scsi: only access SCSIDevice->requests from one thread

Stop depending on the AioContext lock and instead access
SCSIDevice->requests from only one thread at a time:
- When the VM is running only the BlockBackend's AioContext may access
  the requests list.
- When the VM is stopped only the main loop may access the requests
  list.

These constraints protect the requests list without the need for locking
in the I/O code path.

Note that multiple IOThreads are not supported yet because the code
assumes all SCSIRequests are executed from a single AioContext. Leave
that as future work.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-ID: <20231204164259.1515217-2-stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

1 files changed, 6 insertions, 1 deletions

diff --git a/include/hw/scsi/scsi.h b/include/hw/scsi/scsi.h
index 3692ca82f3..10c4e8288d 100644
--- a/include/hw/scsi/scsi.h
+++ b/include/hw/scsi/scsi.h
@@ -69,14 +69,19 @@ struct SCSIDevice
 {
     DeviceState qdev;
     VMChangeStateEntry *vmsentry;
-    QEMUBH *bh;
     uint32_t id;
     BlockConf conf;
     SCSISense unit_attention;
     bool sense_is_ua;
     uint8_t sense[SCSI_SENSE_BUF_SIZE];
     uint32_t sense_len;
+
+    /*
+     * The requests list is only accessed from the AioContext that executes
+     * requests or from the main loop when IOThread processing is stopped.
+     */
     QTAILQ_HEAD(, SCSIRequest) requests;
+
     uint32_t channel;
     uint32_t lun;
     int blocksize;