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SubjectRe: [PATCH v7 4/6] iommu: Add bootup option "iommu.non_strict"
On Fri, Sep 14, 2018 at 03:30:22PM +0100, Robin Murphy wrote:
> From: Zhen Lei <thunder.leizhen@huawei.com>
>
> 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@huawei.com>
> [rm: move handling out of SMMUv3 driver]
> Signed-off-by: Robin Murphy <robin.murphy@arm.com>
> ---
> .../admin-guide/kernel-parameters.txt | 13 ++++++++++
> drivers/iommu/iommu.c | 26 +++++++++++++++++++
> 2 files changed, 39 insertions(+)
>
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index 9871e649ffef..406b91759b62 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -1749,6 +1749,19 @@
> nobypass [PPC/POWERNV]
> Disable IOMMU bypass, using IOMMU for PCI devices.
>
> + iommu.non_strict= [ARM64]
> + Format: { "0" | "1" }
> + 0 - strict mode, default.
> + Release IOVAs after the related TLBs are invalid
> + completely.
> + 1 - non-strict mode.
> + 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.

This text needs help. How about something like:

0 - strict mode, default.
Invalidate the TLB of the IOMMU hardware as part of every
unmap() operation.
1 - lazy mode.
Defer TLB invalidation so that the TLB of the IOMMU hardware
is invalidated periodically, rather than as part of every
unmap() operation.

(generally, I think I'd s/non strict/lazy/ in this patch to avoid the double
negatives)

> +
> iommu.passthrough=
> [ARM64] Configure DMA to bypass the IOMMU by default.
> Format: { "0" | "1" }
> diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c
> index 8c15c5980299..2cabd0c0a4f3 100644
> --- a/drivers/iommu/iommu.c
> +++ b/drivers/iommu/iommu.c
> @@ -41,6 +41,7 @@ static unsigned int iommu_def_domain_type = IOMMU_DOMAIN_IDENTITY;
> #else
> static unsigned int iommu_def_domain_type = IOMMU_DOMAIN_DMA;
> #endif
> +static bool iommu_dma_non_strict __read_mostly;
>
> struct iommu_callback_data {
> const struct iommu_ops *ops;
> @@ -131,6 +132,24 @@ static int __init iommu_set_def_domain_type(char *str)
> }
> early_param("iommu.passthrough", iommu_set_def_domain_type);
>
> +static int __init iommu_dma_setup(char *str)
> +{
> + int ret;
> +
> + ret = kstrtobool(str, &iommu_dma_non_strict);
> + if (ret)
> + return ret;
> +
> + if (iommu_dma_non_strict) {
> + pr_warn("WARNING: iommu non-strict mode is chosen.\n"
> + "It's good for scatter-gather performance but lacks full isolation\n");

Hmm, not sure about this message either and tainting is probably over the
top. Maybe drop the taint and just pr_info something like "IOMMU DMA ops
using lazy TLB invalidation: unable to protect against malicious devices"

> + add_taint(TAINT_WARN, LOCKDEP_STILL_OK);
> + }
> +
> + return 0;
> +}
> +early_param("iommu.non_strict", iommu_dma_setup);
> +
> static ssize_t iommu_group_attr_show(struct kobject *kobj,
> struct attribute *__attr, char *buf)
> {
> @@ -1072,6 +1091,13 @@ struct iommu_group *iommu_group_get_for_dev(struct device *dev)
> group->default_domain = dom;
> if (!group->domain)
> group->domain = dom;
> +
> + if (dom && iommu_dma_non_strict) {
> + int attr = 1;
> + iommu_domain_set_attr(dom,
> + DOMAIN_ATTR_DMA_USE_FLUSH_QUEUE,
> + &attr);
> + }

Hmm, I don't think we can guarantee that we're working with the DMA domain
here. Does this all fall out in the wash for the identity domain?

Will

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