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Re: [PATCH v2] virtio-balloon-pci: Allow setting nvectors, so we can use


From: David Hildenbrand
Subject: Re: [PATCH v2] virtio-balloon-pci: Allow setting nvectors, so we can use MSI-X
Date: Mon, 27 Jan 2025 14:11:06 +0100
User-agent: Mozilla Thunderbird

On 15.01.25 17:14, Reza Arbab wrote:
Most virtio-pci devices allow MSI-X. Add it to virtio-balloon-pci, but
only enable it in new machine types, so we don't break migration of
existing machine types between different qemu versions.

This copies what was done for virtio-rng-pci in:
9ea02e8f1306 ("virtio-rng-pci: Allow setting nvectors, so we can use MSI-X")
bad9c5a5166f ("virtio-rng-pci: fix migration compat for vectors")
62bdb8871512 ("virtio-rng-pci: fix transitional migration compat for vectors")

Acked-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Reza Arbab <arbab@linux.ibm.com>
---
This patch enables the virtio-balloon-pci device to be plugged on s390x.
Before this change, plugging that device would fail with "MSI-X support is
mandatory on s390".

However, the device does not seem to be initialized by the Linux guest:

# lspci -v
0001:00:00.0 Unclassified device [00ff]: Red Hat, Inc. Virtio memory balloon
        Subsystem: Red Hat, Inc. Device 0005
        Physical Slot: 00000000
        Flags: fast devsel, IOMMU group 0
        Memory at 4001000000000000 (32-bit, non-prefetchable) [virtual] 
[size=4K]
        Memory at 4002000000000000 (64-bit, prefetchable) [size=16K]
        Capabilities: [98] MSI-X: Enable- Count=2 Masked-
        Capabilities: [84] Vendor Specific Information: VirtIO: <unknown>
        Capabilities: [70] Vendor Specific Information: VirtIO: Notify
        Capabilities: [60] Vendor Specific Information: VirtIO: DeviceCfg
        Capabilities: [50] Vendor Specific Information: VirtIO: ISR
        Capabilities: [40] Vendor Specific Information: VirtIO: CommonCfg


I tried enabling it:

[  343.718529] pci 0001:00:00.0: enabling device (0000 -> 0002)

Without any change. @s390x folks, is there something missing to make it fly,
or are there additional steps to initialize+use such devices from inside the VM?

--
Cheers,

David / dhildenb




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