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

[Jordan Crouse <jcrouse@codeaurora.org>]

On Wed, Aug 21, 2019 at 01:56:20PM +0100, Robin Murphy wrote:
> 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.

Bumping this as is our tradition in the -rc1 time frame before we get all
distracted with other stuff. It sounds like the last agreement was for a
TTBR1 hint for the EDP and the sign extension in the functions.

Let me know if you need any help. I've got a little time and more than a little
motivation to keep slogging ahead toward a glorious arm-smmu-v2
per-context pagetable future.

Jordan

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