On 21/06/2021 12:59, Robin Murphy wrote:
+ Nadav
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?
For me it boils down to whether there are any realistic workloads where
non-strict mode*would* still perform better under virtualisation. The
only reason for the user to explicitly pass "iommu.strict=0" is because
they expect it to increase unmap performance; if it's only ever going to
lead to an unexpected performance loss, I don't see any value in
overriding the kernel's decision purely for the sake of subservience.
If there*are* certain valid cases for allowing it for people who really
know what they're doing, then we should arguably also log a counterpart
message to say "we're honouring your override but beware it may have the
opposite effect to what you expect" for the benefit of other users who
assume it's a generic go-faster knob. At that point it starts getting
non-trivial enough that I'd want to know for sure it's worthwhile.
The other reason this might be better to revisit later is that an AMD
equivalent is still in flight[1], and there might be more that can
eventually be factored out. I think both series are pretty much good to
merge for 5.14, but time's already tight to sort out the conflicts which
exist as-is, without making them any worse.
ok, fine. Can revisit.
As for getting these merged, I'll dry-run merging both of those series
to see the conflicts. It doesn't look too problematic from a glance.
Cheers,
John
Robin.
[1]
https://lore.kernel.org/linux-iommu/20210616100500.174507-3-namit@xxxxxxxxxx/
------->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@xxxxxxxxxx>
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,
