linux-kernel - Re: [PATCH v2 1/1] iommu: Add config option to set lazy mode as default

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Date:   Wed, 27 Mar 2019 19:18:51 +0000
From:   Robin Murphy <robin.murphy@....com>
To:     Zhen Lei <thunder.leizhen@...wei.com>,
        Jean-Philippe Brucker <jean-philippe.brucker@....com>,
        John Garry <john.garry@...wei.com>,
        Will Deacon <will.deacon@....com>,
        Joerg Roedel <joro@...tes.org>,
        iommu <iommu@...ts.linux-foundation.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Cc:     Hanjun Guo <guohanjun@...wei.com>
Subject: Re: [PATCH v2 1/1] iommu: Add config option to set lazy mode as
 default

On 27/03/2019 15:00, Zhen Lei wrote:
> This allows the default behaviour to be controlled by a kernel config
> option instead of changing the command line for the kernel to include
> "iommu.strict=0" on ARM64 where this is desired.
> 
> This is similar to CONFIG_IOMMU_DEFAULT_PASSTHROUGH.
> 
> Note: At present, intel_iommu, amd_iommu and s390_iommu use lazy mode as
> default, so there is no need to add code for them.

That seems a bit self-contradictory - if there's a real need for TLB 
flush behaviour to be statically configurable and not command-line-based 
then why should users of other architectures be exempt?

> Signed-off-by: Zhen Lei <thunder.leizhen@...wei.com>
> ---
>   drivers/iommu/Kconfig | 14 ++++++++++++++
>   drivers/iommu/iommu.c |  5 +++++
>   2 files changed, 19 insertions(+)
> 
> diff --git a/drivers/iommu/Kconfig b/drivers/iommu/Kconfig
> index 6f07f3b21816c64..5daa110d0e83a07 100644
> --- a/drivers/iommu/Kconfig
> +++ b/drivers/iommu/Kconfig
> @@ -85,6 +85,20 @@ config IOMMU_DEFAULT_PASSTHROUGH
> 
>   	  If unsure, say N here.
> 
> +config IOMMU_DMA_DEFAULT_LAZY_MODE
> +	bool "IOMMU DMA use lazy mode to flush IOTLB and free IOVA"
> +	depends on IOMMU_API
> +	help
> +	  Support lazy mode, where for every IOMMU DMA unmap operation, the
> +	  flush operation of IOTLB and the free operation of IOVA are deferred.
> +	  They are only guaranteed to be done before the related IOVA will be
> +	  reused. Removing the need to pass in kernel parameters through
> +	  command line. For example, iommu.strict=0 on ARM64. If this is
> +	  enabled, you can still disable with kernel parameters, such as
> +	  iommu.strict=1 depending on the architecture.
> +
> +	  If unsure, say N here.
> +
>   config OF_IOMMU
>          def_bool y
>          depends on OF && IOMMU_API
> diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c
> index 33a982e33716369..5acb98e79b5b32d 100644
> --- a/drivers/iommu/iommu.c
> +++ b/drivers/iommu/iommu.c
> @@ -43,7 +43,12 @@
>   #else
>   static unsigned int iommu_def_domain_type = IOMMU_DOMAIN_DMA;
>   #endif
> +
> +#ifdef CONFIG_IOMMU_DMA_DEFAULT_LAZY_MODE
> +static bool iommu_dma_strict __read_mostly;
> +#else
>   static bool iommu_dma_strict __read_mostly = true;
> +#endif

For a straightforward boolean, you can simply do:

	static bool foo = IS_ENABLED(CONFIG_FOO);

but that said, I'm still not particularly convinced that there are a 
significant number of users in a position to build and install a custom 
kernel but not edit /etc/default/grub, and who really value the 
combination of less performance than passthrough with less isolation 
than strict.

It's also not necessarily obvious to the user how this interacts with 
IOMMU_DEFAULT_PASSTHROUGH, so if we really do go down this route, maybe 
it would be better to refactor the whole lot into a single selection of 
something like IOMMU_DEFAULT_MODE anyway.

Robin.

> 
>   struct iommu_callback_data {
>   	const struct iommu_ops *ops;
> --
> 1.8.3
> 
>