[PATCH for-4.10] passthrough/vtd: Don't DMA to the stack in queue_invalidate_wait()

[Andrew Cooper <andrew.cooper3@citrix.com>]

DMA-ing to the stack is generally considered bad practice.  In this case, if a
timeout occurs because of a sluggish device which is processing the request,
the completion notification will corrupt the stack of a subsequent deeper call
tree.

Place the poll_slot in a percpu area and DMA to that instead.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
---
CC: Jan Beulich <JBeulich@suse.com>
CC: Kevin Tian <kevin.tian@intel.com>
CC: Julien Grall <julien.grall@arm.com>

Julien: This wants backporting to all releases, and therefore should be
considered for 4.10 at this point.
---
 xen/drivers/passthrough/vtd/qinval.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/xen/drivers/passthrough/vtd/qinval.c b/xen/drivers/passthrough/vtd/qinval.c
index e95dc54..0ddda00 100644
--- a/xen/drivers/passthrough/vtd/qinval.c
+++ b/xen/drivers/passthrough/vtd/qinval.c
@@ -147,7 +147,8 @@ static int __must_check queue_invalidate_wait(struct iommu *iommu,
                                               u8 iflag, u8 sw, u8 fn,
                                               bool_t flush_dev_iotlb)
 {
-    volatile u32 poll_slot = QINVAL_STAT_INIT;
+    static DEFINE_PER_CPU(u32, poll_slot);
+    volatile u32 *this_poll_slot = &this_cpu(poll_slot);
     unsigned int index;
     unsigned long flags;
     u64 entry_base;
@@ -167,7 +168,7 @@ static int __must_check queue_invalidate_wait(struct iommu *iommu,
     qinval_entry->q.inv_wait_dsc.lo.res_1 = 0;
     qinval_entry->q.inv_wait_dsc.lo.sdata = QINVAL_STAT_DONE;
     qinval_entry->q.inv_wait_dsc.hi.res_1 = 0;
-    qinval_entry->q.inv_wait_dsc.hi.saddr = virt_to_maddr(&poll_slot) >> 2;
+    qinval_entry->q.inv_wait_dsc.hi.saddr = virt_to_maddr(this_poll_slot) >> 2;
 
     unmap_vtd_domain_page(qinval_entries);
     qinval_update_qtail(iommu, index);
@@ -182,7 +183,7 @@ static int __must_check queue_invalidate_wait(struct iommu *iommu,
         timeout = NOW() + MILLISECS(flush_dev_iotlb ?
                                     iommu_dev_iotlb_timeout : VTD_QI_TIMEOUT);
 
-        while ( poll_slot != QINVAL_STAT_DONE )
+        while ( *this_poll_slot != QINVAL_STAT_DONE )
         {
             if ( NOW() > timeout )
             {
-- 
2.1.4


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