[RFC PATCH] x86/irq: Drop redundant irq_{enter,exit}() calls from posted MSI handler

[Sean Christopherson]

Drop the unnecessary irq_{enter,exit}() calls from the posted MSI handler,
as the wrapper provided by DEFINE_IDTENTRY_SYSVEC() does all of the
necessary bookkeeping (the x86 specific entry/exit code provides a
superset of functionality).

Note, in the series[1] that added DEFINE_IDTENTRY_SYSVEC(), the follow-up
patches that actually use DEFINE_IDTENTRY_SYSVEC() drop all
{entering,exiting}_irq() pairs (including entering_ack_irq()), where
{entering,exiting}_irq() were little more than wrappers to
irq_{enter,exit}().

TODO: Explain what, if anything, can go wrong?

Link: https://lore.kernel.org/all/20200521200513.656533920@xxxxxxxxxxxxx [1]
Fixes: 1b03d82ba15e ("x86/irq: Install posted MSI notification handler")
Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
---

RFC purely because I have no idea what can go wrong, and I'd like to document
any potential issues in the changelog.  I'm confident in the change itself,
or I've failed miserably at reading the code. :-)

Found by inspection when looking at a bizarre code fetch from RIP=0 #PF that
traces back to the irq_enter() (no reproducer for the #PF at this time).  I
don't know if the redundant bookkeeping contributed in any way, but the calls
to irq_enter() and irq_exit() really stand out, because AFAICT, these are the
*only* calls to those APIs in x86.

 arch/x86/kernel/irq.c | 2 --
 1 file changed, 2 deletions(-)

diff --git a/arch/x86/kernel/irq.c b/arch/x86/kernel/irq.c
index 30122f0b3af9..0ac63628a585 100644
--- a/arch/x86/kernel/irq.c
+++ b/arch/x86/kernel/irq.c
@@ -430,7 +430,6 @@ DEFINE_IDTENTRY_SYSVEC(sysvec_posted_msi_notification)
 	/* Mark the handler active for intel_ack_posted_msi_irq() */
 	__this_cpu_write(posted_msi_handler_active, true);
 	inc_irq_stat(POSTED_MSI_NOTIFICATION);
-	irq_enter();
 
 	/*
 	 * Loop only MAX_POSTED_MSI_COALESCING_LOOP - 1 times here to take
@@ -456,7 +455,6 @@ DEFINE_IDTENTRY_SYSVEC(sysvec_posted_msi_notification)
 	handle_pending_pir(pid->pir, regs);
 
 	apic_eoi();
-	irq_exit();
 	__this_cpu_write(posted_msi_handler_active, false);
 	set_irq_regs(old_regs);
 }

base-commit: dc59e4fea9d83f03bad6bddf3fa2e52491777482
-- 
2.55.0.rc0.799.gd6f94ed593-goog