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

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Date:   Thu, 5 Nov 2020 00:08:06 -0700
From:   Alex Williamson <alex.williamson@...hat.com>
To:     Vikas Gupta <vikas.gupta@...adcom.com>
Cc:     eric.auger@...hat.com, cohuck@...hat.com, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, vikram.prakash@...adcom.com
Subject: Re: [RFC, v0 1/3] vfio/platform: add support for msi

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