[RFC v2 3/3] cxl: Add memory barrier to guarantee TLBI scope

[Frederic Barrat]

With the hash memory model, all TLBIs become global when the cxl
driver is active, i.e. as soon as one context is open.
It is theoretically possible to send a TLBI with the wrong scope as
there's currently no memory barrier between when the driver is marked
as in use, and attaching a context to the device, therefore we are
exposed to re-ordering. It is highly unlikely as the use count for the
driver is incremented on open() and the attachment to the device
happens on a different system call (ioctl)

Signed-off-by: Frederic Barrat <fbarrat at linux.vnet.ibm.com>
---
 include/misc/cxl-base.h | 18 +++++++++++++++++-
 1 file changed, 17 insertions(+), 1 deletion(-)

diff --git a/include/misc/cxl-base.h b/include/misc/cxl-base.h
index b2ebc91fe09a..dcb6d38ab3ad 100644
--- a/include/misc/cxl-base.h
+++ b/include/misc/cxl-base.h
@@ -25,12 +25,28 @@ extern atomic_t cxl_use_count;
 
 static inline bool cxl_ctx_in_use(void)
 {
-       return (atomic_read(&cxl_use_count) != 0);
+	/*
+	 * This is called when sending an TLBI, to know whether it
+	 * should be global or local.
+	 *
+	 * We need to make sure the PTE update is happening before
+	 * reading the context global flag. Otherwise, reading the
+	 * flag may be re-ordered and happen first, and we could end
+	 * up in a situation where the old PTE is seen by the device,
+	 * but the TLBI is not global.
+	 */
+	smp_mb();
+	return (atomic_read(&cxl_use_count) != 0);
 }
 
 static inline void cxl_ctx_get(void)
 {
        atomic_inc(&cxl_use_count);
+       /*
+	* Barrier guarantees that the device will receive all TLBIs
+	* from that point on
+	*/
+       smp_wmb();
 }
 
 static inline void cxl_ctx_put(void)
-- 
2.11.0