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

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Message-ID: <gsntv85rrvg5.fsf@coltonlewis-kvm.c.googlers.com>
Date: Tue, 12 Mar 2024 21:28:42 +0000
From: Colton Lewis <coltonlewis@...gle.com>
To: Oliver Upton <oliver.upton@...ux.dev>
Cc: kvm@...r.kernel.org, maz@...nel.org, james.morse@....com, 
	suzuki.poulose@....com, yuzenghui@...wei.com, catalin.marinas@....com, 
	will@...nel.org, pbonzini@...hat.com, linux-arm-kernel@...ts.infradead.org, 
	kvmarm@...ts.linux.dev, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: arm64: Add capability for unconditional WFx passthrough

Oliver Upton <oliver.upton@...ux.dev> 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?