Re: [PATCH] KVM: arm64: Add capability for unconditional WFx passthrough

From: Colton Lewis
Date: Tue Mar 12 2024 - 17:28:54 EST


Oliver Upton <oliver.upton@xxxxxxxxx> writes:

Hi Colton,

On Mon, Jan 29, 2024 at 09:39:17PM +0000, Colton Lewis wrote:
Add KVM_CAP_ARM_WFX_PASSTHROUGH capability to always allow WFE/WFI
instructions to run without trapping. Current behavior is to only
allow this if the vcpu is the only task running. This commit keeps the
old behavior when the capability is not set.

This allows userspace to set deterministic behavior and increase
efficiency for platforms with direct interrupt injection support.

Marc and I actually had an offlist conversation (shame on us!) about
this very topic since there are users asking for the _opposite_ of this
patch (unconditionally trap) [*].

I had originally wanted something like this, but Marc made the very good
point that (1) the behavior of WFx traps is in no way user-visible and
(2) it is entirely an IMP DEF behavior. The architecture only requires
the traps be effective if the instruction does not complete in finite
time.

We need to think of an interface that doesn't depend on
implementation-specific behavior, such as a control based on runqueue
depth.

Here's the first thing I came up with after returning to this problem:

We have an ioctl to get/set a threshold value,
wfx_traps_runqueue_depth. If the depth is less than or equal to the
threshold, disable WFx traps. If greater than, enable WFx traps.

Current behavior occurs with a setting of 1. Always trap occurs with a
setting of 0. Never trap occurs with any large enough number.

Of course, having an integer may be more flexible than needed. I can't
imagine a practical use for a number between 1 and UINT_MAX, in which
case it would be better as an enum for different behaviors than a
integer threshold.

What do you think?

Also, do you mean runqueue depth for the current CPU or globally?