[PATCH v6 38/48] KVM: PPC: Book3S HV: Remove support for dependent threads mode on P9

[Nicholas Piggin]

Excerpts from Paul Mackerras's message of April 7, 2021 4:51 pm:
> On Mon, Apr 05, 2021 at 11:19:38AM +1000, Nicholas Piggin wrote:
>> Radix guest support will be removed from the P7/8 path, so disallow
>> dependent threads mode on P9.
> 
> Dependent threads mode on P9 was added in order to support the mode
> where for security reasons you want to restrict the vcpus that run on
> a core to all be from the same VM, without requiring all guests to run
> single-threaded.  This was (at least at one stage) thought to be a
> useful mode for users that are worried about side-channel data leaks.

Right.

> 
> Now it's possible that that mode is not practically useful for some
> reason, or that no-one actually wants it, but its removal should be
> discussed.  Also, the fact that we are losing that mode should be
> explicit in the commit message.

Let's discuss. Did / does anyone really use it or ask for it that you
know of? What do other archs do? Compared with using standard options
that would achive this kind of security (disable SMT, I guess?) how
much is this worth keeping?

It was pretty simple to support when the P8 dependent theads code had
to support P9 anyway. After this series, now all that code is only for
that one feature, so it would be pretty nice to be able to remove it.
How do we reach a point where you'd be okay to remove this and tell 
people to just turn off SMT?

Thanks,
Nick