On Fri, Apr 29, 2022 at 3:29 PM Dave Hansen <dave.hansen@xxxxxxxxx> wrote: > > On 4/29/22 14:32, Jue Wang wrote: > > On Fri, Apr 29, 2022 at 2:10 PM Dave Hansen <dave.hansen@xxxxxxxxx> wrote: > >> I wouldn't go that far. The unaccepted TDX guest memory thing is just > >> the obvious one at the moment. There are a whole ton of other guest > >> ballooning mechanisms out there and I'm not sure that all of them are > >> happy to let you touch ballooned-away memory. > > > > This type of scanning is intended to be run on the host side. That > > should avoid concerns around the guest ballooning or any effects to > > the host side reclaim that's based on the guest's working set. > > Hint: Talk is cheap. Just saying how it is intended doesn't avoid > concerns. > > Saying how it is intended, then backing up that intent with code and > deliberate design that matches that intent would be nice. > > > I don't know why a guest wants to spend its CPU cycles and pollute its > > caches etc to run this scanner, anyway. This should be a benefit > > provide by the cloud platform transparently to the guest. > > "This should only be used by and made available by cloud providers!" ... > says the cloud provider. ;) This is a much better way to put it. How to express in design that some kernel component that is "best to be used by and made available by cloud providers" is what I like to get some feedback on. :-) > > Also, who said anything about polluting the caches? Aren't there lots > of reasons for a memory poison detector to intentionally not use the > caches? First, you really *do* always want to go to memory. That's > kinda the point. If this code hits the caches, it's kinda pointless. > > Second, you want this code to have a low profile. Not polluting the > caches seems like a good way to have a low profile. > We were experimenting with some non-temporal prefetch hint (prefetchnta) that worked as intended based on perf measurement. The pollution to LLC is minimal but non-zero. This is definitely an area we want to keep iterating on, love to hear feedback. Thanks, -Jue
