Re: [PATCH v2 3/3] AMD/IOMMU: iommu_enable vs iommu_intremap

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Tue, Nov 02, 2021 at 11:13:08AM +0100, Jan Beulich wrote:
> On 25.10.2021 12:28, Roger Pau Monné wrote:
> > On Thu, Oct 21, 2021 at 11:59:02AM +0200, Jan Beulich wrote:
> >> The two are really meant to be independent settings; iov_supports_xt()
> >> using || instead of && was simply wrong. The corrected check is,
> >> however, redundant, just like the (correct) one in iov_detect(): These
> >> hook functions are unreachable without acpi_ivrs_init() installing the
> >> iommu_init_ops pointer, which it does only upon success. (Unlike for
> >> VT-d there is no late clearing of iommu_enable due to quirks, and any
> >> possible clearing of iommu_intremap happens only after iov_supports_xt()
> >> has run.)
> >>
> >> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> >> ---
> >> In fact in iov_detect() it could be iommu_enable alone which gets
> >> checked, but this felt overly aggressive to me. Instead I'm getting the
> >> impression that the function may wrongly not get called when "iommu=off"
> >> but interrupt remapping is in use: We'd not get the interrupt handler
> >> installed, and hence interrupt remapping related events would never get
> >> reported. (Same on VT-d, FTAOD.)
> > 
> > I've spend a non-trivial amount of time looking into this before
> > reading this note. AFAICT you could set iommu=off and still get x2APIC
> > enabled and relying on interrupt remapping.
> 
> Right, contrary to ...
> 
> >> For iov_supports_xt() the question is whether, like VT-d's
> >> intel_iommu_supports_eim(), it shouldn't rather check iommu_intremap
> >> alone (in which case it would need to remain a check rather than getting
> >> converted to ASSERT()).
> > 
> > Hm, no, I don't think so. I think iommu_enable should take precedence
> > over iommu_intremap, and having iommu_enable == false should force
> > interrupt remapping to be reported as disabled. Note that disabling it
> > in iommu_setup is too late, as the APIC initialization will have
> > already taken place.
> > 
> > It's my reading of the command line parameter documentation that
> > setting iommu=off should disable all usage of the IOMMU, and that
> > includes the interrupt remapping support (ie: a user should not need
> > to set iommu=off,no-intremap)
> 
> ... that documentation. But I think it's the documentation that
> wants fixing, such that iommu=off really only control DMA remap.

IMO I think it's confusing to have sub-options that could be enabled
when you set the global one to off. I would expect `iommu=off` to
disable all the iommu related options, and I think it's fair for
people to expect that behavior.

I'm unsure whether it's fair to change the documentation now, we
should instead fix the code, so that people using `iommu=off` get the
expected behavior. Then we would likely need to introduce a way to
disable just dma remapping (dmaremap, similar to intremap). That
would make a much better and saner interface IMO.

Thanks, Roger.