Messages in this thread Patch in this message | | | From | Laurent Vivier <> | Subject | Re: [PATCH] virtio-mmio: read[wl]()/write[wl] are already little-endian | Date | Sat, 13 Mar 2021 18:10:29 +0100 |
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Le 11/03/2021 à 16:44, Michael S. Tsirkin a écrit : > On Tue, Mar 09, 2021 at 11:43:13PM +0100, Laurent Vivier wrote: >> read[wl]()/write[wl] already access memory in little-endian mode. > > But then they convert it to CPU right? We just convert it back ...
In fact the problem is in QEMU.
On a big-endian guest, the readw() returns a byte-swapped value, This means QEMU doesn't provide a little-endian value.
I found in QEMU virtio_mmio_read() provides a value with byte-swapped bytes.
The problem comes from virtio_config_readX() functions that read the value using ldX_p accessors.
Is it normal not to use the modern variant here if we are not in legacy mode?
I think we should have something like this in virtio_mmio_read (and write):
--- a/hw/virtio/virtio-mmio.c +++ b/hw/virtio/virtio-mmio.c @@ -112,15 +112,28 @@ static uint64_t virtio_mmio_read(void *opaque, hwaddr offset, unsigned size)
if (offset >= VIRTIO_MMIO_CONFIG) { offset -= VIRTIO_MMIO_CONFIG; - switch (size) { - case 1: - return virtio_config_readb(vdev, offset); - case 2: - return virtio_config_readw(vdev, offset); - case 4: - return virtio_config_readl(vdev, offset); - default: - abort(); + if (proxy->legacy) { + switch (size) { + case 1: + return virtio_config_readb(vdev, offset); + case 2: + return virtio_config_readw(vdev, offset); + case 4: + return virtio_config_readl(vdev, offset); + default: + abort(); + } + } else { + switch (size) { + case 1: + return virtio_config_modern_readb(vdev, offset); + case 2: + return virtio_config_modern_readw(vdev, offset); + case 4: + return virtio_config_modern_readl(vdev, offset); + default: + abort(); + } } } if (size != 4) { And we need the same thing in virtio_pci_config_read() (and write).
And this could explain why it works with virtio-pci and not with virtio-mmio with the big-endian guest:
with virtio-pci the bytes are swapped twice (once in virtio-mmio and then in virtio-pci), so they are restored to the initial value, whereas with direct virtio-mmio they are swapped only once.
Thanks, Laurent
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