linux-kernel - Re: [PATCH v2] vfio iommu: Add dma available capability

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Date:   Tue, 15 Sep 2020 11:44:01 +0200
From:   Cornelia Huck <cohuck@...hat.com>
To:     Matthew Rosato <mjrosato@...ux.ibm.com>
Cc:     alex.williamson@...hat.com, pmorel@...ux.ibm.com,
        schnelle@...ux.ibm.com, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] vfio iommu: Add dma available capability

On Mon, 14 Sep 2020 18:25:31 -0400
Matthew Rosato <mjrosato@...ux.ibm.com> wrote:

> Commit 492855939bdb ("vfio/type1: Limit DMA mappings per container")
> added the ability to limit the number of memory backed DMA mappings.
> However on s390x, when lazy mapping is in use, we use a very large
> number of concurrent mappings.  Let's provide the current allowable
> number of DMA mappings to userspace via the IOMMU info chain so that
> userspace can take appropriate mitigation.
> 
> Signed-off-by: Matthew Rosato <mjrosato@...ux.ibm.com>
> ---
>  drivers/vfio/vfio_iommu_type1.c | 17 +++++++++++++++++
>  include/uapi/linux/vfio.h       | 16 ++++++++++++++++
>  2 files changed, 33 insertions(+)

(...)

> diff --git a/include/uapi/linux/vfio.h b/include/uapi/linux/vfio.h
> index 9204705..a8cc4a5 100644
> --- a/include/uapi/linux/vfio.h
> +++ b/include/uapi/linux/vfio.h
> @@ -1039,6 +1039,22 @@ struct vfio_iommu_type1_info_cap_migration {
>  	__u64	max_dirty_bitmap_size;		/* in bytes */
>  };
>  
> +/*
> + * The DMA available capability allows to report the current number of
> + * simultaneously outstanding DMA mappings that are allowed.
> + *
> + * The structures below define version 1 of this capability.

"The structure below defines..." ?

> + *
> + * max: specifies the maximum number of outstanding DMA mappings allowed.

I think you forgot to tweak that one:

"avail: specifies the current number of outstanding DMA mappings allowed."

?

> + */
> +#define VFIO_IOMMU_TYPE1_INFO_DMA_AVAIL 3
> +
> +struct vfio_iommu_type1_info_dma_avail {
> +	struct	vfio_info_cap_header header;
> +	__u32	avail;
> +};
> +
> +
>  #define VFIO_IOMMU_GET_INFO _IO(VFIO_TYPE, VFIO_BASE + 12)
>  
>  /**