Re: [PATCH 3/4] iommu/io-pgtable-arm: Rationalise TCR handling

[Robin Murphy <robin.murphy@arm.com>]

On 21/08/2019 13:11, Will Deacon wrote:
> On Tue, Aug 20, 2019 at 07:41:52PM +0100, Robin Murphy wrote:
>> On 20/08/2019 17:07, Will Deacon wrote:
>>> On Tue, Aug 20, 2019 at 04:25:56PM +0100, Robin Murphy wrote:
>>>> On 20/08/2019 11:31, Will Deacon wrote:
>>>>> On Mon, Aug 19, 2019 at 07:19:30PM +0100, Robin Murphy wrote:
>>>>>> Although it's conceptually nice for the io_pgtable_cfg to provide a
>>>>>> standard VMSA TCR value, the reality is that no VMSA-compliant IOMMU
>>>>>> looks exactly like an Arm CPU, and they all have various other TCR
>>>>>> controls which io-pgtable can't be expected to understand. Thus since
>>>>>> there is an expectation that drivers will have to add to the given TCR
>>>>>> value anyway, let's strip it down to just the essentials that are
>>>>>> directly relevant to io-pgatble's inner workings - namely the address
>>>>>> sizes, walk attributes, and where appropriate, format selection.
>>>>>>
>>>>>> Signed-off-by: Robin Murphy <robin.murphy@arm.com>
>>>>>> ---
>>>>>>     drivers/iommu/arm-smmu-v3.c        | 7 +------
>>>>>>     drivers/iommu/arm-smmu.c           | 1 +
>>>>>>     drivers/iommu/arm-smmu.h           | 2 ++
>>>>>>     drivers/iommu/io-pgtable-arm-v7s.c | 6 ++----
>>>>>>     drivers/iommu/io-pgtable-arm.c     | 4 ----
>>>>>>     drivers/iommu/qcom_iommu.c         | 2 +-
>>>>>>     6 files changed, 7 insertions(+), 15 deletions(-)
>>>>>
>>>>> Hmm, so I'm a bit nervous about this one since I think we really should
>>>>> be providing a TCR with EPD1 set if we're only giving you TTBR0. Relying
>>>>> on the driver to do this worries me. See my comments on the next patch.
>>>>
>>>> The whole idea is that we already know we can't provide a *complete* TCR
>>>> value (not least because anything above bit 31 is the wild west), thus
>>>> there's really no point in io-pgtable trying to provide anything other than
>>>> the parts it definitely controls. It makes sense to provide this partial TCR
>>>> value "as if" for TTBR0, since that's the most common case, but ultimately
>>>> io-pgatble doesn't know (or need to) which TTBR the caller intends to
>>>> actually use for this table. Even if the caller *is* allocating it for
>>>> TTBR0, io-pgtable doesn't know that they haven't got something live in TTBR1
>>>> already, so it still wouldn't be in a position to make the EPD1 call either
>>>> way.
>>>
>>> Ok, but the driver can happily rewrite/ignore what it gets back. I suppose
>>> an alternative would be scrapped the 'u64 tcr' and instead having a bunch
>>> of named bitfields for the stuff we're actually providing, although I'd
>>> still like EPDx to be in there.
>>
>> I like the bitfield idea; it would certainly emphasise the "you have to do
>> something more with this" angle that I'm pushing towards here, but still
>> leave things framed in TCR terms without having to go to some more general
>> abstraction. It really doesn't play into your EPD argument though - such a
>> config would be providing TxSZ/TGx/IRGNx/ORGNx/SHx, but EPDy, for y = !x.
>> For a driver to understand that and do the right thing with it is even more
>> involved than for the driver to just set EPD1 by itself anyway.
> 
> Having considered the bitfield idea some more, I'm less attached to EPDx
> because we simply wouldn't be making a statement about them, rather than a
> (dangerous) zero value and expecting it to be ignored. So I think we're in
> agreement on that.

Cool, I'll give bitfields a go for v2.

> The only part I'm still stuck to is that I think io-pgtable should know
> whether it's targetting TTBR0 or TTBR1 so that it can sanitise input
> addresses correctly. Doing this in the driver code is possible, but I'd
> rather not start from that position, particularly as it would require things
> like sign-extension in the TLBI callbacks.

Good point, and thanks for the prod that the way I end up propagating 
masked-off IOVAs through to the TLBI calls here is busted either way. 
I'm OK with introducing an explicit TTBR1 quirk to begin with, as it 
should make things a little easier to reason about, and we can always 
revisit later if and when we do find a need for more flexibility.

>> If only LPAE had created these bits as enables rather than disables then
>> things would be logical and we could all be happy, but here we are...
> 
> I'm happy! :D:D:D

Yeah, but I refuse to believe it's ever because of pagetables ;)

Robin.
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