On Fri, May 17, 2024 at 9:16 PM Isaku Yamahata <isaku.yamahata@xxxxxxxxx> wrote: > > On Fri, May 17, 2024 at 06:16:26PM +0000, > "Edgecombe, Rick P" <rick.p.edgecombe@xxxxxxxxx> wrote: > > > On Fri, 2024-05-17 at 02:03 -0700, Isaku Yamahata wrote: > > > > > > On top of your patch, I created the following patch to remove > > > kvm_gfn_for_root(). > > > Although I haven't tested it yet, I think the following shows my idea. > > > > > > - Add gfn_shared_mask to struct tdp_iter. > > > - Use iter.gfn_shared_mask to determine the starting sptep in the root. > > > - Remove kvm_gfn_for_root() > > > > I investigated it. > > Thanks for looking at it. > > > After this, gfn_t's never have shared bit. It's a simple rule. The MMU mostly > > thinks it's operating on a shared root that is mapped at the normal GFN. Only > > the iterator knows that the shared PTEs are actually in a different location. > > > > There are some negative side effects: > > 1. The struct kvm_mmu_page's gfn doesn't match it's actual mapping anymore. > > 2. As a result of above, the code that flushes TLBs for a specific GFN will be > > confused. It won't functionally matter for TDX, just look buggy to see flushing > > code called with the wrong gfn. > > flush_remote_tlbs_range() is only for Hyper-V optimization. In other cases, > x86_op.flush_remote_tlbs_range = NULL or the member isn't defined at compile > time. So the remote tlb flush falls back to flushing whole range. I don't > expect TDX in hyper-V guest. I have to admit that the code looks superficially > broken and confusing. You could add an "&& kvm_has_private_root(kvm)" to kvm_available_flush_remote_tlbs_range(), since kvm_has_private_root(kvm) is sort of equivalent to "there is no 1:1 correspondence between gfn and PTE to be flushed". I am conflicted myself, but the upsides below are pretty substantial. Paolo > > On the positive effects side: > > 1. There is code that passes sp->gfn into things that it shouldn't (if it has > > shared bits) like memslot lookups. > > 2. Also code that passes iter.gfn into things it shouldn't like > > kvm_mmu_max_mapping_level(). > > > > These places are not called by TDX, but if you know that gfn's might include > > shared bits, then that code looks buggy. > > > > I think the solution in the diff is more elegant then before, because it hides > > what is really going on with the shared root. That is both good and bad. Can we > > accept the downsides? > > Kai, do you have any thoughts? > -- > Isaku Yamahata <isaku.yamahata@xxxxxxxxx> >
