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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
Date: Thu, 16 Nov 2023 15:22:10 -0800	[thread overview]
Message-ID: <ZVakIv5mw6YUlHms@thinky-boi> (raw)
In-Reply-To: <ZVZoKlWrjV1L3CBo@shell.armlinux.org.uk>

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

WARNING: multiple messages have this Message-ID (diff)
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
Date: Thu, 16 Nov 2023 15:22:10 -0800	[thread overview]
Message-ID: <ZVakIv5mw6YUlHms@thinky-boi> (raw)
In-Reply-To: <ZVZoKlWrjV1L3CBo@shell.armlinux.org.uk>

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

_______________________________________________
linux-arm-kernel mailing list
linux-arm-kernel@lists.infradead.org
http://lists.infradead.org/mailman/listinfo/linux-arm-kernel

  reply	other threads:[~2023-11-16 23:22 UTC|newest]

Thread overview: 18+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2023-11-16 11:41 [PATCH] arm64/kvm: Introduce feature extension for SMCCC filter Jianyong Wu
2023-11-16 11:41 ` Jianyong Wu
2023-11-16 13:08 ` Cornelia Huck
2023-11-16 13:08   ` Cornelia Huck
2023-11-16 14:06   ` Salil Mehta
2023-11-16 14:06     ` Salil Mehta
2023-11-21  1:58     ` Jianyong Wu
2023-11-21  1:58       ` Jianyong Wu
2023-11-16 14:21   ` Marc Zyngier
2023-11-16 14:21     ` Marc Zyngier
2023-11-21  2:01     ` Jianyong Wu
2023-11-21  2:01       ` Jianyong Wu
2023-11-16 19:06 ` Russell King (Oracle)
2023-11-16 19:06   ` Russell King (Oracle)
2023-11-16 23:22   ` Oliver Upton [this message]
2023-11-16 23:22     ` Oliver Upton
2023-11-21 10:57     ` Salil Mehta
2023-11-21 10:57       ` Salil Mehta

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