Re: [RFC 0/3] Export APICv-related state via binary stats interface

[Sean Christopherson <seanjc@google.com>]

On Thu, Feb 15, 2024, Alejandro Jimenez wrote:
> The goal of this RFC is to agree on a mechanism for querying the state (and
> related stats) of APICv/AVIC. I clearly have an AVIC bias when approaching this
> topic since that is the side that I have mostly looked at, and has the greater
> number of possible inhibits, but I believe the argument applies for both
> vendor's technologies.
> 
> Currently, a user or monitoring app trying to determine if APICv is actually
> being used needs implementation-specific knowlegde in order to look for specific
> types of #VMEXIT (i.e. AVIC_INCOMPLETE_IPI/AVIC_NOACCEL), checking GALog events
> by watching /proc/interrupts for AMD-Vi*-GA, etc. There are existing tracepoints
> (e.g. kvm_apicv_accept_irq, kvm_avic_ga_log) that make this task easier, but
> tracefs is not viable in some scenarios. Adding kvm debugfs entries has similar
> downsides. Suravee has previously proposed a new IOCTL interface[0] to expose
> this information, but there has not been any development in that direction.
> Sean has mentioned a preference for using BPF to extract info from the current
> tracepoints, which would require reworking existing structs to access some
> desired data, but as far as I know there isn't any work done on that approach
> yet.
> 
> Recently Joao mentioned another alternative: the binary stats framework that is
> already supported by kernel[1] and QEMU[2].

The hiccup with stats are that they are ABI, e.g. we can't (easily) ditch stats
once they're added, and KVM needs to maintain the exact behavior.

Tracepoints are explicitly not ABI, and so we can be much more permissive when it
comes to adding/expanding tracepoints, specifically because there are no guarantees
provided to userspace.