netdev - Re: [PATCH net-next v3 01/13] virtio: add packed ring types and macros

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Date:   Fri, 30 Nov 2018 14:01:06 +0100
From:   Maxime Coquelin <maxime.coquelin@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>,
        Tiwei Bie <tiwei.bie@...el.com>
Cc:     Jason Wang <jasowang@...hat.com>,
        virtualization@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
        virtio-dev@...ts.oasis-open.org, wexu@...hat.com,
        jfreimann@...hat.com
Subject: Re: [PATCH net-next v3 01/13] virtio: add packed ring types and
 macros



On 11/30/18 1:47 PM, Michael S. Tsirkin wrote:
> On Fri, Nov 30, 2018 at 05:53:40PM +0800, Tiwei Bie wrote:
>> On Fri, Nov 30, 2018 at 04:10:55PM +0800, Jason Wang wrote:
>>>
>>> On 2018/11/21 下午6:03, Tiwei Bie wrote:
>>>> Add types and macros for packed ring.
>>>>
>>>> Signed-off-by: Tiwei Bie <tiwei.bie@...el.com>
>>>> ---
>>>>    include/uapi/linux/virtio_config.h |  3 +++
>>>>    include/uapi/linux/virtio_ring.h   | 52 ++++++++++++++++++++++++++++++++++++++
>>>>    2 files changed, 55 insertions(+)
>>>>
>>>> diff --git a/include/uapi/linux/virtio_config.h b/include/uapi/linux/virtio_config.h
>>>> index 449132c76b1c..1196e1c1d4f6 100644
>>>> --- a/include/uapi/linux/virtio_config.h
>>>> +++ b/include/uapi/linux/virtio_config.h
>>>> @@ -75,6 +75,9 @@
>>>>     */
>>>>    #define VIRTIO_F_IOMMU_PLATFORM		33
>>>> +/* This feature indicates support for the packed virtqueue layout. */
>>>> +#define VIRTIO_F_RING_PACKED		34
>>>> +
>>>>    /*
>>>>     * Does the device support Single Root I/O Virtualization?
>>>>     */
>>>> diff --git a/include/uapi/linux/virtio_ring.h b/include/uapi/linux/virtio_ring.h
>>>> index 6d5d5faa989b..2414f8af26b3 100644
>>>> --- a/include/uapi/linux/virtio_ring.h
>>>> +++ b/include/uapi/linux/virtio_ring.h
>>>> @@ -44,6 +44,13 @@
>>>>    /* This means the buffer contains a list of buffer descriptors. */
>>>>    #define VRING_DESC_F_INDIRECT	4
>>>> +/*
>>>> + * Mark a descriptor as available or used in packed ring.
>>>> + * Notice: they are defined as shifts instead of shifted values.
>>>
>>>
>>> This looks inconsistent to previous flags, any reason for using shifts?
>>
>> Yeah, it was suggested to use shifts, as _F_ should be a bit
>> number, not a shifted value:
>>
>> https://patchwork.ozlabs.org/patch/942296/#1989390
> 
> But let's add a _SPLIT_ variant that uses shifts consistently.

Maybe we could avoid adding SPLIT and PACKED, but define as follow:

#define VRING_DESC_F_INDIRECT_SHIFT 2
#define VRING_DESC_F_INDIRECT (1ull <<  VRING_DESC_F_INDIRECT_SHIFT)

#define VRING_DESC_F_AVAIL_SHIFT 7
#define VRING_DESC_F_AVAIL (1ull << VRING_DESC_F_AVAIL_SHIFT)

I think it would be better for consistency.
> 
> 
>>>
>>>
>>>> + */
>>>> +#define VRING_PACKED_DESC_F_AVAIL	7
>>>> +#define VRING_PACKED_DESC_F_USED	15
>>>> +
>>>>    /* The Host uses this in used->flags to advise the Guest: don't kick me when
>>>>     * you add a buffer.  It's unreliable, so it's simply an optimization.  Guest
>>>>     * will still kick if it's out of buffers. */
>>>> @@ -53,6 +60,23 @@
>>>>     * optimization.  */
>>>>    #define VRING_AVAIL_F_NO_INTERRUPT	1
>>>> +/* Enable events in packed ring. */
>>>> +#define VRING_PACKED_EVENT_FLAG_ENABLE	0x0
>>>> +/* Disable events in packed ring. */
>>>> +#define VRING_PACKED_EVENT_FLAG_DISABLE	0x1
>>>> +/*
>>>> + * Enable events for a specific descriptor in packed ring.
>>>> + * (as specified by Descriptor Ring Change Event Offset/Wrap Counter).
>>>> + * Only valid if VIRTIO_RING_F_EVENT_IDX has been negotiated.
>>>> + */
>>>> +#define VRING_PACKED_EVENT_FLAG_DESC	0x2
>>>
>>>
>>> Any reason for using _FLAG_ instead of _F_?
>>
>> Yeah, it was suggested to not use _F_, as these are values,
>> should not have _F_:
>>
>> https://patchwork.ozlabs.org/patch/942296/#1989390
>>
>> Regards,
>> Tiwei
>>
>>>
>>> Thanks
>>>
>>>
>>>> +
>>>> +/*
>>>> + * Wrap counter bit shift in event suppression structure
>>>> + * of packed ring.
>>>> + */
>>>> +#define VRING_PACKED_EVENT_F_WRAP_CTR	15
>>>> +
>>>>    /* We support indirect buffer descriptors */
>>>>    #define VIRTIO_RING_F_INDIRECT_DESC	28
>>>> @@ -171,4 +195,32 @@ static inline int vring_need_event(__u16 event_idx, __u16 new_idx, __u16 old)
>>>>    	return (__u16)(new_idx - event_idx - 1) < (__u16)(new_idx - old);
>>>>    }
>>>> +struct vring_packed_desc_event {
>>>> +	/* Descriptor Ring Change Event Offset/Wrap Counter. */
>>>> +	__le16 off_wrap;
>>>> +	/* Descriptor Ring Change Event Flags. */
>>>> +	__le16 flags;
>>>> +};
>>>> +
>>>> +struct vring_packed_desc {
>>>> +	/* Buffer Address. */
>>>> +	__le64 addr;
>>>> +	/* Buffer Length. */
>>>> +	__le32 len;
>>>> +	/* Buffer ID. */
>>>> +	__le16 id;
>>>> +	/* The flags depending on descriptor type. */
>>>> +	__le16 flags;
>>>> +};
>>>> +
>>>> +struct vring_packed {
>>>> +	unsigned int num;
>>>> +
>>>> +	struct vring_packed_desc *desc;
>>>> +
>>>> +	struct vring_packed_desc_event *driver;
>>>> +
>>>> +	struct vring_packed_desc_event *device;
>>>> +};
>>>> +
>>>>    #endif /* _UAPI_LINUX_VIRTIO_RING_H */