Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::3:7 as permitted sender) client-ip=2620:137:e000::3:7;
Date:   Thu, 16 Nov 2023 15:22:10 -0800
From:   Oliver Upton <oliver.upton@linux.dev>
To:     "Russell King (Oracle)" <linux@armlinux.org.uk>
Cc:     Jianyong Wu <jianyong.wu@arm.com>, maz@kernel.org,
        james.morse@arm.com, will@kernel.org, salil.mehta@huawei.com,
        suzuki.poulose@arm.com, linux-arm-kernel@lists.infradead.org,
        kvmarm@lists.linux.dev, linux-kernel@vger.kernel.org,
        justin.he@arm.com
Subject: Re: [PATCH] arm64/kvm: Introduce feature extension for SMCCC filter
Message-ID: <ZVakIv5mw6YUlHms@thinky-boi>
References: <20231116114152.912344-1-jianyong.wu@arm.com>
 <ZVZoKlWrjV1L3CBo@shell.armlinux.org.uk>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <ZVZoKlWrjV1L3CBo@shell.armlinux.org.uk>
Precedence: bulk

On Thu, Nov 16, 2023 at 07:06:18PM +0000, Russell King (Oracle) wrote:
> On Thu, Nov 16, 2023 at 11:41:52AM +0000, Jianyong Wu wrote:
> > 821d935c87b introduces support for userspace SMCCC filtering, but lack
> > of a way to tell userspace if we have this feature. Add a corresponding
> > feature extension can resolve this issue.
> > 
> > For example, the incoming feature Vcpu Hotplug needs the SMCCC filter.
> > As there is no way to check this feature, VMM will run into error when
> > it calls this feature on an old kernel. It's bad for backward compatible.
> 
> Can't you just attempt to use the SMCCC filtering, and if it errors out
> with the appropriate error code, decide that SMCCC filtering is not
> available?

That would also work, as we return ENXIO for the unsupported ioctl.

> That's how most things like kernel syscalls work - if they're not
> implemented they return -ENOSYS. glibc can detect that and use a
> fallback.

I generally agree, but KVM has gone in the other direction of providing
auxiliary interfaces for discovering new UAPI. ENXIO has been slightly
overloaded to imply that a given ioctl is non-existent or otherwise
unsupported due to some dynamic configuration.

Is it ideal? Of course not. With that said userspace may as well use the
preferred / documented discoverability mechanism. And in Jianyong's case
the KVM documentation is rather unambiguous (for once) about how you
discover device attributes.

https://docs.kernel.org/virt/kvm/api.html#kvm-has-device-attr

-- 
Thanks,
Oliver