linux-kernel - Re: [PATCH 2/2] iommu/virtio: Add ops->flush_iotlb

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Date:   Mon, 4 Sep 2023 16:34:03 +0100
From:   Jean-Philippe Brucker <jean-philippe@...aro.org>
To:     Niklas Schnelle <schnelle@...ux.ibm.com>
Cc:     Joerg Roedel <joro@...tes.org>, Will Deacon <will@...nel.org>,
        Robin Murphy <robin.murphy@....com>,
        virtualization@...ts.linux-foundation.org, iommu@...ts.linux.dev,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] iommu/virtio: Add ops->flush_iotlb_all and enable
 deferred flush

On Fri, Aug 25, 2023 at 05:21:26PM +0200, Niklas Schnelle wrote:
> Add ops->flush_iotlb_all operation to enable virtio-iommu for the
> dma-iommu deferred flush scheme. This results inn a significant increase

in

> in performance in exchange for a window in which devices can still
> access previously IOMMU mapped memory. To get back to the prior behavior
> iommu.strict=1 may be set on the kernel command line.

Maybe add that it depends on CONFIG_IOMMU_DEFAULT_DMA_{LAZY,STRICT} as
well, because I've seen kernel configs that enable either.

> 
> Link: https://lore.kernel.org/lkml/20230802123612.GA6142@myrica/
> Signed-off-by: Niklas Schnelle <schnelle@...ux.ibm.com>
> ---
>  drivers/iommu/virtio-iommu.c | 12 ++++++++++++
>  1 file changed, 12 insertions(+)
> 
> diff --git a/drivers/iommu/virtio-iommu.c b/drivers/iommu/virtio-iommu.c
> index fb73dec5b953..1b7526494490 100644
> --- a/drivers/iommu/virtio-iommu.c
> +++ b/drivers/iommu/virtio-iommu.c
> @@ -924,6 +924,15 @@ static int viommu_iotlb_sync_map(struct iommu_domain *domain,
>  	return viommu_sync_req(vdomain->viommu);
>  }
>  
> +static void viommu_flush_iotlb_all(struct iommu_domain *domain)
> +{
> +	struct viommu_domain *vdomain = to_viommu_domain(domain);
> +
> +	if (!vdomain->nr_endpoints)
> +		return;

As for patch 1, a NULL check in viommu_sync_req() would allow dropping
this one

Thanks,
Jean

> +	viommu_sync_req(vdomain->viommu);
> +}
> +
>  static void viommu_get_resv_regions(struct device *dev, struct list_head *head)
>  {
>  	struct iommu_resv_region *entry, *new_entry, *msi = NULL;
> @@ -1049,6 +1058,8 @@ static bool viommu_capable(struct device *dev, enum iommu_cap cap)
>  	switch (cap) {
>  	case IOMMU_CAP_CACHE_COHERENCY:
>  		return true;
> +	case IOMMU_CAP_DEFERRED_FLUSH:
> +		return true;
>  	default:
>  		return false;
>  	}
> @@ -1069,6 +1080,7 @@ static struct iommu_ops viommu_ops = {
>  		.map_pages		= viommu_map_pages,
>  		.unmap_pages		= viommu_unmap_pages,
>  		.iova_to_phys		= viommu_iova_to_phys,
> +		.flush_iotlb_all	= viommu_flush_iotlb_all,
>  		.iotlb_sync		= viommu_iotlb_sync,
>  		.iotlb_sync_map		= viommu_iotlb_sync_map,
>  		.free			= viommu_domain_free,
> 
> -- 
> 2.39.2
>