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

[Jacob Pan]

Hi Baolu,

On Tue, 9 Apr 2024 11:12:20 +0800, Baolu Lu <baolu.lu@xxxxxxxxxxxxxxx>
wrote:

> On 4/9/24 5:03 AM, Jacob Pan wrote:
> > Hi Lu,  
> 
> Hi Jacob,
> 
> > 
> > On Sun,  7 Apr 2024 22:42:32 +0800, Lu Baolu<baolu.lu@xxxxxxxxxxxxxxx>
> > 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.
> >>  
> > I agree with the changes below, what about this CM check:
> > 
> > /* Notification for newly created mappings */
> > static void __mapping_notify_one(struct intel_iommu *iommu, struct
> > dmar_domain *domain, unsigned long pfn, unsigned int pages)
> > {
> > 	/*
> > 	 * It's a non-present to present mapping. Only flush if caching
> > mode
> > 	 * and second level.
> > 	 */
> > 	if (cap_caching_mode(iommu->cap) && !domain->use_first_level)
> > 		iommu_flush_iotlb_psi(iommu, domain, pfn, pages, 0, 1);
> > 
> > We are still tying devTLB flush to CM=1, no?  
> 
> __mapping_notify_one() is called in the path where some PTEs are changed
> from non-present to present.
> 
> In this scenario,
> 
> - if CM is set and first-stage translation is not used, the IOTLB caches
>    are required to be explicitly flushed.
> - else if hardware requires write buffer flushing, do it.
> - Otherwise, no op.
> - devtlb invalidation is irrelevant to this path.
> 
> The code after the fix appears to do the right thing. devTLB is not
> invalidated in iommu_flush_iotlb_psi() since it's a map (map == 1).
> 
> Or perhaps I overlooked anything?
My confusion is that, on one side, this patch is saying devTLB flush has
nothing to do with CM. But here, if CMD==1, we don't flush devTLB since
map==1.

If the guest uses SL page tables in vIOMMU, we don;t expose ATS to the
guest. So ATS is not relevant here, does't matter map or unmap.

Can we remove the map argument in iommu_flush_iotlb_psi(iommu, domain,pfn,
pages, 0, 1)?

Then devTLB flush will naturally be skipped in the guest (CM=1, SL) since
ATS is not enabled.
iommu_flush_dev_iotlb(domain, addr, mask);

i.e.

diff --git a/drivers/iommu/intel/iommu.c b/drivers/iommu/intel/iommu.c
index 50eb9aed47cc..ee3e5a1af0c5 100644
--- a/drivers/iommu/intel/iommu.c
+++ b/drivers/iommu/intel/iommu.c
@@ -1483,7 +1483,7 @@ static void __iommu_flush_iotlb_psi(struct intel_iommu *iommu, u16 did,
 static void iommu_flush_iotlb_psi(struct intel_iommu *iommu,
                                  struct dmar_domain *domain,
                                  unsigned long pfn, unsigned int pages,
-                                 int ih, int map)
+                                 int ih)
 {
        unsigned int aligned_pages = __roundup_pow_of_two(pages);
        unsigned int mask = ilog2(aligned_pages);
@@ -1501,12 +1501,7 @@ static void iommu_flush_iotlb_psi(struct intel_iommu *iommu,
        else
                __iommu_flush_iotlb_psi(iommu, did, pfn, pages, ih);
 
-       /*
-        * In caching mode, changes of pages from non-present to present require
-        * flush. However, device IOTLB doesn't need to be flushed in this case.
-        */
-       if (!cap_caching_mode(iommu->cap) || !map)
-               iommu_flush_dev_iotlb(domain, addr, mask);
+       iommu_flush_dev_iotlb(domain, addr, mask);
 }
 
 /* Notification for newly created mappings */
@@ -1518,7 +1513,7 @@ static void __mapping_notify_one(struct intel_iommu *iommu, struct dmar_domain *
         * and second level.
         */
        if (cap_caching_mode(iommu->cap) && !domain->use_first_level)
-               iommu_flush_iotlb_psi(iommu, domain, pfn, pages, 0, 1);
+               iommu_flush_iotlb_psi(iommu, domain, pfn, pages, 0);
        else
                iommu_flush_write_buffer(iommu);
 }



> > 
> > If we are running in the guest with second level page table (shadowed),
> > can we decide if devTLB flush is needed based on ATS enable just as the
> > rest of the cases?  
> 
> I think the ATS check should be consistent. It's generic no matter how
> the IOMMU is implemented (in hardware or emulated in software).
> 
> Best regards,
> baolu


Thanks,

Jacob