x86/irq: Install posted MSI notification handler

All MSI vectors are multiplexed into a single notification vector when
posted MSI is enabled. It is the responsibility of the notification vector
handler to demultiplex MSI vectors. In the handler the MSI vector handlers
are dispatched without IDT delivery for each pending MSI interrupt.

For example, the interrupt flow will change as follows:
(3 MSIs of different vectors arrive in a a high frequency burst)

BEFORE:
interrupt(MSI)
    irq_enter()
    handler() /* EOI */
    irq_exit()
        process_softirq()
interrupt(MSI)
    irq_enter()
    handler() /* EOI */
    irq_exit()
        process_softirq()
interrupt(MSI)
    irq_enter()
    handler() /* EOI */
    irq_exit()
        process_softirq()

AFTER:
interrupt /* Posted MSI notification vector */
    irq_enter()
	atomic_xchg(PIR)
	handler()
	handler()
	handler()
	pi_clear_on()
    apic_eoi()
    irq_exit()
        process_softirq()

Except for the leading MSI, CPU notifications are skipped/coalesced.

For MSIs which arrive at a low frequency, the demultiplexing loop does not
wait for more interrupts to coalesce. Therefore, there's no additional
latency other than the processing time.

Signed-off-by: Jacob Pan <jacob.jun.pan@linux.intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20240423174114.526704-9-jacob.jun.pan@linux.intel.com

1 files changed, 2 insertions, 0 deletions

diff --git a/arch/x86/entry/entry_fred.c b/arch/x86/entry/entry_fred.c
index 89c1476fcdd9..f004a4dc74c2 100644
--- a/arch/x86/entry/entry_fred.c
+++ b/arch/x86/entry/entry_fred.c
@@ -117,6 +117,8 @@ static idtentry_t sysvec_table[NR_SYSTEM_VECTORS] __ro_after_init = {
 	SYSVEC(POSTED_INTR_VECTOR,		kvm_posted_intr_ipi),
 	SYSVEC(POSTED_INTR_WAKEUP_VECTOR,	kvm_posted_intr_wakeup_ipi),
 	SYSVEC(POSTED_INTR_NESTED_VECTOR,	kvm_posted_intr_nested_ipi),
+
+	SYSVEC(POSTED_MSI_NOTIFICATION_VECTOR,	posted_msi_notification),
 };
 
 static bool fred_setup_done __initdata;