netdev - Re: [RFC PATCH 3/4] virtio: introudce a mdev based transport

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Date:   Wed, 11 Sep 2019 05:36:47 -0400
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Jason Wang <jasowang@...hat.com>
Cc:     kvm@...r.kernel.org, virtualization@...ts.linux-foundation.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        kwankhede@...dia.com, alex.williamson@...hat.com,
        cohuck@...hat.com, tiwei.bie@...el.com, maxime.coquelin@...hat.com,
        cunming.liang@...el.com, zhihong.wang@...el.com,
        rob.miller@...adcom.com, idos@...lanox.com, xiao.w.wang@...el.com,
        haotian.wang@...ive.com
Subject: Re: [RFC PATCH 3/4] virtio: introudce a mdev based transport

On Wed, Sep 11, 2019 at 10:38:39AM +0800, Jason Wang wrote:
> 
> On 2019/9/10 下午9:52, Michael S. Tsirkin wrote:
> > On Tue, Sep 10, 2019 at 09:13:02PM +0800, Jason Wang wrote:
> > > On 2019/9/10 下午6:01, Michael S. Tsirkin wrote:
> > > > > +#ifndef _LINUX_VIRTIO_MDEV_H
> > > > > +#define _LINUX_VIRTIO_MDEV_H
> > > > > +
> > > > > +#include <linux/interrupt.h>
> > > > > +#include <linux/vringh.h>
> > > > > +#include <uapi/linux/virtio_net.h>
> > > > > +
> > > > > +/*
> > > > > + * Ioctls
> > > > > + */
> > > > Pls add a bit more content here. It's redundant to state these
> > > > are ioctls. Much better to document what does each one do.
> > > 
> > > Ok.
> > > 
> > > 
> > > > > +
> > > > > +struct virtio_mdev_callback {
> > > > > +	irqreturn_t (*callback)(void *);
> > > > > +	void *private;
> > > > > +};
> > > > > +
> > > > > +#define VIRTIO_MDEV 0xAF
> > > > > +#define VIRTIO_MDEV_SET_VQ_CALLBACK _IOW(VIRTIO_MDEV, 0x00, \
> > > > > +					 struct virtio_mdev_callback)
> > > > > +#define VIRTIO_MDEV_SET_CONFIG_CALLBACK _IOW(VIRTIO_MDEV, 0x01, \
> > > > > +					struct virtio_mdev_callback)
> > > > Function pointer in an ioctl parameter? How does this ever make sense?
> > > 
> > > I admit this is hacky (casting).
> > > 
> > > 
> > > > And can't we use a couple of registers for this, and avoid ioctls?
> > > 
> > > Yes, how about something like interrupt numbers for each virtqueue and
> > > config?
> > Should we just reuse VIRTIO_PCI_COMMON_Q_XXX then?
> 
> 
> You mean something like VIRTIO_PCI_COMMON_Q_MSIX? Then it becomes a PCI
> transport in fact. And using either MSIX or irq number is actually another
> layer of indirection. So I think we can just write callback function and
> parameter through registers.

I just realized, all these registers are just encoded so you
can pass stuff through read/write. But it can instead be
just a normal C function call with no messy encoding.
So why do we want to do this encoding?


> 
> > 
> > 
> > > > > +
> > > > > +#define VIRTIO_MDEV_DEVICE_API_STRING		"virtio-mdev"
> > > > > +
> > > > > +/*
> > > > > + * Control registers
> > > > > + */
> > > > > +
> > > > > +/* Magic value ("virt" string) - Read Only */
> > > > > +#define VIRTIO_MDEV_MAGIC_VALUE		0x000
> > > > > +
> > > > > +/* Virtio device version - Read Only */
> > > > > +#define VIRTIO_MDEV_VERSION		0x004
> > > > > +
> > > > > +/* Virtio device ID - Read Only */
> > > > > +#define VIRTIO_MDEV_DEVICE_ID		0x008
> > > > > +
> > > > > +/* Virtio vendor ID - Read Only */
> > > > > +#define VIRTIO_MDEV_VENDOR_ID		0x00c
> > > > > +
> > > > > +/* Bitmask of the features supported by the device (host)
> > > > > + * (32 bits per set) - Read Only */
> > > > > +#define VIRTIO_MDEV_DEVICE_FEATURES	0x010
> > > > > +
> > > > > +/* Device (host) features set selector - Write Only */
> > > > > +#define VIRTIO_MDEV_DEVICE_FEATURES_SEL	0x014
> > > > > +
> > > > > +/* Bitmask of features activated by the driver (guest)
> > > > > + * (32 bits per set) - Write Only */
> > > > > +#define VIRTIO_MDEV_DRIVER_FEATURES	0x020
> > > > > +
> > > > > +/* Activated features set selector - Write Only */
> > > > > +#define VIRTIO_MDEV_DRIVER_FEATURES_SEL	0x024
> > > > > +
> > > > > +/* Queue selector - Write Only */
> > > > > +#define VIRTIO_MDEV_QUEUE_SEL		0x030
> > > > > +
> > > > > +/* Maximum size of the currently selected queue - Read Only */
> > > > > +#define VIRTIO_MDEV_QUEUE_NUM_MAX	0x034
> > > > > +
> > > > > +/* Queue size for the currently selected queue - Write Only */
> > > > > +#define VIRTIO_MDEV_QUEUE_NUM		0x038
> > > > > +
> > > > > +/* Ready bit for the currently selected queue - Read Write */
> > > > > +#define VIRTIO_MDEV_QUEUE_READY		0x044
> > > > Is this same as started?
> > > 
> > > Do you mean "status"?
> > I really meant "enabled", didn't remember the correct name.
> > As in:  VIRTIO_PCI_COMMON_Q_ENABLE
> 
> 
> Yes, it's the same.
> 
> Thanks
> 
> 
> > 
> > > > > +
> > > > > +/* Alignment of virtqueue - Read Only */
> > > > > +#define VIRTIO_MDEV_QUEUE_ALIGN		0x048
> > > > > +
> > > > > +/* Queue notifier - Write Only */
> > > > > +#define VIRTIO_MDEV_QUEUE_NOTIFY	0x050
> > > > > +
> > > > > +/* Device status register - Read Write */
> > > > > +#define VIRTIO_MDEV_STATUS		0x060
> > > > > +
> > > > > +/* Selected queue's Descriptor Table address, 64 bits in two halves */
> > > > > +#define VIRTIO_MDEV_QUEUE_DESC_LOW	0x080
> > > > > +#define VIRTIO_MDEV_QUEUE_DESC_HIGH	0x084
> > > > > +
> > > > > +/* Selected queue's Available Ring address, 64 bits in two halves */
> > > > > +#define VIRTIO_MDEV_QUEUE_AVAIL_LOW	0x090
> > > > > +#define VIRTIO_MDEV_QUEUE_AVAIL_HIGH	0x094
> > > > > +
> > > > > +/* Selected queue's Used Ring address, 64 bits in two halves */
> > > > > +#define VIRTIO_MDEV_QUEUE_USED_LOW	0x0a0
> > > > > +#define VIRTIO_MDEV_QUEUE_USED_HIGH	0x0a4
> > > > > +
> > > > > +/* Configuration atomicity value */
> > > > > +#define VIRTIO_MDEV_CONFIG_GENERATION	0x0fc
> > > > > +
> > > > > +/* The config space is defined by each driver as
> > > > > + * the per-driver configuration space - Read Write */
> > > > > +#define VIRTIO_MDEV_CONFIG		0x100
> > > > Mixing device and generic config space is what virtio pci did,
> > > > caused lots of problems with extensions.
> > > > It would be better to reserve much more space.
> > > 
> > > I see, will do this.
> > > 
> > > Thanks
> > > 
> > > 
> > > > 
> > > > > +
> > > > > +#endif
> > > > > +
> > > > > +
> > > > > +/* Ready bit for the currently selected queue - Read Write */
> > > > > -- 
> > > > > 2.19.1