[Xen-devel] [PATCH v7 3/5] 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>
---
Cc: Paul Durrant <paul.durrant@xxxxxxxxxx>
Cc: Jan Beulich <jbeulich@xxxxxxxx>
Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>

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 | 35 +++++++++++++++++++++++++++++++++++
 1 file changed, 35 insertions(+)

diff --git a/xen/arch/x86/hvm/emulate.c b/xen/arch/x86/hvm/emulate.c
index fb56f7b..9744dcb 100644
--- a/xen/arch/x86/hvm/emulate.c
+++ b/xen/arch/x86/hvm/emulate.c
@@ -94,6 +94,26 @@ 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)
@@ -197,6 +217,10 @@ static int hvmemul_do_io(
          *   - If the IOREQ_MEM_ACCESS_WRITE flag is not set, treat it
          *   like a normal PIO or MMIO that doesn't have an ioreq
          *   server (i.e., by ignoring it).
+         *
+         *   - If the accesss is a read, this could be part of a
+         *   read-modify-write instruction, emulate the read so that we
+         *   have it.
          */
         struct hvm_ioreq_server *s = NULL;
         p2m_type_t p2mt = p2m_invalid;
@@ -226,6 +250,17 @@ static int hvmemul_do_io(
                 }
 
                 /*
+                 * This is part of a read-modify-write instruction.
+                 * Emulate the read part so we have the value cached.
+                 */
+                if ( dir == IOREQ_READ )
+                {
+                    rc = hvm_process_io_intercept(&ioreq_server_handler, &p);
+                    vio->io_req.state = STATE_IOREQ_NONE;
+                    break;
+                }
+
+                /*
                  * If the IOREQ_MEM_ACCESS_WRITE flag is not set,
                  * we should set s to NULL, and just ignore such
                  * access.
-- 
1.9.1


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