Re: [PATCH v5 3/3] KVM: x86: Rescan I/O APIC routes after EOI interception for old routing

["Huang, Kai" <kai.huang@intel.com>]

On Mon, 2025-03-03 at 17:33 -0800, Sean Christopherson wrote:
> From: weizijie <zijie.wei@linux.alibaba.com>
> 
> 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 change routing from its
						   ^
						   changes ?

> 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 intercepted, as IRQs
							^
							be intercepted

> 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 db2bdcbbbd32 ("KVM: x86: fix edge EOI and IOAPIC reconfig
> race") and commit 0fc5a36dd6b3 ("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]
> Signed-off-by: Sean Christopherson <seanjc@google.com>

Reviewed-by: Kai Huang <kai.huang@intel.com>