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iommu/vt-d: Handle volatile descriptor status read

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[ Upstream commit b5e86a95541cea737394a1da967df4cd4d8f7182 ]

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>
(cherry picked from commit 75a34515eb1be431819ec00cd09fe3a3eb369cdb)
Signed-off-by: Harshit Mogalapalli <harshit.m.mogalapalli@oracle.com>
Signed-off-by: Vegard Nossum <vegard.nossum@oracle.com>
This commit is contained in:
Jacob Pan 2024-07-02 21:08:33 +08:00 committed by Vegard Nossum
parent 1c1073bf74
commit b3774dcec0
1 changed files with 1 additions and 1 deletions
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2
drivers/iommu/dmar.c
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@ -1289,7 +1289,7 @@ restart:
*/
writel(qi->free_head << DMAR_IQ_SHIFT, iommu->reg + DMAR_IQT_REG);
while (qi->desc_status[wait_index] != QI_DONE) {
while (READ_ONCE(qi->desc_status[wait_index]) != QI_DONE) {
/*
* We will leave the interrupts disabled, to prevent interrupt
* context to queue another cmd while a cmd is already submitted