Re: [Xen-devel] [PATCH 2/2] IOMMU/MMU: Adjust low level functions for VT-d Device-TLB flush error.

["Xu, Quan" <quan.xu@xxxxxxxxx>]

On April 11, 2016 11:10am, <quan.xu@xxxxxxxxx> wrote:
> On March 29, 2016 3:37pm, Jan Beulich <JBeulich@xxxxxxxx> wrote:
> > >>> On 25.03.16 at 10:27, <quan.xu@xxxxxxxxx> wrote:
> > > On March 18, 2016 6:20pm, <JBeulich@xxxxxxxx> wrote:
> > >> >>> On 17.03.16 at 07:54, <quan.xu@xxxxxxxxx> wrote:
> > >> > --- a/xen/drivers/passthrough/iommu.c
> > >> > +++ b/xen/drivers/passthrough/iommu.c
> > >> > @@ -554,11 +555,24 @@ static void iommu_flush_all(void)
> > >> >          iommu = drhd->iommu;
> > >> >          iommu_flush_context_global(iommu, 0);
> > >> >          flush_dev_iotlb = find_ats_dev_drhd(iommu) ? 1 : 0;
> > >> > -        iommu_flush_iotlb_global(iommu, 0, flush_dev_iotlb);
> > >> > +        rc = iommu_flush_iotlb_global(iommu, 0,
> > >> > + flush_dev_iotlb);
> > >> > +
> > >> > +        if ( rc > 0 )
> > >> > +        {
> > >> > +            iommu_flush_write_buffer(iommu);
> > >>
> > >> Why is this needed all of the sudden?
> > >
> > > As there may be multiple IOMMUs. .e.g, there are 2 IOMMUs in my
> > > machine, and I can find the following log message:
> > > """
> > > (XEN) Intel VT-d iommu 0 supported page sizes: 4kB, 2MB, 1GB.
> > > (XEN) Intel VT-d iommu 1 supported page sizes: 4kB, 2MB, 1GB.
> > > """
> > > __iiuc__, iommu_flush_write_buffer() is per IOMMU, so It should be
> > > called to flush every IOMMU.
> >
> > For one what you say suggests that right now this is being done for
> > some (one?) IOMMU(s), which I don't see being the case. And then what
> > you say _still_ doesn't say _why_ this is now needed all of the
> > sudden. If, in the course of doing your re-work here, you find
> > pre-existing issues with the code, please split the necessary fixes
> > out of your re-work and submit them separately with proper explanations in
> their commit messages.
> >
> 
> I find out it is no need modification for this function.
Sorry, this modification refers to as:
"
+        if ( rc > 0 )
+        {
+            iommu_flush_write_buffer(iommu);
+            rc = 0;
+        }
"

at least errors need to be propagated.


Quan

> I overlooked the parameter 'flush_non_present_entry', which is '0' to call
> iommu_flush_iotlb_global().
> At the bottom of the call tree,
> (Existing code)
> >>
> In flush->iotlb():
> {
> ....
>     if ( flush_non_present_entry )
>     {
>         if ( !cap_caching_mode(iommu->cap) )
>             return 1;
>         else
>             did = 0;
>     }
> 
> ....
> }
> <<
> 
> 
> it is impossible to return '1', which is used to indicate caller needs to 
> flush
> cache.

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