[Xen-devel] [PATCH v11 4/6] x86/ioreq server: Handle read-modify-write cases for p2m_ioreq_server pages.

[Yu Zhang <yu.c.zhang@xxxxxxxxxxxxxxx>]

In ept_handle_violation(), write violations are also treated as
read violations. And when a VM is accessing a write-protected
address with read-modify-write instructions, the read emulation
process is triggered first.

For p2m_ioreq_server pages, current ioreq server only forwards
the write operations to the device model. Therefore when such page
is being accessed by a read-modify-write instruction, the read
operations should be emulated here in hypervisor. This patch provides
such a handler to copy the data to the buffer.

Note: MMIOs with p2m_mmio_dm type do not need such special treatment
because both reads and writes will go to the device mode.

Signed-off-by: Paul Durrant <paul.durrant@xxxxxxxxxx>
Signed-off-by: Yu Zhang <yu.c.zhang@xxxxxxxxxxxxxxx>
Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
---
Cc: Paul Durrant <paul.durrant@xxxxxxxxxx>
Cc: Jan Beulich <jbeulich@xxxxxxxx>
Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>

changes in v4: 
  - Added "Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>" with one comment
    change in hvmemul_do_io().

changes in v3: 
  - According to comments from Jan: clarify comments in hvmemul_do_io().

changes in v2: 
  - According to comments from Jan: rename mem_ops to ioreq_server_ops.
  - According to comments from Jan: use hvm_copy_from_guest_phys() in
    ioreq_server_read(), instead of do it by myself.
---
 xen/arch/x86/hvm/emulate.c | 34 ++++++++++++++++++++++++++++++++++
 1 file changed, 34 insertions(+)

diff --git a/xen/arch/x86/hvm/emulate.c b/xen/arch/x86/hvm/emulate.c
index dc6f1f2..4de3936 100644
--- a/xen/arch/x86/hvm/emulate.c
+++ b/xen/arch/x86/hvm/emulate.c
@@ -95,6 +95,26 @@ static const struct hvm_io_handler null_handler = {
     .ops = &null_ops
 };
 
+static int ioreq_server_read(const struct hvm_io_handler *io_handler,
+                    uint64_t addr,
+                    uint32_t size,
+                    uint64_t *data)
+{
+    if ( hvm_copy_from_guest_phys(data, addr, size) != HVMCOPY_okay )
+        return X86EMUL_UNHANDLEABLE;
+
+    return X86EMUL_OKAY;
+}
+
+static const struct hvm_io_ops ioreq_server_ops = {
+    .read = ioreq_server_read,
+    .write = null_write
+};
+
+static const struct hvm_io_handler ioreq_server_handler = {
+    .ops = &ioreq_server_ops
+};
+
 static int hvmemul_do_io(
     bool_t is_mmio, paddr_t addr, unsigned long *reps, unsigned int size,
     uint8_t dir, bool_t df, bool_t data_is_addr, uintptr_t data)
@@ -195,6 +215,9 @@ static int hvmemul_do_io(
          *   a race with an unmap operation on the ioreq server, so re-try the
          *   instruction.
          *
+         *   - If the accesss is a read, this could be part of a
+         *   read-modify-write instruction, emulate the read first.
+         *
          * Note: Even when an ioreq server is found, its value could become
          * stale later, because it is possible that
          *
@@ -228,6 +251,17 @@ static int hvmemul_do_io(
                     vio->io_req.state = STATE_IOREQ_NONE;
                     break;
                 }
+
+                /*
+                 * This is part of a read-modify-write instruction.
+                 * Emulate the read part so we have the value available.
+                 */
+                if ( dir == IOREQ_READ )
+                {
+                    rc = hvm_process_io_intercept(&ioreq_server_handler, &p);
+                    vio->io_req.state = STATE_IOREQ_NONE;
+                    break;
+                }
             }
         }
 
-- 
1.9.1


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