> From: David Gibson <david@xxxxxxxxxxxxxxxxxxxxx> > Sent: Tuesday, June 8, 2021 8:50 AM > > On Thu, Jun 03, 2021 at 06:49:20AM +0000, Tian, Kevin wrote: > > > From: David Gibson > > > Sent: Thursday, June 3, 2021 1:09 PM > > [...] > > > > > In this way the SW mode is the same as a HW mode with an infinite > > > > > cache. > > > > > > > > > > The collaposed shadow page table is really just a cache. > > > > > > > > > > > > > OK. One additional thing is that we may need a 'caching_mode" > > > > thing reported by /dev/ioasid, indicating whether invalidation is > > > > required when changing non-present to present. For hardware > > > > nesting it's not reported as the hardware IOMMU will walk the > > > > guest page table in cases of iotlb miss. For software nesting > > > > caching_mode is reported so the user must issue invalidation > > > > upon any change in guest page table so the kernel can update > > > > the shadow page table timely. > > > > > > For the fist cut, I'd have the API assume that invalidates are > > > *always* required. Some bypass to avoid them in cases where they're > > > not needed can be an additional extension. > > > > > > > Isn't a typical TLB semantics is that non-present entries are not > > cached thus invalidation is not required when making non-present > > to present? > > Usually, but not necessarily. > > > It's true to both CPU TLB and IOMMU TLB. > > I don't think it's entirely true of the CPU TLB on all ppc MMU models > (of which there are far too many). > > > In reality > > I feel there are more usages built on hardware nesting than software > > nesting thus making default following hardware TLB behavior makes > > more sense... > > I'm arguing for always-require-invalidate because it's strictly more > general. Requiring the invalidate will support models that don't > require it in all cases; we just make the invalidate a no-op. The > reverse is not true, so we should tackle the general case first, then > optimize. > It makes sense. Will adopt this way.
