Messages in this thread | | | Date | Thu, 16 Nov 2023 15:22:10 -0800 | From | Oliver Upton <> | Subject | Re: [PATCH] arm64/kvm: Introduce feature extension for SMCCC filter |
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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
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