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

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Date: Mon, 19 Feb 2024 17:38:15 +0800
From: "sundongxu (A)" <sundongxu3@...wei.com>
To: Oliver Upton <oliver.upton@...ux.dev>, Marc Zyngier <maz@...nel.org>
CC: James Morse <james.morse@....com>, Suzuki K Poulose
	<suzuki.poulose@....com>, Zenghui Yu <yuzenghui@...wei.com>, Catalin Marinas
	<catalin.marinas@....com>, Will Deacon <will@...nel.org>, Paolo Bonzini
	<pbonzini@...hat.com>, <linux-arm-kernel@...ts.infradead.org>,
	<kvmarm@...ts.linux.dev>, <linux-kernel@...r.kernel.org>, Colton Lewis
	<coltonlewis@...gle.com>, <wanghaibin.wang@...wei.com>
Subject: Re: [PATCH] KVM: arm64: Add capability for unconditional WFx
 passthrough

Hi Marc, Oliver,

On 2024/1/30 7:17, Oliver Upton wrote:
> 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.

If I understand correctly, this run queue belongs to the scheduler, right?
And I will be appreciated if you can share any more detail information
about this.

> 
> [*] https://lore.kernel.org/kvmarm/a481ef04-ddd2-dfc1-41b1-d2ec45c6a3b5@huawei.com/
> 

Thanks,
Dongxu