[PATCH] docs/design: introduce HVMMEM_ioreq_serverX types

[Paul Durrant <paul.durrant@citrix.com>]

This patch adds a new 'designs' subdirectory under docs as a repository
for this and future design proposals.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
---

For convenience this document can also be viewed in PDF at:

http://xenbits.xen.org/people/pauldu/hvmmem_ioreq_server.pdf
---
 docs/designs/hvmmem_ioreq_server.md | 63 +++++++++++++++++++++++++++++++++++++
 1 file changed, 63 insertions(+)
 create mode 100755 docs/designs/hvmmem_ioreq_server.md

diff --git a/docs/designs/hvmmem_ioreq_server.md b/docs/designs/hvmmem_ioreq_server.md
new file mode 100755
index 0000000..47fa715
--- /dev/null
+++ b/docs/designs/hvmmem_ioreq_server.md
@@ -0,0 +1,63 @@
+HVMMEM\_ioreq\_serverX
+----------------------
+
+Background
+==========
+
+The concept of the IOREQ server was introduced to allow multiple distinct
+device emulators to a single VM. The XenGT project uses an IOREQ server to
+provide mediated pass-through of Intel GPUs to guests and, as part of the
+mediation, needs to intercept accesses to GPU page-tables (or GTTs) that
+reside in guest RAM.
+
+The current implementation of this sets the type of GTT pages to type
+HVMMEM\_mmio\_write\_dm, which causes Xen to emulate writes to such pages,
+and then maps the guest physical addresses of those pages to the XenGT
+IOREQ server using the HVMOP\_map\_io\_range\_to\_ioreq\_server hypercall.
+However, because the number of GTTs is potentially large, using this
+approach does not scale well.
+
+Proposal
+========
+
+Because the number of spare types available in the P2M type-space is
+currently very limited it is proposed that HVMMEM\_mmio\_write\_dm be
+replaced by a single new type HVMMEM\_ioreq\_server. In future, if the
+P2M type-space is increased, this can be renamed to HVMMEM\_ioreq\_server0
+and new HVMMEM\_ioreq\_server1, HVMMEM\_ioreq\_server2, etc. types
+can be added.
+
+Accesses to a page of type HVMMEM\_ioreq\_serverX should be the same as
+HVMMEM\_ram\_rw until the type is _claimed_ by an IOREQ server. Furthermore
+it should only be possible to set the type of a page to
+HVMMEM\_ioreq\_serverX if that page is currently of type HVMMEM\_ram\_rw.
+
+To allow an IOREQ server to claim or release a claim to a type a new pair
+of hypercalls will be introduced:
+
+- HVMOP\_map\_mem\_type\_to\_ioreq\_server
+- HVMOP\_unmap\_mem\_type\_from\_ioreq\_server
+
+and an associated argument structure:
+
+		struct hvm_ioreq_mem_type {
+			domid_t domid;      /* IN - domain to be serviced */
+			ioservid_t id;      /* IN - server id */
+			hvmmem_type_t type; /* IN - memory type */
+			uint32_t flags;     /* IN - types of access to be
+	                                    intercepted */
+
+	#define _HVMOP_IOREQ_MEM_ACCESS_READ 0
+	#define HVMOP_IOREQ_MEM_ACCESS_READ \
+	        (1 << _HVMOP_IOREQ_MEM_ACCESS_READ)
+
+	#define _HVMOP_IOREQ_MEM_ACCESS_WRITE 1
+	#define HVMOP_IOREQ_MEM_ACCESS_WRITE \
+	        (1 << _HVMOP_IOREQ_MEM_ACCESS_WRITE)
+
+		};
+
+
+Once the type has been claimed then the requested types of access to any
+page of the claimed type will be passed to the IOREQ server for handling.
+Only HVMMEM\_ioreq\_serverX types may be claimed.
-- 
2.1.4


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