[xen stable-4.20] x86/HPET: deal with unused channels

[patchbot@xxxxxxx]

commit afc035d6c9527d21e12ba8cc508c8239491c4f10
Author:     Jan Beulich <jbeulich@xxxxxxxx>
AuthorDate: Mon Nov 10 15:59:33 2025 +0100
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Mon Nov 10 15:59:33 2025 +0100

    x86/HPET: deal with unused channels
    
    Keeping channels enabled when they're unused is only causing problems:
    Extra interrupts harm performance, and extra nested interrupts could even
    have caused worse problems. However, on all Intel hardware I looked at
    closely, a 0->1 transition of the enable bit causes an immediate IRQ.
    Hence disabling channels isn't a good idea there. Set a "long" timeout
    instead.
    
    Along with that also "clear" the channel's "next event", for it to be
    properly written by whatever the next user is going to want (possibly
    avoiding too early an IRQ).
    
    Further, along the same lines, don't enable channels early when starting
    up an IRQ. This doesn't need to happen earlier than from
    set_channel_irq_affinity() (once a channel goes into use the very first
    time). This eliminates a single instance of
    
    (XEN) [VT-D]INTR-REMAP: Request device [0000:00:1f.0] fault index 0
    (XEN) [VT-D]INTR-REMAP: reason 25 - Blocked a compatibility format 
interrupt request
    
    during boot. (Why exactly there's only one instance, when we use multiple
    counters and hence multiple IRQs, I can't tell. My understanding would be
    that this was due to __hpet_setup_msi_irq() being called only after
    request_irq() [and hence the .startup handler], yet that should have
    affected all channels.)
    
    Fixes: 3ba523ff957c ("CPUIDLE: enable MSI capable HPET for timer broadcast")
    Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
    Reviewed-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
    master commit: 24f608dc909ed6b6fefb6c35b6de39e851bba6eb
    master date: 2025-10-27 15:51:03 +0100
---
 xen/arch/x86/hpet.c | 10 +++-------
 1 file changed, 3 insertions(+), 7 deletions(-)

diff --git a/xen/arch/x86/hpet.c b/xen/arch/x86/hpet.c
index 7e9d996436..0c788b0f38 100644
--- a/xen/arch/x86/hpet.c
+++ b/xen/arch/x86/hpet.c
@@ -291,12 +291,6 @@ static int hpet_msi_write(struct hpet_event_channel *ch, 
struct msi_msg *msg)
     return 0;
 }
 
-static unsigned int cf_check hpet_msi_startup(struct irq_desc *desc)
-{
-    hpet_msi_unmask(desc);
-    return 0;
-}
-
 #define hpet_msi_shutdown hpet_msi_mask
 
 static void cf_check hpet_msi_set_affinity(
@@ -322,7 +316,7 @@ static void cf_check hpet_msi_set_affinity(
  */
 static hw_irq_controller hpet_msi_type = {
     .typename   = "HPET-MSI",
-    .startup    = hpet_msi_startup,
+    .startup    = irq_startup_none,
     .shutdown   = hpet_msi_shutdown,
     .enable        = hpet_msi_unmask,
     .disable    = hpet_msi_mask,
@@ -522,6 +516,8 @@ static void hpet_detach_channel(unsigned int cpu,
         spin_unlock_irq(&ch->lock);
     else if ( (next = cpumask_first(ch->cpumask)) >= nr_cpu_ids )
     {
+        hpet_write32(hpet_read32(HPET_COUNTER), HPET_Tn_CMP(ch->idx));
+        ch->next_event = STIME_MAX;
         ch->cpu = -1;
         clear_bit(HPET_EVT_USED_BIT, &ch->flags);
         spin_unlock_irq(&ch->lock);
--
generated by git-patchbot for /home/xen/git/xen.git#stable-4.20