linux-kernel - Re: [PATCH v14 6/6] iommu: Remove mode argument from iommu_set_dma

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Date:   Mon, 21 Jun 2021 11:34:25 +0100
From:   John Garry <john.garry@...wei.com>
To:     Lu Baolu <baolu.lu@...ux.intel.com>, <joro@...tes.org>,
        <will@...nel.org>, <dwmw2@...radead.org>, <robin.murphy@....com>,
        <corbet@....net>
CC:     <linux-kernel@...r.kernel.org>, <iommu@...ts.linux-foundation.org>,
        <linuxarm@...wei.com>, <thunder.leizhen@...wei.com>,
        <chenxiang66@...ilicon.com>, <linux-doc@...r.kernel.org>
Subject: Re: [PATCH v14 6/6] iommu: Remove mode argument from
 iommu_set_dma_strict()

On 21/06/2021 11:00, Lu Baolu wrote:
>> void iommu_set_dma_strict(bool force)
>> {
>>           if (force == true)
>>          iommu_dma_strict = true;
>>      else if (!(iommu_cmd_line & IOMMU_CMD_LINE_STRICT))
>>          iommu_dma_strict = true;
>> }
>>
>> So we would use iommu_set_dma_strict(true) for a) and b), but 
>> iommu_set_dma_strict(false) for c).
> 
> Yes. We need to distinguish the "must" and "nice-to-have" cases of
> setting strict mode.
> 
>>
>> Then I am not sure what you want to do with the accompanying print for 
>> c). It was:
>> "IOMMU batching is disabled due to virtualization"
>>
>> And now is from this series:
>> "IOMMU batching disallowed due to virtualization"
>>
>> Using iommu_get_dma_strict(domain) is not appropriate here to know the 
>> current mode (so we know whether to print).
>>
>> Note that this change would mean that the current series would require 
>> non-trivial rework, which would be unfortunate so late in the cycle.
> 
> This patch series looks good to me and I have added by reviewed-by.
> Probably we could make another patch series to improve it so that the
> kernel optimization should not override the user setting.

On a personal level I would be happy with that approach, but I think 
it's better to not start changing things right away in a follow-up series.

So how about we add this patch (which replaces 6/6 "iommu: Remove mode 
argument from iommu_set_dma_strict()")?

Robin, any opinion?

------->8---------

[PATCH] iommu/vt-d: Make "iommu.strict" override batching due to
  virtualization

As a change in policy, make iommu.strict cmdline argument override 
whether we disable batching due to virtualization.

The API of iommu_set_dma_strict() is changed to accept a "force" 
argument, which means that we always set iommu_dma_strict true, 
regardless of whether we already set via cmdline. Also return a boolean, 
to tell whether iommu_dma_strict was set or not.

Note that in all pre-existing callsites of iommu_set_dma_strict(), 
argument strict was true, so this argument is dropped.

Signed-off-by: John Garry <john.garry@...wei.com>

diff --git a/drivers/iommu/intel/iommu.c b/drivers/iommu/intel/iommu.c
index 06666f9d8116..e8d65239b359 100644
--- a/drivers/iommu/intel/iommu.c
+++ b/drivers/iommu/intel/iommu.c
@@ -4380,10 +4380,8 @@ int __init intel_iommu_init(void)
  		 * is likely to be much lower than the overhead of synchronizing
  		 * the virtual and physical IOMMU page-tables.
  		 */
-		if (cap_caching_mode(iommu->cap)) {
+		if (cap_caching_mode(iommu->cap) && iommu_set_dma_strict(false))
  			pr_info_once("IOMMU batching disallowed due to virtualization\n");
-			iommu_set_dma_strict(true);
-		}
  		iommu_device_sysfs_add(&iommu->iommu, NULL,
  				       intel_iommu_groups,
  				       "%s", iommu->name);
diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c
index 60b1ec42e73b..1434bee64af3 100644
--- a/drivers/iommu/iommu.c
+++ b/drivers/iommu/iommu.c
@@ -349,10 +349,14 @@ static int __init iommu_dma_setup(char *str)
  }
  early_param("iommu.strict", iommu_dma_setup);

-void iommu_set_dma_strict(bool strict)
+/* Return true if we set iommu_dma_strict */
+bool iommu_set_dma_strict(bool force)
  {
-	if (strict || !(iommu_cmd_line & IOMMU_CMD_LINE_STRICT))
-		iommu_dma_strict = strict;
+	if (force || !(iommu_cmd_line & IOMMU_CMD_LINE_STRICT)) {
+		iommu_dma_strict = true;
+		return true;
+	}
+	return false;
  }

  bool iommu_get_dma_strict(struct iommu_domain *domain)
diff --git a/include/linux/iommu.h b/include/linux/iommu.h
index 32d448050bf7..f17b20234296 100644
--- a/include/linux/iommu.h
+++ b/include/linux/iommu.h
@@ -476,7 +476,7 @@ int iommu_enable_nesting(struct iommu_domain *domain);
  int iommu_set_pgtable_quirks(struct iommu_domain *domain,
  		unsigned long quirks);

-void iommu_set_dma_strict(bool val);
+bool iommu_set_dma_strict(bool force);
  bool iommu_get_dma_strict(struct iommu_domain *domain);

  extern int report_iommu_fault(struct iommu_domain *domain, struct 
device *dev,
-- 
2.26.2