Re: [PATCH 2/2] iommu/vt-d: Remove caching mode check before devtlb flush

[Ethan Zhao]

On 4/8/2024 3:23 PM, Baolu Lu wrote:
> On 2024/4/8 15:21, Ethan Zhao wrote:
> > On 4/7/2024 10:42 PM, Lu Baolu wrote:
> > > The Caching Mode (CM) of the Intel IOMMU indicates if the hardware
> > > implementation caches not-present or erroneous translation-structure
> > > entries except the first-stage translation. The caching mode is
> > > unrelated to the device TLB , therefore there is no need to check
> > > it before a device TLB invalidation operation.
> > >
> > > Before the scalable mode is introduced, caching mode is treated as
> > > an indication that the driver is running in a VM guest. This is just
> > > a software contract as shadow page table is the only way to implement
> > > a virtual IOMMU. But the VT-d spec doesn't state this anywhere. After
> > > the scalable mode is introduced, this doesn't stand for anymore, as
> > > caching mode is not relevant for the first-stage translation. A virtual
> > > IOMMU implementation is free to support first-stage translation only
> > > with caching mode cleared.
> > >
> > > Remove the caching mode check before device TLB invalidation to ensure
> > > compatibility with the scalable mode use cases.
> > >
> > > Fixes: 792fb43ce2c9 ("iommu/vt-d: Enable Intel IOMMU scalable mode 
> > > by default")
> > > Signed-off-by: Lu Baolu <baolu.lu@xxxxxxxxxxxxxxx>
> > > ---
> > >   drivers/iommu/intel/iommu.c | 5 ++---
> > >   1 file changed, 2 insertions(+), 3 deletions(-)
> > >
> > > diff --git a/drivers/iommu/intel/iommu.c b/drivers/iommu/intel/iommu.c
> > > index 493b6a600394..681789b1258d 100644
> > > --- a/drivers/iommu/intel/iommu.c
> > > +++ b/drivers/iommu/intel/iommu.c
> > > @@ -1501,7 +1501,7 @@ static void iommu_flush_iotlb_psi(struct 
> > > intel_iommu *iommu,
> > >       else
> > >           __iommu_flush_iotlb_psi(iommu, did, pfn, pages, ih);
> > > -    if (!cap_caching_mode(iommu->cap) && !map)
> > > +    if (!map)
> >
> > My understanding, we don't need patch[1/2] at all, and customer is 
> > just asking
> > about the CM & tlb flushing, it is great to have this commit [2/2].
>
> Actually they fix different problems.

Yup, progress view.

Thanks,
Ethan

> Best regards,
> baolu