Re: [PATCH] virtio_mmio: fix endian-ness for mmio

[Rusty Russell]

"Michael S. Tsirkin" <mst@xxxxxxxxxx> writes:
> On Thu, Mar 12, 2015 at 12:33:36PM +1030, Rusty Russell wrote:
>> "Michael S. Tsirkin" <mst@xxxxxxxxxx> writes:
>> > Going over the virtio mmio code, I noticed that it doesn't correctly
>> > return device config values in LE format when using virtio 1.0.
>> > Borrow code from virtio_pci_modern to do this correctly.
>> 
>> AFAICT, it doesn't need to.  The endian correction is done by the
>> callers.
>> 
>> The only reason that virtio_pci_modern() does it is because readl() etc
>> do endian conversion, and we don't want them to.  And the PCI part of
>> the spec says to use "natural" accessors, so we don't do byte-at-a-time.
>> 
>> Cheers,
>> Rusty.
>
> You are right, the endina-ness is not an issue, so the commit log I
> wrote is wrong, but I still think the patch is required, because MMIO
> spec says the same as PCI.
> 	The driver MUST only use 32 bit wide and aligned reads and writes to
> 	access the control registers described
> 	in table 4.1. For the device-specific configuration space, the driver
> 	MUST use 8 bit wide accesses for 8 bit
> 	wide fields, 16 bit wide and aligned accesses for 16 bit wide fields and
> 	32 bit wide and aligned accesses for
> 	32 and 64 bit wide fields.

Ah, good point.

I'm sure Pawel is on a beach somewhere sipping cocktails, so I'll apply
this immediately (with your updated commit message) to my
pending-rebases and give him the weekend to Ack.

Thanks,
Rusty.


>
>
> Here's a better commit log:
>
> --->
>
> virtio_mmio: fix access width for mmio
>
> Going over the virtio mmio code, I noticed that it doesn't correctly
> access modern device config values using "natural" accessors: it uses
> readb to get/set them byte by byte, while the virtio 1.0 spec explicitly states:
>
> 	4.2.2.2 Driver Requirements: MMIO Device Register Layout
>
> 	...
>
> 	The driver MUST only use 32 bit wide and aligned reads and writes to
> 	access the control registers described in table 4.1.
> 	For the device-specific configuration space, the driver MUST use
> 	8 bit wide accesses for 8 bit wide fields, 16 bit wide and aligned
> 	accesses for 16 bit wide fields and 32 bit wide and aligned accesses for
> 	32 and 64 bit wide fields.
>
> Borrow code from virtio_pci_modern to do this correctly.
>
> Signed-off-by: Michael S. Tsirkin <mst@xxxxxxxxxx>
>
> Makes sense now, right?
> Want me to repost or can you just tweak the commit log?
>
>
>> > Signed-off-by: Michael S. Tsirkin <mst@xxxxxxxxxx>
>> > ---
>> >
>> > Note: untested: QEMU doesn't support virtio 1.0 for virtio-mmio.
>> > Pawel, could you please confirm this patch makes sense?
>> >
>> >  drivers/virtio/virtio_mmio.c | 79 +++++++++++++++++++++++++++++++++++++++-----
>> >  1 file changed, 71 insertions(+), 8 deletions(-)
>> >
>> > diff --git a/drivers/virtio/virtio_mmio.c b/drivers/virtio/virtio_mmio.c
>> > index cad5698..0375456 100644
>> > --- a/drivers/virtio/virtio_mmio.c
>> > +++ b/drivers/virtio/virtio_mmio.c
>> > @@ -156,22 +156,85 @@ static void vm_get(struct virtio_device *vdev, unsigned offset,
>> >  		   void *buf, unsigned len)
>> >  {
>> >  	struct virtio_mmio_device *vm_dev = to_virtio_mmio_device(vdev);
>> > -	u8 *ptr = buf;
>> > -	int i;
>> > +	void __iomem *base = vm_dev->base + VIRTIO_MMIO_CONFIG;
>> > +	u8 b;
>> > +	__le16 w;
>> > +	__le32 l;
>> >  
>> > -	for (i = 0; i < len; i++)
>> > -		ptr[i] = readb(vm_dev->base + VIRTIO_MMIO_CONFIG + offset + i);
>> > +	if (vm_dev->version == 1) {
>> > +		u8 *ptr = buf;
>> > +		int i;
>> > +
>> > +		for (i = 0; i < len; i++)
>> > +			ptr[i] = readb(base + offset + i);
>> > +		return;
>> > +	}
>> > +
>> > +	switch (len) {
>> > +	case 1:
>> > +		b = readb(base + offset);
>> > +		memcpy(buf, &b, sizeof b);
>> > +		break;
>> > +	case 2:
>> > +		w = cpu_to_le16(readw(base + offset));
>> > +		memcpy(buf, &w, sizeof w);
>> > +		break;
>> > +	case 4:
>> > +		l = cpu_to_le32(readl(base + offset));
>> > +		memcpy(buf, &l, sizeof l);
>> > +		break;
>> > +	case 8:
>> > +		l = cpu_to_le32(readl(base + offset));
>> > +		memcpy(buf, &l, sizeof l);
>> > +		l = cpu_to_le32(ioread32(base + offset + sizeof l));
>> > +		memcpy(buf + sizeof l, &l, sizeof l);
>> > +		break;
>> > +	default:
>> > +		BUG();
>> > +	}
>> >  }
>> >  
>> >  static void vm_set(struct virtio_device *vdev, unsigned offset,
>> >  		   const void *buf, unsigned len)
>> >  {
>> >  	struct virtio_mmio_device *vm_dev = to_virtio_mmio_device(vdev);
>> > -	const u8 *ptr = buf;
>> > -	int i;
>> > +	void __iomem *base = vm_dev->base + VIRTIO_MMIO_CONFIG;
>> > +	u8 b;
>> > +	__le16 w;
>> > +	__le32 l;
>> >  
>> > -	for (i = 0; i < len; i++)
>> > -		writeb(ptr[i], vm_dev->base + VIRTIO_MMIO_CONFIG + offset + i);
>> > +	if (vm_dev->version == 1) {
>> > +		const u8 *ptr = buf;
>> > +		int i;
>> > +
>> > +		for (i = 0; i < len; i++)
>> > +			writeb(ptr[i], base + offset + i);
>> > +
>> > +		return;
>> > +	}
>> > +
>> > +	switch (len) {
>> > +	case 1:
>> > +		memcpy(&b, buf, sizeof b);
>> > +		writeb(b, base + offset);
>> > +		break;
>> > +	case 2:
>> > +		memcpy(&w, buf, sizeof w);
>> > +		writew(le16_to_cpu(w), base + offset);
>> > +		break;
>> > +	case 4:
>> > +		memcpy(&l, buf, sizeof l);
>> > +		writel(le32_to_cpu(l), base + offset);
>> > +		break;
>> > +	case 8:
>> > +		memcpy(&l, buf, sizeof l);
>> > +		writel(le32_to_cpu(l), base + offset);
>> > +		memcpy(&l, buf + sizeof l, sizeof l);
>> > +		writel(le32_to_cpu(l), base + offset + sizeof l);
>> > +		break;
>> > +	default:
>> > +		BUG();
>> > +	}
>> >  }
>> >  
>> >  static u8 vm_get_status(struct virtio_device *vdev)
>> > -- 
>> > MST
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