Hi Robin,
On 2021/6/21 19:59, Robin Murphy wrote:
On 2021-06-21 11:34, John Garry wrote:
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?
For me it boils down to whether there are any realistic workloads where
non-strict mode *would* still perform better under virtualisation. The
At present, we see that strict mode has better performance in the
virtualization environment because it will make the shadow page table
management more efficient. When the hardware supports nested
translation, we may have to re-evaluate this since there's no need for
a shadowing page table anymore.
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.
Agreed. We could revisit it later.
Best regards,
baolu
