independent.
>>>
>>>> I really think UAPI should reflect the hardware and encode TG and TTL
>>>> directly. Especially since there's technically a flaw in the current
>>>> driver where we assume TTL in cases where it isn't actually known, thus
>>>> may potentially fail to invalidate level 2 block entries when removing a
>>>> level 1 table, since io-pgtable passes the level 3 granule in that case.
>>>
>>> Do you mean something like hw_info forwarding pgsize_bitmap/tg
>>> to the guest? Or the other direction?
>>
>> I mean if the interface wants to support range invalidations in a way
>> which works correctly, then it should ideally carry both the TG and TTL
>> fields from the guest command straight through to the host. If not, then
>> at the very least the host must always assume TTL=0, because it cannot
>> correctly infer otherwise once the guest command's original intent has
>> been lost.
> 
> Oh, it's about hypervisor simply forwarding the entire CMD to
> the host side. Jason is suggesting a fast approach by letting
> host kernel read the CMDQ directly to get the raw CMD. Perhaps
> that would address this comments about TG/TTL too.

That did cross my mind, but given the usage model, having host userspace 
give guest memory whose contents it can't control (unless it pauses the 
whole VM on all CPUs) directly to the host kernel just seems to invite 
more potential problems than necessary. Commands aren't big, so I think 
it's fair to expect the VMM to marshal them into host memory, and save 
the host kernel from ever having to reason about any races or other 
emulation details which may exist between a VM and its VMM.

> I wonder if there could be other case than a WAR, where TG/TTL
> fields from the guest's aren't supported by the host. And then
> should the host handle it with a different CMD?

As Eric found previously, there's a clear benefit in emulating range 
invalidation for guests even if the underlying hardware doesn't support 
it, to minimise trapping. But that's not hard, and the patch as-is is 
already achieving it. All we need to be careful to avoid is issuing 
hardware commands with *less* scope than guest originally asked for - if 
the guest asks for a nonsense TG/TTL which doesn't match its current 
context, that's fine. The interface just has to ensure that a VMM's SMMU 
emulation *is* able to make a nested S1 context behave as expected by 
the architecture; we don't need to care if a guest uses the architecture 
wrong, since it's only hurting itself.
>>>> When range invalidation came along, the distinction between "all leaves
>>>> are definitely at the last level" and "use last-level granularity to
>>>> make sure everything at at any level is hit" started to matter, but the
>>>> interface never caught up. It hasn't seemed desperately urgent to fix
>>>> (who does 1GB+ unmaps outside of VFIO teardown anyway?), but we must
>>>> definitely not bake the same mistake into user ABI.
>>>>
>>>> Of course, there might then be cases where we need to transform
>>>> non-range commands into range commands for the sake of workarounds, but
>>>> that's our own problem to deal with.
>>>
>>> Noted it down.
>>>
>>>>>> What about NSNH_ALL? That still needs to invalidate all the S1 context
>>>>>> that the guest *thinks* it's invalidating.
>>>>>
>>>>> NSNH_ALL is translated to NH_ALL at the guest level. But maybe
>>>>> it should have been done here instead.
>>>>
>>>> Yes. It seems the worst of both worlds to have an interface which takes
>>>> raw opcodes rather than an enum of supported commands, but still
>>>> requires userspace to know which opcodes are supported and which ones
>>>> don't work as expected even though they are entirely reasonable to use
>>>> in the context of the stage-1-only SMMU being emulated.
>>>
>>> Maybe a list of supported TLBI commands via the hw_info uAPI?
>>
>> I don't think it's all that difficult to implicitly support all commands
>> that are valid for a stage-1-only SMMU, it just needs the right
>> interface design to be capable of encoding them all completely and
>> unambiguously. Coming back to the previous point about the address
>> encoding, I think that means basing it more directly on the actual
>> SMMUv3 commands, rather than on io-pgtable's abstraction of invalidation
>> with SMMUv3 opcodes bolted on.
> 
> Yea, with the actual commands from the guest, the host can do
> something with its supported commands, I think.

The one slightly fiddly case, of course, is CMD_SYNC, but I think that's 
just a matter for clear documentation of the expectations and behaviour.

Thanks,
Robin.

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