On Wed, 2024-05-15 at 12:48 -0700, Sean Christopherson wrote: > > It's just another little quirk in an already complicated solution. They > > third > > thing we discussed was somehow rejecting or not supporting non-coherent DMA. > > This seemed simpler than that. > > Again, huh? This has _nothing_ to do with non-coherent DMA. Devices can't > DMA > into TDX private memory. Hmm... I'm confused how you are confused... :) For normal VMs (after that change you linked), guests will honor guest PAT on newer HW. On older HW it will only honor guest PAT if non-coherent DMA is attached. For TDX we can't honor guest PAT for private memory. So we can either have: 1. Have shared honor PAT and private not. 2. Have private and shared both not honor PAT and be consistent. Unless non- coherent DMA is attached. In that case KVM could zap shared only and switch to 1. The only benefit of 2 is that in normal conditions the guest will have consistent cache behavior between private and shared. FWIW, there was at one time a use for private uncacheable memory proposed. It was for keeping non-performance sensitive secret data protected from speculative access. (not for TDX, a general kernel thing). This isn't a real thing today, but it's an example of how the private/shared split is quirky, when you ask "do TDs support PAT?". 1 is a little quirky, but 2 is too complex and also quirky. 1 is the best option. If it's obvious we can trim down the log. There was a bit of hand wringing on this one, so seemed relevant to discussion. The other point was to describe why we don't need to support kvm_zap_gfn_range(). I think that point is worth review. The KVM_BUG_ON() is not super critical so we could even drop the patch if its all settled.
