Re: [PATCH v2 15/20] KVM: x86/xen: Convert kvm_xen_set_evtchn_fast() to gpc's CLASS() APIs

[Sean Christopherson <seanjc@google.com>]

On Tue, Jun 02, 2026, David Woodhouse wrote:
> On Fri, 29 May 2026 12:11:37 -0700, Sean Christopherson wrote:
> > On Fri, May 29, 2026, sashiko-bot@kernel.org wrote:
> > > [Severity: High]
> > > Does this unintentionally hold the gpc lock across the IPI kick?
> > 
> > No, it intentionally holds the gpc lock across _sending_ the IPI kick.
> > 
> > > [Severity: High]
> > > Does this now hold the VM-wide shinfo_cache lock while calling
> > > __kvm_xen_set_evtchn_fast() and kicking the vCPU?
> > > 
> > > Since shinfo_map is a function-scoped CLASS() variable, its destructor
> > > won't release the lock until after __kvm_xen_set_evtchn_fast() returns.
> > > This creates a nested locking dependency and holds locks over expensive
> > > cross-vCPU operations, potentially serializing event channel deliveries
> > > across the entire VM on the fast path.
> > 
> > __kvm_vcpu_kick() is neither expensive nor cross-vCPU.  In the wait=false case,
> > which is the behavior of kvm_vcpu_kick(), it sends IPIs via smp_send_reschedule(),
> > i.e. it's more or less just __apic_send_IPI(cpu, RESCHEDULE_VECTOR), which is a
> > single WRMSR on modern harware.
> 
> Hm, if we really are going to use a raw rwlock then I suppose we should
> take care not to spend any extra cycles with the lock held unless we
> actually *need* to?
> 
> In which case I wonder if we really need the scoped CLASS() thing (that
> encourages people to run to the end of the function under the lock), or
> if the helpers you add for that are sufficient on their own?

Eh, I would argue that "the code is easier to read" is sufficient justification
so long as holding the lock for a few tens of extra cycles doesn't cause problems
in practice.