habanalabs/gaudi: fix a race condition causing DMAR error

There is a rare race condition in CB completion mechanism, that can
occur under a very high pressure of command submissions.
The preconditions for this to happen are:

 1. There should be enough command submissions for the pre-allocated
    patched CB pool to run out of commands. At this stage we start
    allocating new patched CBs as they arrive.
 2. CB size has to be exactly (128*n + 104)B for some n, i.e. 24B below
    a cache line end.

The flow:

 1. Two command buffers being completed on different streams, at the
    same time. Denote those CB1 and CB2.
 2. Each command buffer is injected with two messages, 16B each - one
    for a HBW update of the completion queue, another to raise
    interrupt.
 3. Assume CB1 updated the completion queue and raise the interrupt.
 4. Assume CB2 updated the completion queue but did not raise the
    interrupt yet.
 5. The host receives the interrupt. It goes over the completion queue
    and sees two completions - CB1 and CB2. Release them both.
 6. CB2 performs the last command. The problem is that the last command
    is split between 2 cache lines. So to read the last 8B of the last
    command, it has to access the host again. Problem is - CB2 is
    already released. This causes a DMAR error.

The solution to this problem is simply to make sure the last two
commands in the CB are always in the same cache line, using NOP padding.

Signed-off-by: Yuri Nudelman <ynudelman@habana.ai>
Reviewed-by: Oded Gabbay <ogabbay@kernel.org>
Signed-off-by: Oded Gabbay <ogabbay@kernel.org>

1 files changed, 34 insertions, 12 deletions

diff --git a/drivers/misc/habanalabs/gaudi/gaudi.c b/drivers/misc/habanalabs/gaudi/gaudi.c
index 172c21f262e7..453de3d27d0c 100644
--- a/drivers/misc/habanalabs/gaudi/gaudi.c
+++ b/drivers/misc/habanalabs/gaudi/gaudi.c
@@ -1423,6 +1423,19 @@ static int gaudi_collective_wait_init_cs(struct hl_cs *cs)
 	return 0;
 }
 
+static u32 gaudi_get_patched_cb_extra_size(u32 user_cb_size)
+{
+	u32 cacheline_end, additional_commands;
+
+	cacheline_end = round_up(user_cb_size, DEVICE_CACHE_LINE_SIZE);
+	additional_commands = sizeof(struct packet_msg_prot) * 2;
+
+	if (user_cb_size + additional_commands > cacheline_end)
+		return cacheline_end - user_cb_size + additional_commands;
+	else
+		return additional_commands;
+}
+
 static int gaudi_collective_wait_create_job(struct hl_device *hdev,
 		struct hl_ctx *ctx, struct hl_cs *cs,
 		enum hl_collective_mode mode, u32 queue_id, u32 wait_queue_id,
@@ -1443,7 +1456,7 @@ static int gaudi_collective_wait_create_job(struct hl_device *hdev,
 		 * 1 fence packet
 		 * 4 msg short packets for monitor 2 configuration
 		 * 1 fence packet
-		 * 2 msg prot packets for completion and MSI-X
+		 * 2 msg prot packets for completion and MSI
 		 */
 		cb_size = sizeof(struct packet_msg_short) * 8 +
 				sizeof(struct packet_fence) * 2 +
@@ -5337,11 +5350,13 @@ static int gaudi_validate_cb(struct hl_device *hdev,
 
 	/*
 	 * The new CB should have space at the end for two MSG_PROT packets:
-	 * 1. A packet that will act as a completion packet
-	 * 2. A packet that will generate MSI-X interrupt
+	 * 1. Optional NOP padding for cacheline alignment
+	 * 2. A packet that will act as a completion packet
+	 * 3. A packet that will generate MSI interrupt
 	 */
 	if (parser->completion)
-		parser->patched_cb_size += sizeof(struct packet_msg_prot) * 2;
+		parser->patched_cb_size += gaudi_get_patched_cb_extra_size(
+			parser->patched_cb_size);
 
 	return rc;
 }
@@ -5564,13 +5579,14 @@ static int gaudi_parse_cb_mmu(struct hl_device *hdev,
 	int rc;
 
 	/*
-	 * The new CB should have space at the end for two MSG_PROT pkt:
-	 * 1. A packet that will act as a completion packet
-	 * 2. A packet that will generate MSI interrupt
+	 * The new CB should have space at the end for two MSG_PROT packets:
+	 * 1. Optional NOP padding for cacheline alignment
+	 * 2. A packet that will act as a completion packet
+	 * 3. A packet that will generate MSI interrupt
 	 */
 	if (parser->completion)
 		parser->patched_cb_size = parser->user_cb_size +
-				sizeof(struct packet_msg_prot) * 2;
+				gaudi_get_patched_cb_extra_size(parser->user_cb_size);
 	else
 		parser->patched_cb_size = parser->user_cb_size;
 
@@ -5745,18 +5761,24 @@ static int gaudi_cs_parser(struct hl_device *hdev, struct hl_cs_parser *parser)
 		return gaudi_parse_cb_no_mmu(hdev, parser);
 }
 
-static void gaudi_add_end_of_cb_packets(struct hl_device *hdev,
-					void *kernel_address, u32 len,
-					u64 cq_addr, u32 cq_val, u32 msi_vec,
-					bool eb)
+static void gaudi_add_end_of_cb_packets(struct hl_device *hdev, void *kernel_address,
+				u32 len, u32 original_len, u64 cq_addr, u32 cq_val,
+				u32 msi_vec, bool eb)
 {
 	struct gaudi_device *gaudi = hdev->asic_specific;
 	struct packet_msg_prot *cq_pkt;
+	struct packet_nop *cq_padding;
 	u64 msi_addr;
 	u32 tmp;
 
+	cq_padding = kernel_address + original_len;
 	cq_pkt = kernel_address + len - (sizeof(struct packet_msg_prot) * 2);
 
+	while ((void *)cq_padding < (void *)cq_pkt) {
+		cq_padding->ctl = FIELD_PREP(GAUDI_PKT_CTL_OPCODE_MASK, PACKET_NOP);
+		cq_padding++;
+	}
+
 	tmp = FIELD_PREP(GAUDI_PKT_CTL_OPCODE_MASK, PACKET_MSG_PROT);
 	tmp |= FIELD_PREP(GAUDI_PKT_CTL_MB_MASK, 1);