Re: [RFC] Expose a memory poison detector ioctl to user space.

[Jue Wang <juew@google.com>]

On Fri, Apr 29, 2022 at 3:29 PM Dave Hansen <dave.hansen@intel.com> wrote:
>
> On 4/29/22 14:32, Jue Wang wrote:
> > On Fri, Apr 29, 2022 at 2:10 PM Dave Hansen <dave.hansen@intel.com> 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