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 <jbeul...@suse.com> > >> --- > >> 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.
