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

[Will Deacon <will@kernel.org>]

On Thu, Oct 03, 2019 at 11:33:52AM -0600, Jordan Crouse wrote:
> 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.

If somebody respins this using bitfields and an explicit TTBR1 quirk then
I'll merge it.

Will

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