linux-kernel - Re: [RFC PATCH] vfio/pci: Add ioeventfd support

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Message-ID: <6014d60c-9bdb-4dc0-7cd7-9299005d9c5a@ozlabs.ru>
Date:   Wed, 7 Feb 2018 15:48:26 +1100
From:   Alexey Kardashevskiy <aik@...abs.ru>
To:     Alex Williamson <alex.williamson@...hat.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        qemu-devel@...gnu.org
Subject: Re: [RFC PATCH] vfio/pci: Add ioeventfd support

On 07/02/18 15:25, Alex Williamson wrote:
> On Wed, 7 Feb 2018 15:09:22 +1100
> Alexey Kardashevskiy <aik@...abs.ru> wrote:
>> On 07/02/18 11:08, Alex Williamson wrote:
>>> diff --git a/include/uapi/linux/vfio.h b/include/uapi/linux/vfio.h
>>> index e3301dbd27d4..07966a5f0832 100644
>>> --- a/include/uapi/linux/vfio.h
>>> +++ b/include/uapi/linux/vfio.h
>>> @@ -503,6 +503,30 @@ struct vfio_pci_hot_reset {
>>>  
>>>  #define VFIO_DEVICE_PCI_HOT_RESET	_IO(VFIO_TYPE, VFIO_BASE + 13)
>>>  
>>> +/**
>>> + * VFIO_DEVICE_IOEVENTFD - _IOW(VFIO_TYPE, VFIO_BASE + 14,
>>> + *                              struct vfio_device_ioeventfd)
>>> + *
>>> + * Perform a write to the device at the specified device fd offset, with
>>> + * the specified data and width when the provided eventfd is triggered.
>>> + *
>>> + * Return: 0 on success, -errno on failure.
>>> + */
>>> +struct vfio_device_ioeventfd {
>>> +	__u32	argsz;
>>> +	__u32	flags;
>>> +#define VFIO_DEVICE_IOEVENTFD_8		(1 << 0) /* 1-byte write */
>>> +#define VFIO_DEVICE_IOEVENTFD_16	(1 << 1) /* 2-byte write */
>>> +#define VFIO_DEVICE_IOEVENTFD_32	(1 << 2) /* 4-byte write */
>>> +#define VFIO_DEVICE_IOEVENTFD_64	(1 << 3) /* 8-byte write */
>>> +#define VFIO_DEVICE_IOEVENTFD_SIZE_MASK	(0xf)
>>> +	__u64	offset;			/* device fd offset of write */
>>> +	__u64	data;			/* data to be written */
>>> +	__s32	fd;			/* -1 for de-assignment */
>>> +};
>>> +
>>> +#define VFIO_DEVICE_IOEVENTFD		_IO(VFIO_TYPE, VFIO_BASE + 14)  
>>
>>
>> Is this a first ioctl with endianness fixed to little-endian? I'd suggest
>> to comment on that as things like vfio_info_cap_header do use the host
>> endianness.
> 
> Look at our current read and write interface, we call leXX_to_cpu
> before calling iowriteXX there and I think a user would logically
> expect to use the same data format here as they would there.

If the data is "char data[8]" (i.e. bytestream), then it can be expected to
be device/bus endian (i.e. PCI == little endian), but if it is u64 - then I
am not so sure really, and this made me look around. It could be "__le64
data" too.

> Also note
> that iowriteXX does a cpu_to_leXX, so are we really defining the
> interface as little-endian or are we just trying to make ourselves
> endian neutral and counter that implicit conversion?  Thanks,

Defining it LE is fine, I just find it a bit confusing when
vfio_info_cap_header is host endian but vfio_device_ioeventfd is not.


-- 
Alexey