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From a9862ff2c205aa289b62abcb7ddd03c7630f5c7a Mon Sep 17 00:00:00 2001
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From: "Michael S. Tsirkin" <mst@redhat.com>
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Date: Thu, 11 Sep 2014 18:45:33 +0200
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Subject: [PATCH] virtio-pci: enable bus master for old guests
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commit cc943c36faa192cd4b32af8fe5edb31894017d35
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pci: Use bus master address space for delivering MSI/MSI-X messages
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breaks virtio-net for rhel6.[56] x86 guests because they don't
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enable bus mastering for virtio PCI devices. For the same reason,
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rhel6.[56] ppc64 guests cannot boot on a virtio-blk disk anymore.
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Old guests forgot to enable bus mastering, enable it automatically on
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DRIVER (guests use some devices before DRIVER_OK).
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Reported-by: Greg Kurz <gkurz@linux.vnet.ibm.com>
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Reviewed-by: Greg Kurz <gkurz@linux.vnet.ibm.com>
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Tested-by: Greg Kurz <gkurz@linux.vnet.ibm.com>
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Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
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(cherry picked from commit e43c0b2ea5574efb0bedebf6a7d05916eefeba52)
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---
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hw/virtio/virtio-pci.c | 10 ++++++++++
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1 file changed, 10 insertions(+)
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diff --git a/hw/virtio/virtio-pci.c b/hw/virtio/virtio-pci.c
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index 3007319..58ebbcf 100644
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--- a/hw/virtio/virtio-pci.c
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+++ b/hw/virtio/virtio-pci.c
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@@ -314,6 +314,16 @@ static void virtio_ioport_write(void *opaque, uint32_t addr, uint32_t val)
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msix_unuse_all_vectors(&proxy->pci_dev);
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}
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+ /* Linux before 2.6.34 drives the device without enabling
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+ the PCI device bus master bit. Enable it automatically
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+ for the guest. This is a PCI spec violation but so is
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+ initiating DMA with bus master bit clear. */
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+ if (val == (VIRTIO_CONFIG_S_ACKNOWLEDGE | VIRTIO_CONFIG_S_DRIVER)) {
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+ pci_default_write_config(&proxy->pci_dev, PCI_COMMAND,
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+ proxy->pci_dev.config[PCI_COMMAND] |
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+ PCI_COMMAND_MASTER, 1);
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+ }
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+
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/* Linux before 2.6.34 sets the device as OK without enabling
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the PCI device bus master bit. In this case we need to disable
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some safety checks. */
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