KVM: x86: Rescan I/O APIC routes after EOI interception for old routing

Rescan I/O APIC routes for a vCPU after handling an intercepted I/O APIC
EOI for an IRQ that is not targeting said vCPU, i.e. after handling what's
effectively a stale EOI VM-Exit.  If a level-triggered IRQ is in-flight
when IRQ routing changes, e.g. because the guest changes routing from its
IRQ handler, then KVM intercepts EOIs on both the new and old target vCPUs,
so that the in-flight IRQ can be de-asserted when it's EOI'd.

However, only the EOI for the in-flight IRQ needs to be intercepted, as
IRQs on the same vector with the new routing are coincidental, i.e. occur
only if the guest is reusing the vector for multiple interrupt sources.
If the I/O APIC routes aren't rescanned, KVM will unnecessarily intercept
EOIs for the vector and negative impact the vCPU's interrupt performance.

Note, both commit db2bdcbb ("KVM: x86: fix edge EOI and IOAPIC reconfig
race") and commit 0fc5a36d ("KVM: x86: ioapic: Fix level-triggered EOI
and IOAPIC reconfigure race") mentioned this issue, but it was considered
a "rare" occurrence thus was not addressed.  However in real environments,
this issue can happen even in a well-behaved guest.

Cc: Kai Huang <kai.huang@intel.com>
Co-developed-by: xuyun <xuyun_xy.xy@linux.alibaba.com>
Signed-off-by: xuyun <xuyun_xy.xy@linux.alibaba.com>
Signed-off-by: weizijie <zijie.wei@linux.alibaba.com>
[sean: massage changelog and comments, use int/-1, reset at scan]
Reviewed-by: Kai Huang <kai.huang@intel.com>
Link: https://lore.kernel.org/r/20250304013335.4155703-4-seanjc@google.com


Signed-off-by: Sean Christopherson <seanjc@google.com>