linux-kernel - Re: [RFC, v0 1/3] vfio/platform: add support for msi

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Date:   Mon, 9 Nov 2020 16:05:02 +0100
From:   Auger Eric <eric.auger@...hat.com>
To:     Vikas Gupta <vikas.gupta@...adcom.com>,
        Alex Williamson <alex.williamson@...hat.com>
Cc:     Cornelia Huck <cohuck@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Vikram Prakash <vikram.prakash@...adcom.com>
Subject: Re: [RFC, v0 1/3] vfio/platform: add support for msi

Hi Vikas,

On 11/6/20 3:54 AM, Vikas Gupta wrote:
> Hi Alex,
> 
> On Thu, Nov 5, 2020 at 12:38 PM Alex Williamson
> <alex.williamson@...hat.com> wrote:
>>
>> On Thu,  5 Nov 2020 11:32:55 +0530
>> Vikas Gupta <vikas.gupta@...adcom.com> wrote:
>>
>>> diff --git a/include/uapi/linux/vfio.h b/include/uapi/linux/vfio.h
>>> index 2f313a238a8f..aab051e8338d 100644
>>> --- a/include/uapi/linux/vfio.h
>>> +++ b/include/uapi/linux/vfio.h
>>> @@ -203,6 +203,7 @@ struct vfio_device_info {
>>>  #define VFIO_DEVICE_FLAGS_AP (1 << 5)        /* vfio-ap device */
>>>  #define VFIO_DEVICE_FLAGS_FSL_MC (1 << 6)    /* vfio-fsl-mc device */
>>>  #define VFIO_DEVICE_FLAGS_CAPS       (1 << 7)        /* Info supports caps */
>>> +#define VFIO_DEVICE_FLAGS_MSI        (1 << 8)        /* Device supports msi */
>>>       __u32   num_regions;    /* Max region index + 1 */
>>>       __u32   num_irqs;       /* Max IRQ index + 1 */
>>>       __u32   cap_offset;     /* Offset within info struct of first cap */
>>
>> This doesn't make any sense to me, MSIs are just edge triggered
>> interrupts to userspace, so why isn't this fully described via
>> VFIO_DEVICE_GET_IRQ_INFO?  If we do need something new to describe it,
>> this seems incomplete, which indexes are MSI (IRQ_INFO can describe
>> that)?  We also already support MSI with vfio-pci, so a global flag for
>> the device advertising this still seems wrong.  Thanks,
>>
>> Alex
>>
> Since VFIO platform uses indexes for IRQ numbers so I think MSI(s)
> cannot be described using indexes.
> In the patch set there is no difference between MSI and normal
> interrupt for VFIO_DEVICE_GET_IRQ_INFO.
in vfio_platform_irq_init() we first iterate on normal interrupts using
get_irq(). Can't we add an MSI index at the end of this list with
vdev->irqs[i].count > 1 and set vdev->num_irqs accordingly?

Thanks

Eric
> The patch set adds MSI(s), say as an extension, to the normal
> interrupts and handled accordingly. Do you see this is a violation? If
> yes, then we`ll think of other possible ways to support MSI for the
> platform devices.
> Macro VFIO_DEVICE_FLAGS_MSI can be changed to any other name if it
> collides with an already supported vfio-pci or if not necessary, we
> can remove this flag.
> 
> Thanks,
> Vikas
>