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iommu/vt-d: Handle volatile descriptor status read
[ Upstream commit b5e86a9554
]
Queued invalidation wait descriptor status is volatile in that IOMMU
hardware writes the data upon completion.
Use READ_ONCE() to prevent compiler optimizations which ensures memory
reads every time. As a side effect, READ_ONCE() also enforces strict
types and may add an extra instruction. But it should not have negative
performance impact since we use cpu_relax anyway and the extra time(by
adding an instruction) may allow IOMMU HW request cacheline ownership
easier.
e.g. gcc 12.3
BEFORE:
81 38 ad de 00 00 cmpl $0x2,(%rax)
AFTER (with READ_ONCE())
772f: 8b 00 mov (%rax),%eax
7731: 3d ad de 00 00 cmp $0x2,%eax
//status data is 32 bit
Signed-off-by: Jacob Pan <jacob.jun.pan@linux.intel.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Reviewed-by: Yi Liu <yi.l.liu@intel.com>
Link: https://lore.kernel.org/r/20240607173817.3914600-1-jacob.jun.pan@linux.intel.com
Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
Link: https://lore.kernel.org/r/20240702130839.108139-2-baolu.lu@linux.intel.com
Signed-off-by: Will Deacon <will@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
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1 changed files with 1 additions and 1 deletions
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@ -1422,7 +1422,7 @@ int qi_submit_sync(struct intel_iommu *iommu, struct qi_desc *desc,
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*/
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writel(qi->free_head << shift, iommu->reg + DMAR_IQT_REG);
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while (qi->desc_status[wait_index] != QI_DONE) {
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while (READ_ONCE(qi->desc_status[wait_index]) != QI_DONE) {
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/*
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* We will leave the interrupts disabled, to prevent interrupt
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* context to queue another cmd while a cmd is already submitted
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