Re: [PATCH] arm64/kvm: Introduce feature extension for SMCCC filter

[Oliver Upton]

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