Hi Ingo,

On Thu, Sep 15, 2016 at 07:44:35AM +0200, Ingo Molnar wrote:
> Yeah, I can still remove it - just zapped it in fact.

Thanks, and sorry for the hassle. Here is the v2 patch that has the
correct locking. I tested it with and without lockdep enabled and also
under some load. Looks all fine.

So here is the patch:

>From 9becedb853c682c3fc3dd5574b933df216d91acd Mon Sep 17 00:00:00 2001
From: Joerg Roedel <jroe...@suse.de>
Date: Wed, 14 Sep 2016 11:41:59 +0200
Subject: [PATCH] iommu/amd: Don't put completion-wait semaphore on stack

The semaphore used by the AMD IOMMU to signal command
completion lived on the stack until now, which was safe as
the driver busy-waited on the semaphore with IRQs disabled,
so the stack can't go away under the driver.

But the recently introduced vmap-based stacks break this as
the physical address of the semaphore can't be determinded
easily anymore. The driver used the __pa() macro, but that
only works in the direct-mapping. The result were
Completion-Wait timeout errors seen by the IOMMU driver,
breaking system boot.

Since putting the semaphore on the stack is bad design
anyway, move the semaphore into 'struct amd_iommu'. It is
protected by the per-iommu lock and now in the direct
mapping again. This fixes the Completion-Wait timeout errors
and makes AMD IOMMU systems boot again with vmap-based
stacks enabled.

Reported-by: Borislav Petkov <b...@alien8.de>
Signed-off-by: Joerg Roedel <jroe...@suse.de>
---
 drivers/iommu/amd_iommu.c       | 51 ++++++++++++++++++++++++++++-------------
 drivers/iommu/amd_iommu_types.h |  2 ++
 2 files changed, 37 insertions(+), 16 deletions(-)

diff --git a/drivers/iommu/amd_iommu.c b/drivers/iommu/amd_iommu.c
index 96de97a..4025291 100644
--- a/drivers/iommu/amd_iommu.c
+++ b/drivers/iommu/amd_iommu.c
@@ -940,15 +940,13 @@ static void build_inv_irt(struct iommu_cmd *cmd, u16 
devid)
  * Writes the command to the IOMMUs command buffer and informs the
  * hardware about the new command.
  */
-static int iommu_queue_command_sync(struct amd_iommu *iommu,
-                                   struct iommu_cmd *cmd,
-                                   bool sync)
+static int __iommu_queue_command_sync(struct amd_iommu *iommu,
+                                     struct iommu_cmd *cmd,
+                                     bool sync)
 {
        u32 left, tail, head, next_tail;
-       unsigned long flags;
 
 again:
-       spin_lock_irqsave(&iommu->lock, flags);
 
        head      = readl(iommu->mmio_base + MMIO_CMD_HEAD_OFFSET);
        tail      = readl(iommu->mmio_base + MMIO_CMD_TAIL_OFFSET);
@@ -957,15 +955,14 @@ again:
 
        if (left <= 2) {
                struct iommu_cmd sync_cmd;
-               volatile u64 sem = 0;
                int ret;
 
-               build_completion_wait(&sync_cmd, (u64)&sem);
-               copy_cmd_to_buffer(iommu, &sync_cmd, tail);
+               iommu->cmd_sem = 0;
 
-               spin_unlock_irqrestore(&iommu->lock, flags);
+               build_completion_wait(&sync_cmd, (u64)&iommu->cmd_sem);
+               copy_cmd_to_buffer(iommu, &sync_cmd, tail);
 
-               if ((ret = wait_on_sem(&sem)) != 0)
+               if ((ret = wait_on_sem(&iommu->cmd_sem)) != 0)
                        return ret;
 
                goto again;
@@ -976,9 +973,21 @@ again:
        /* We need to sync now to make sure all commands are processed */
        iommu->need_sync = sync;
 
+       return 0;
+}
+
+static int iommu_queue_command_sync(struct amd_iommu *iommu,
+                                   struct iommu_cmd *cmd,
+                                   bool sync)
+{
+       unsigned long flags;
+       int ret;
+
+       spin_lock_irqsave(&iommu->lock, flags);
+       ret = __iommu_queue_command_sync(iommu, cmd, sync);
        spin_unlock_irqrestore(&iommu->lock, flags);
 
-       return 0;
+       return ret;
 }
 
 static int iommu_queue_command(struct amd_iommu *iommu, struct iommu_cmd *cmd)
@@ -993,19 +1002,29 @@ static int iommu_queue_command(struct amd_iommu *iommu, 
struct iommu_cmd *cmd)
 static int iommu_completion_wait(struct amd_iommu *iommu)
 {
        struct iommu_cmd cmd;
-       volatile u64 sem = 0;
+       unsigned long flags;
        int ret;
 
        if (!iommu->need_sync)
                return 0;
 
-       build_completion_wait(&cmd, (u64)&sem);
 
-       ret = iommu_queue_command_sync(iommu, &cmd, false);
+       build_completion_wait(&cmd, (u64)&iommu->cmd_sem);
+
+       spin_lock_irqsave(&iommu->lock, flags);
+
+       iommu->cmd_sem = 0;
+
+       ret = __iommu_queue_command_sync(iommu, &cmd, false);
        if (ret)
-               return ret;
+               goto out_unlock;
+
+       ret = wait_on_sem(&iommu->cmd_sem);
 
-       return wait_on_sem(&sem);
+out_unlock:
+       spin_unlock_irqrestore(&iommu->lock, flags);
+
+       return ret;
 }
 
 static int iommu_flush_dte(struct amd_iommu *iommu, u16 devid)
diff --git a/drivers/iommu/amd_iommu_types.h b/drivers/iommu/amd_iommu_types.h
index caf5e38..9652848 100644
--- a/drivers/iommu/amd_iommu_types.h
+++ b/drivers/iommu/amd_iommu_types.h
@@ -524,6 +524,8 @@ struct amd_iommu {
        struct irq_domain *ir_domain;
        struct irq_domain *msi_domain;
 #endif
+
+       volatile u64 __aligned(8) cmd_sem;
 };
 
 #define ACPIHID_UID_LEN 256
-- 
2.6.2

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