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Re: [RFC 2/8] s390x/pci: MSI-X isn't strictly required for passthrough

From: Matthew Rosato
Subject: Re: [RFC 2/8] s390x/pci: MSI-X isn't strictly required for passthrough
Date: Thu, 17 Dec 2020 10:12:37 -0500
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.4.0

On 12/17/20 8:08 AM, Cornelia Huck wrote:
On Thu, 10 Dec 2020 10:13:29 -0500
Matthew Rosato <mjrosato@linux.ibm.com> wrote:

On 12/10/20 5:28 AM, Cornelia Huck wrote:
On Wed,  9 Dec 2020 15:34:20 -0500
Matthew Rosato <mjrosato@linux.ibm.com> wrote:
s390 PCI currently disallows PCI devices without the MSI-X capability.
However, this fence doesn't make sense for passthrough devices.  Move
the check to only fence emulated devices (e.g., virtio).

Signed-off-by: Matthew Rosato <mjrosato@linux.ibm.com>
Reviewed-by: Pierre Morel <pmorel@linux.ibm.com>
   hw/s390x/s390-pci-bus.c | 14 ++++++++------
   1 file changed, 8 insertions(+), 6 deletions(-)

diff --git a/hw/s390x/s390-pci-bus.c b/hw/s390x/s390-pci-bus.c
index 05f7460..afad048 100644
--- a/hw/s390x/s390-pci-bus.c
+++ b/hw/s390x/s390-pci-bus.c
@@ -1028,12 +1028,12 @@ static void s390_pcihost_plug(HotplugHandler 
*hotplug_dev, DeviceState *dev,
           } else {
               pbdev->fh |= FH_SHM_EMUL;
-        }
- if (s390_pci_msix_init(pbdev)) {
-            error_setg(errp, "MSI-X support is mandatory "
-                       "in the S390 architecture");
-            return;
+            if (s390_pci_msix_init(pbdev)) {
+                error_setg(errp, "MSI-X support is mandatory "
+                           "in the S390 architecture");
+                return;
+            }
if (dev->hotplugged) {
@@ -1073,7 +1073,9 @@ static void s390_pcihost_unplug(HotplugHandler 
*hotplug_dev, DeviceState *dev,
           devfn = pci_dev->devfn;
- s390_pci_msix_free(pbdev);
+        if (pbdev->fh & FH_SHM_EMUL) {
+            s390_pci_msix_free(pbdev);
+        }
           s390_pci_iommu_free(s, bus, devfn);
           pbdev->pdev = NULL;
           pbdev->state = ZPCI_FS_RESERVED;

Remind me: Wasn't it only msi that was strictly required (i.e., not msi-x?)

Can we generally relax this requirement, possibly with some changes in
the adapter interrupt mapping? I might misremember, though.

Yes, but even so our current emulation support only sets up for MSI-X,
it does not have an msi_init() equivalent.  I do believe that this
requirement can be relaxed at some point for the emulation support as
well, but the focus on this set was to at least stop fencing passthrough
for no reason.

Looking back to when this was introduced, I see that 857cc71985dc
("s390x/pci: merge msix init functions") actually makes this mandatory
and states that nothing changes for passthrough. Has anything changed
regarding msi-x in the architecture in the meantime?

I don't believe anything has changed from the architecture or kernel support perspective (Maybe I'm wrong about the latter; Niklas feel free to correct me in that case) -- Otherwise, as far as I can, tell the statement from 857cc71985dc was just incorrect and the restriction was placed unnecessarily for passthrough. vfio-pci sets up for both, and our base s390 kernel support allows for both (and I can drive ISM traffic and see interrupts being routed to the guest with these patches applied of course); it's just our qemu emulation support that only sets up for MSI-X.

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