According to the spec (ARM IHI 0070 F.b), in
"5.5 Fault configuration (A, R, S bits)":
    A STE with stage 2 translation enabled and STE.S2S == 0 is
    considered ILLEGAL if SMMU_IDR0.STALL_MODEL == 0b10.

Also described in the pseudocode “SteIllegal()”
    if STE.Config == '11x' then
        [..]
        if eff_idr0_stall_model == '10' && STE.S2S == '0' then
            // stall_model forcing stall, but S2S == 0
            return TRUE;

Which means, S2S must be set when stall model is
"ARM_SMMU_FEAT_STALL_FORCE", but currently the driver ignores that.

Although, the driver can do the minimum and only set S2S for
“ARM_SMMU_FEAT_STALL_FORCE”, it is more consistent to match S1
behaviour, which also sets it for “ARM_SMMU_FEAT_STALL” if the
master has requested stalls.

Also, since S2 stalls are enabled now, report them to the IOMMU layer
and for VFIO devices it will fail anyway as VFIO doesn’t register an
iopf handler.
Signed-off-by: Mostafa Saleh <smostafa@google.com>
Link: https://lore.kernel.org/r/20240830110349.797399-2-smostafa@google.comSigned-off-by: Will Deacon <will@kernel.org>