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Re: [PATCH qemu] spapr_pci: Disable IRQFD resampling on XIVE

From: Cédric Le Goater
Subject: Re: [PATCH qemu] spapr_pci: Disable IRQFD resampling on XIVE
Date: Thu, 28 Apr 2022 09:31:57 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.7.0

On 4/28/22 09:26, Alexey Kardashevskiy wrote:

On 4/28/22 16:25, Cédric Le Goater wrote:
On 4/28/22 07:32, Alexey Kardashevskiy wrote:

On 4/27/22 17:36, Cédric Le Goater wrote:
Hello Alexey,

On 4/27/22 06:36, Alexey Kardashevskiy wrote:
VFIO-PCI has an "KVM_IRQFD_FLAG_RESAMPLE" optimization for INTx EOI
handling when KVM can unmask PCI INTx (level triggered interrupt) without
switching to the userspace (==QEMU).

Unfortunately XIVE does not support level interrupts,

That's not correctly phrased I think.

My bad, I meant "XIVE hardware".

ok. It makes more sense.

PSIHB and PHBs have internal latches to maintain the assertion level.
XIVE has none.

The QEMU XIVE device support LSIs but the POWER9 kernel-irqchips,
KVM XICS-on-XIVE and XIVE native devices, are broken with respect
to passthrough adapters using INTx.

QEMU emulates them
and therefore there is no existing code path to kick the resamplefd.
The problem appears when passing through a PCI adapter with
the "pci=nomsi" kernel parameter - the adapter's interrupt interrupt
count in /proc/interrupts will stuck at "1".

This disables resampler when the XIVE interrupt controller is configured.
This should not be very visible though KVM already exits to QEMU for INTx
and XIVE-capable boxes (POWER9 and newer) do not seem to have
performance-critical INTx-only capable devices.

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>

Cédric, this is what I meant when I said that spapr_pci.c was unaware of
the interrupt controller type, neither xics nor xive was mentioned
in the file before.

  hw/ppc/spapr_pci.c | 14 +++++++++++---
  1 file changed, 11 insertions(+), 3 deletions(-)

diff --git a/hw/ppc/spapr_pci.c b/hw/ppc/spapr_pci.c
index 5bfd4aa9e5aa..2675052601db 100644
--- a/hw/ppc/spapr_pci.c
+++ b/hw/ppc/spapr_pci.c
@@ -729,11 +729,19 @@ static void pci_spapr_set_irq(void *opaque, int irq_num, 
int level)
  static PCIINTxRoute spapr_route_intx_pin_to_irq(void *opaque, int pin)
+    SpaprMachineState *spapr = SPAPR_MACHINE(qdev_get_machine());
      SpaprPhbState *sphb = SPAPR_PCI_HOST_BRIDGE(opaque);
-    PCIINTxRoute route;
+    PCIINTxRoute route = { .mode = PCI_INTX_DISABLED };
-    route.mode = PCI_INTX_ENABLED;
-    route.irq = sphb->lsi_table[pin].irq;
+    /*
+     * Disable IRQFD resampler on XIVE as it does not support LSI and QEMU
+     * emulates those so the KVM kernel resamplefd kick is skipped and EOI
+     * is not delivered to VFIO-PCI.
+     */
+    if (!spapr->xive) {

This is testing the availability of the XIVE interrupt mode, but not
the activate controller. See spapr_irq_init() which is called very
early in the machine initialization.

Is that what we want ? Is everything fine if we start the machine with
ic-mode=xics ? On a POWER9 host, this would use the KVM XICS-on-XIVE
device which is broken also AFAICT.

I should probably fix that in KVM, just not quite sure yet how for the realmode 
handlers, or just drop those on P9 and then the fix is trivial.

You should extend the SpaprInterruptControllerClass (for a routine) or
simply SpaprIrq (for a bool) if you need to handle IRQ matters from a
device model.

It is a property of KVM rather than the interrupt controller so it probably 
makes more sense to just stop advertising KVM_CAP_IRQFD_RESAMPLE. Hmmm...

I would fix the realmode handlers of the the KVM XICS-on-XIVE device
first. The problem has been there for a while.

Are they really used on POWER9? TCE ones are not.

The HCALLs should be.

Then, for the XIVE native mode, I would simply handle it at the QEMU
level with a 'resample' bool in SpaprIrq. It  would be tested in spapr
pci when configuring the INTx routing.

But there is a dedicated CAP advertised by the KVM already which is not correct 
as we know that KVM won't resample.

You know more that I do in that area now.




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