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Date: Mon, 29 Jan 2024 23:17:06 +0000
From: Oliver Upton <oliver.upton@linux.dev>
To: Colton Lewis <coltonlewis@google.com>
Cc: kvm@vger.kernel.org, Marc Zyngier <maz@kernel.org>,
	James Morse <james.morse@arm.com>,
	Suzuki K Poulose <suzuki.poulose@arm.com>,
	Zenghui Yu <yuzenghui@huawei.com>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will@kernel.org>, Paolo Bonzini <pbonzini@redhat.com>,
	linux-arm-kernel@lists.infradead.org, kvmarm@lists.linux.dev,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH] KVM: arm64: Add capability for unconditional WFx
 passthrough
Message-ID: <Zbgx8hZgWCmtzMjH@linux.dev>
References: <20240129213918.3124494-1-coltonlewis@google.com>
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In-Reply-To: <20240129213918.3124494-1-coltonlewis@google.com>

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.

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

-- 
Thanks,
Oliver