linux-kernel - Re: [PATCH 1/5] iommu: Add capability IOMMU_CAP

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Date:   Wed, 19 Jul 2017 11:58:04 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     Anup Patel <anup.patel@...adcom.com>,
        Will Deacon <will.deacon@....com>,
        Joerg Roedel <joro@...tes.org>,
        Baptiste Reynal <b.reynal@...tualopensystems.com>,
        Alex Williamson <alex.williamson@...hat.com>
Cc:     Scott Branden <sbranden@...adcom.com>,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        iommu@...ts.linux-foundation.org, kvm@...r.kernel.org,
        bcm-kernel-feedback-list@...adcom.com
Subject: Re: [PATCH 1/5] iommu: Add capability IOMMU_CAP_BYPASS

On 19/07/17 10:33, Anup Patel wrote:
> Some of the IOMMUs (such as ARM SMMU) are capable of bypassing
> transactions for which no IOMMU domain is configured.
> 
> This patch adds IOMMU_CAP_BYPASS which can be used by IOMMU
> drivers to advertise transation bypass capability of an IOMMU.

Whatever the intended semantics of this are, I can't help thinking it
would be better served by allowing callers to explicitly allocate their
own IOMMU_DOMAIN_IDENTITY domains. That would also be useful for the
problem we have with legacy virtio devices behind real IOMMUs.

Robin.

> Signed-off-by: Anup Patel <anup.patel@...adcom.com>
> ---
>  include/linux/iommu.h | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/include/linux/iommu.h b/include/linux/iommu.h
> index 2cb54ad..6bbb4cc 100644
> --- a/include/linux/iommu.h
> +++ b/include/linux/iommu.h
> @@ -101,6 +101,10 @@ enum iommu_cap {
>  					   transactions */
>  	IOMMU_CAP_INTR_REMAP,		/* IOMMU supports interrupt isolation */
>  	IOMMU_CAP_NOEXEC,		/* IOMMU_NOEXEC flag */
> +	IOMMU_CAP_BYPASS,		/*
> +					 * IOMMU can bypass transactions for
> +					 * which domain is not configured
> +					 */
>  };
>  
>  /*
>