RE: [PATCH 1/3] iommu/vt-d: Setup scalable mode context entry in probe path

[Tian, Kevin]

> From: Baolu Lu <baolu.lu@xxxxxxxxxxxxxxx>
> Sent: Monday, March 4, 2024 4:24 PM
> 
> On 2024/3/4 15:48, Tian, Kevin wrote:
> >> From: Lu Baolu <baolu.lu@xxxxxxxxxxxxxxx>
> >> Sent: Thursday, February 29, 2024 5:48 PM
> >>
> >> +	/*
> >> +	 * For kdump case, at this point, the device is supposed to finish
> >> +	 * reset at its driver probe stage, so no in-flight DMA will exist,
> >> +	 * and we don't need to worry anymore hereafter.
> >> +	 */
> >> +	if (context_copied(iommu, bus, devfn)) {
> >> +		context_clear_entry(context);
> >> +		if (!ecap_coherent(iommu->ecap))
> >> +			clflush_cache_range(context, sizeof(*context));
> >> +		sm_context_flush_caches(dev);
> >> +		clear_context_copied(iommu, bus, devfn);
> >> +	}
> >
> > it's unclear to me why this doesn't need refer to old did as done in the
> > existing code. If scalable mode makes any difference could you extend
> > the comment to explain so people can avoid similar confusion when
> > comparing the different paths between legacy and sm?
> 
> The previous code gets the domain ID from the copied context entry:
> 
> u16 did_old = context_domain_id(context);
> 
> This makes no sense for scalable mode, as the domain ID has been moved
> to the PASID entry in scalable mode. As the result, did_old always gets
> 0.

The point is whether the driver requires to invalidate cache for old did
which is orthogonal to using legacy or sm. If yes, then we should fix the
code to find the right did instead of ignoring it. If no then the legacy path
should be cleared too to avoid unnecessary burden.

> 
> > anyway it's kind of a semantics change probably worth a separate patch
> > to special case sm for bisect and then doing cleanup...
> 
> This change doesn't impact anything as the hardware will skip domain id
> field in the Context-cache Invalidate Descriptor in scalable mode.
> 

no semantics change but if old code has bug we should fix it instead
of carrying the behavior. 😊