[Xen-devel] [RFC PATCH] xen-block: introduces extra request to pass-through SCSI commands

[Bob Liu <bob.liu@xxxxxxxxxx>]

1) What is this patch about?
This patch introduces an new block operation (BLKIF_OP_EXTRA_FLAG).
A request with BLKIF_OP_EXTRA_FLAG set means the following request is an
extra request which is used to pass through SCSI commands.
This is like a simplified version of XEN_NETIF_EXTRA_* in netif.h.
It can be extended easily to transmit other per-request/bio data from frontend
to backend e.g Data Integrity Field per bio.

2) Why we need this?
Currently only raw data segments are transmitted from blkfront to blkback, which
means some advanced features are lost.
 * Guest knows nothing about features of the real backend storage.
        For example, on bare-metal environment INQUIRY SCSI command can be used
        to query storage device information. If it's a SSD or flash device we
        can have the option to use the device as a fast cache.
        But this can't happen in current domU guests, because blkfront only
        knows it's just a normal virtual disk

 * Failover Clusters in Windows
        Failover clusters require SCSI-3 persistent reservation target disks,
        but now this can't work in domU.

3) Known issues:
 * Security issues, how to 'validate' this extra request payload.
   E.g SCSI operates on LUN bases (the whole disk) while we really just want to
   operate on partitions

 * Can't pass SCSI commands through if the backend storage driver is bio-based
   instead of request-based.

4) Alternative approach: Using PVSCSI instead:
 * Doubt PVSCSI can support as many type of backend storage devices as 
Xen-block.

 * Much longer path:
   ioctl() -> SCSI upper layer -> Middle layer -> PVSCSI-frontend -> 
PVSCSI-backend -> Target framework(LIO?) ->

   With xen-block we only need:
   ioctl() -> blkfront -> blkback ->

 * xen-block has been existed for many years, widely used and more stable.

Welcome any input, thank you!

Signed-off-by: Bob Liu <bob.liu@xxxxxxxxxx>
---
 xen/include/public/io/blkif.h |   73 +++++++++++++++++++++++++++++++++++++++++
 1 file changed, 73 insertions(+)

diff --git a/xen/include/public/io/blkif.h b/xen/include/public/io/blkif.h
index 99f0326..7c10bce 100644
--- a/xen/include/public/io/blkif.h
+++ b/xen/include/public/io/blkif.h
@@ -635,6 +635,28 @@
 #define BLKIF_OP_INDIRECT          6
 
 /*
+ * Recognised only if "feature-extra-request" is present in backend xenbus 
info.
+ * A request with BLKIF_OP_EXTRA_FLAG indicates an extra request is followed
+ * in the shared ring buffer.
+ *
+ * By this way, extra data like SCSI command, DIF/DIX and other per-request/bio
+ * data can be transmitted from frontend to backend.
+ *
+ * The 'wire' format is like:
+ *  Request 1: xen_blkif_request
+ * [Request 2: xen_blkif_extra_request]    (only if request 1 has 
BLKIF_OP_EXTRA_FLAG)
+ *  Request 3: xen_blkif_request
+ *  Request 4: xen_blkif_request
+ * [Request 5: xen_blkif_extra_request]    (only if request 4 has 
BLKIF_OP_EXTRA_FLAG)
+ *  ...
+ *  Request N: xen_blkif_request
+ *
+ * If a backend does not recognize BLKIF_OP_EXTRA_FLAG, it should *not* create 
the
+ * "feature-extra-request" node!
+ */
+#define BLKIF_OP_EXTRA_FLAG (0x80)
+
+/*
  * Maximum scatter/gather segments per request.
  * This is carefully chosen so that sizeof(blkif_ring_t) <= PAGE_SIZE.
  * NB. This could be 12 if the ring indexes weren't stored in the same page.
@@ -703,10 +725,61 @@ struct blkif_request_indirect {
 };
 typedef struct blkif_request_indirect blkif_request_indirect_t;
 
+enum blkif_extra_request_type {
+       BLKIF_EXTRA_TYPE_SCSI_CMD = 1,          /* Transmit SCSI command.  */
+};
+
+struct scsi_cmd_req {
+       /*
+        * Grant mapping for transmiting SCSI command to backend, and
+        * also receive sense data from backend.
+        * One 4KB page is enough.
+        */
+       grant_ref_t cmd_gref;
+       /* Length of SCSI command in the grant mapped page. */
+       unsigned int cmd_len;
+
+       /*
+        * SCSI command may require transmiting data segment length less
+        * than a sector(512 bytes).
+        * Record num_sg and last segment length in extra request so that
+        * backend can know about them.
+        */
+       unsigned int num_sg;
+       unsigned int last_sg_len;
+};
+
+/*
+ * Extra request, must follow a normal-request and a normal-request can
+ * only be followed by one extra request.
+ */
+struct blkif_request_extra {
+       uint8_t type;           /* BLKIF_EXTRA_TYPE_* */
+       uint16_t _pad1;
+#ifndef CONFIG_X86_32
+       uint32_t _pad2;         /* offsetof(blkif_...,u.extra.id) == 8 */
+#endif
+       uint64_t id;
+       struct scsi_cmd_req scsi_cmd;
+} __attribute__((__packed__));
+typedef struct blkif_request_extra blkif_request_extra_t;
+
+struct scsi_cmd_res {
+       unsigned int resid_len;
+       /* Length of sense data returned in grant mapped page. */
+       unsigned int sense_len;
+};
+
+struct blkif_response_extra {
+       uint8_t type;  /* BLKIF_EXTRA_TYPE_* */
+       struct scsi_cmd_res scsi_cmd;
+} __attribute__((__packed__));
+
 struct blkif_response {
     uint64_t        id;              /* copied from request */
     uint8_t         operation;       /* copied from request */
     int16_t         status;          /* BLKIF_RSP_???       */
+    struct blkif_response_extra extra;
 };
 typedef struct blkif_response blkif_response_t;
 
-- 
1.7.10.4


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