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

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

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.

> 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)

> ---
> v2: New.
> 
> --- a/xen/drivers/passthrough/amd/iommu_intr.c
> +++ b/xen/drivers/passthrough/amd/iommu_intr.c
> @@ -731,8 +731,7 @@ bool __init iov_supports_xt(void)
>  {
>      unsigned int apic;
>  
> -    if ( !iommu_enable || !iommu_intremap )
> -        return false;
> +    ASSERT(iommu_enable || iommu_intremap);

I think this should be && in order to match my comments above.

Thanks, Roger.