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

[Will Deacon <will@kernel.org>]

On Thu, Oct 24, 2019 at 12:23:46PM +0100, Robin Murphy wrote:
> On 2019-10-24 11:51 am, Will Deacon wrote:
> > 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.
> 
> Oops, the ping did register, I just didn't react outwardly ;)
> 
> I have been working on v2, and plan to have something ready next week - the
> holdup was that I started refactoring all the argument passing since the
> number of things we have to carry through from one end of map/unmap to the
> other is getting a bit silly, but I think I can still finish the TTBR1 quirk
> without that, so if I don't get it cracked imminently then I'll put it aside
> to revisit later.

Cheers, Robin! I'll keep an eye out for it.

Will
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