linux-kernel - Re: [PATCH v4 5/5] iommu/arm-smmu-v3: add bootup option "arm

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Date:   Thu, 9 Aug 2018 12:08:36 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     Zhen Lei <thunder.leizhen@...wei.com>,
        Will Deacon <will.deacon@....com>,
        Joerg Roedel <joro@...tes.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        iommu <iommu@...ts.linux-foundation.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Cc:     LinuxArm <linuxarm@...wei.com>, Hanjun Guo <guohanjun@...wei.com>,
        Libin <huawei.libin@...wei.com>
Subject: Re: [PATCH v4 5/5] iommu/arm-smmu-v3: add bootup option "arm_iommu"

On 06/08/18 13:27, Zhen Lei wrote:
> Add a bootup option to make the system manager can choose which mode to
> be used. The default mode is strict.
> 
> Signed-off-by: Zhen Lei <thunder.leizhen@...wei.com>
> ---
>   Documentation/admin-guide/kernel-parameters.txt |  9 +++++++++
>   drivers/iommu/arm-smmu-v3.c                     | 17 ++++++++++++++++-
>   2 files changed, 25 insertions(+), 1 deletion(-)
> 
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index 533ff5c..426e989 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -1720,6 +1720,15 @@
>   		nobypass	[PPC/POWERNV]
>   			Disable IOMMU bypass, using IOMMU for PCI devices.
> 
> +	arm_iommu=	[ARM64]
> +		non-strict	[Default Off]

Again, I'd much rather have "iommu.non_strict= { "0" | "1" }" in line 
with the passthrough option.

Robin.

> +			Put off TLBs invalidation and release memory first.
> +			It's good for scatter-gather performance but lacks full
> +			isolation, an untrusted device can access the reused
> +			memory because the TLBs may still valid. Please take
> +			full consideration before choosing this mode.
> +			Note that, VFIO will always use strict mode.
> +
>   	iommu.passthrough=
>   			[ARM64] Configure DMA to bypass the IOMMU by default.
>   			Format: { "0" | "1" }
> diff --git a/drivers/iommu/arm-smmu-v3.c b/drivers/iommu/arm-smmu-v3.c
> index 904bc1e..9a30892 100644
> --- a/drivers/iommu/arm-smmu-v3.c
> +++ b/drivers/iommu/arm-smmu-v3.c
> @@ -631,6 +631,21 @@ struct arm_smmu_option_prop {
>   	{ 0, NULL},
>   };
> 
> +static u32 smmu_non_strict __read_mostly;
> +
> +static int __init arm_smmu_setup(char *str)
> +{
> +	if (!strncmp(str, "non-strict", 10)) {
> +		smmu_non_strict = 1;
> +		pr_warn("WARNING: iommu non-strict mode is chosen.\n"
> +			"It's good for scatter-gather performance but lacks full isolation\n");
> +		add_taint(TAINT_WARN, LOCKDEP_STILL_OK);
> +	}
> +
> +	return 0;
> +}
> +early_param("arm_iommu", arm_smmu_setup);
> +
>   static inline void __iomem *arm_smmu_page1_fixup(unsigned long offset,
>   						 struct arm_smmu_device *smmu)
>   {
> @@ -1622,7 +1637,7 @@ static int arm_smmu_domain_finalise(struct iommu_domain *domain)
>   	if (smmu->features & ARM_SMMU_FEAT_COHERENCY)
>   		pgtbl_cfg.quirks = IO_PGTABLE_QUIRK_NO_DMA;
> 
> -	if (domain->type == IOMMU_DOMAIN_DMA) {
> +	if ((domain->type == IOMMU_DOMAIN_DMA) && smmu_non_strict) {
>   		domain->non_strict = 1;
>   		pgtbl_cfg.quirks |= IO_PGTABLE_QUIRK_NON_STRICT;
>   	}
> --
> 1.8.3
> 
>