Re: [PATCH] xen/arm: Enhance IPMMU-VMSA driver robustness and debug output

[Julien Grall <julien@xxxxxxx>]

Hi Jahan,

On 07/07/2025 12:24, Jahan Murudi wrote:
> > On 30/06/2025 13:44, Julien Grall wrote:
> > > On 25/06/2025 16:53, Julien Grall wrote:
> > > Hi Jahan,
> > > > > > > +    dsb(sy);
> > > > > > Any clue why Linux (mainline) does not do that?
> > I understand for the PCI passthrough, Xen will be using stage-2, so in theory 
> > the stage-1 could be used by the guest OS. But ultimately, this is the same PCI 
> > device behind. So if it is not coherent, it should be for both stages. Do you 
> > have any pointer to the documentation that would state otherwise?
> You're right - coherency characteristics are identical for both stages. My 
> earlier understanding was incorrect.
>
> > Note, I just noticed that IOMMU_FEAT_COHERENT_WALK is not set for the IPMMU. So the 
> > "dsb sy" is coherent. However, I find doubful an IOMMU would have a difference 
> > of coherency between two stages. So maybe we should set the flag either unconditionally 
> > or based on a register.
> Excellent observation. Current R-car IPMMU doesn't supports coherent walks - we 
> should indeed set this flag unconditionally.
To clarify, the R-car IPMMU doesn't support coherent walk, then the flag 
IOMMU_FEAT_COHERENT_WALK *must not* be set.
Cheers,

--
Julien Grall