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From:   Oliver Upton <oupton@google.com>
Date:   Mon, 19 Jul 2021 12:37:52 -0700
Message-ID: <CAOQ_QsjyP0PMGOorTss2Fpn011mHPwVqQ72x26Gs2L0bg2amsQ@mail.gmail.com>
Subject: Re: [RFC PATCH 0/5] KVM: arm64: Pass PSCI to userspace
To:     Jean-Philippe Brucker <jean-philippe@linaro.org>
Cc:     Alexandru Elisei <Alexandru.Elisei@arm.com>,
        salil.mehta@huawei.com, lorenzo.pieralisi@arm.com,
        kvm@vger.kernel.org, corbet@lwn.net, maz@kernel.org,
        linux-kernel@vger.kernel.org, jonathan.cameron@huawei.com,
        catalin.marinas@arm.com, pbonzini@redhat.com, will@kernel.org,
        kvmarm@lists.cs.columbia.edu, linux-arm-kernel@lists.infradead.org
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On Mon, Jul 19, 2021 at 11:02 AM Jean-Philippe Brucker
<jean-philippe@linaro.org> wrote:
> We forward the whole PSCI function range, so it's either KVM or userspace.
> If KVM manages PSCI and the guest calls an unimplemented function, that
> returns directly to the guest without going to userspace.
>
> The concern is valid for any other range, though. If userspace enables the
> HVC cap it receives function calls that at some point KVM might need to
> handle itself. So we need some negotiation between user and KVM about the
> specific HVC ranges that userspace can and will handle.

Are we going to use KVM_CAPs for every interesting HVC range that
userspace may want to trap? I wonder if a more generic interface for
hypercall filtering would have merit to handle the aforementioned
cases, and whatever else a VMM will want to intercept down the line.

For example, x86 has the concept of 'MSR filtering', wherein userspace
can specify a set of registers that it wants to intercept. Doing
something similar for HVCs would avoid the need for a kernel change
each time a VMM wishes to intercept a new hypercall.

--
Thanks,
Oliver