Re: [PATCH v3] s390x/pci: set kvm_msi_via_irqfd_allowed

[Farhan Ali]

Reviewed-by: Farhan Ali <al...@linux.ibm.com>

On 10/8/2025 1:33 PM, Matthew Rosato wrote:

Allow irqfd to be used for virtio-pci on s390x if the kernel supports
it.  This improves s390x virtio-pci performance when using kvm
acceleration by allowing kvm to deliver interrupts instead of QEMU.

Signed-off-by: Matthew Rosato <mjros...@linux.ibm.com>
---
v3:
  - Fix builds when !KVM (Thomas). Tested build on x86 and s390.
  - Note: Dropped all review tags due to the code re-organization.
---
  hw/s390x/s390-pci-bus.c         | 1 +
  include/hw/s390x/s390-pci-kvm.h | 7 +++++++
  2 files changed, 8 insertions(+)

diff --git a/hw/s390x/s390-pci-bus.c b/hw/s390x/s390-pci-bus.c
index e8e41c8a9a..52820894fa 100644
--- a/hw/s390x/s390-pci-bus.c
+++ b/hw/s390x/s390-pci-bus.c
@@ -900,6 +900,7 @@ static void s390_pcihost_realize(DeviceState *dev, Error 
**errp)
      s390_pci_init_default_group();
      css_register_io_adapters(CSS_IO_ADAPTER_PCI, true, false,
                               S390_ADAPTER_SUPPRESSIBLE, errp);
+    s390_pcihost_kvm_realize();
  }

static void s390_pcihost_unrealize(DeviceState *dev)

diff --git a/include/hw/s390x/s390-pci-kvm.h b/include/hw/s390x/s390-pci-kvm.h
index 933814a402..c33f2833a3 100644
--- a/include/hw/s390x/s390-pci-kvm.h
+++ b/include/hw/s390x/s390-pci-kvm.h
@@ -14,12 +14,19 @@

#include "hw/s390x/s390-pci-bus.h"

  #include "hw/s390x/s390-pci-inst.h"
+#include "system/kvm.h"

#ifdef CONFIG_KVM

+static inline void s390_pcihost_kvm_realize(void)
+{
+    kvm_msi_via_irqfd_allowed = kvm_irqfds_enabled();
+}
+
  bool s390_pci_kvm_interp_allowed(void);
  int s390_pci_kvm_aif_enable(S390PCIBusDevice *pbdev, ZpciFib *fib, bool 
assist);
  int s390_pci_kvm_aif_disable(S390PCIBusDevice *pbdev);
  #else
+static inline void s390_pcihost_kvm_realize(void) {}
  static inline bool s390_pci_kvm_interp_allowed(void)
  {
      return false;