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

[Baolu Lu]

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.

Best regards,
baolu