[Xen-devel] [PATCH v6 3/4] 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>
---
 xen/arch/x86/hvm/emulate.c | 45 ++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 44 insertions(+), 1 deletion(-)

diff --git a/xen/arch/x86/hvm/emulate.c b/xen/arch/x86/hvm/emulate.c
index a33346e..f92de48 100644
--- a/xen/arch/x86/hvm/emulate.c
+++ b/xen/arch/x86/hvm/emulate.c
@@ -95,6 +95,41 @@ static const struct hvm_io_handler null_handler = {
     .ops = &null_ops
 };
 
+static int mem_read(const struct hvm_io_handler *io_handler,
+                    uint64_t addr,
+                    uint32_t size,
+                    uint64_t *data)
+{
+    struct domain *currd = current->domain;
+    unsigned long gmfn = paddr_to_pfn(addr);
+    unsigned long offset = addr & ~PAGE_MASK;
+    struct page_info *page = get_page_from_gfn(currd, gmfn, NULL, P2M_UNSHARE);
+    uint8_t *p;
+
+    ASSERT(offset + size < PAGE_SIZE);
+
+    if ( !page )
+        return X86EMUL_UNHANDLEABLE;
+
+    p = __map_domain_page(page);
+    p += offset;
+    memcpy(data, p, size);
+
+    unmap_domain_page(p);
+    put_page(page);
+
+    return X86EMUL_OKAY;
+}
+
+static const struct hvm_io_ops mem_ops = {
+    .read = mem_read,
+    .write = null_write
+};
+
+static const struct hvm_io_handler mem_handler = {
+    .ops = &mem_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)
@@ -204,7 +239,15 @@ static int hvmemul_do_io(
         /* If there is no suitable backing DM, just ignore accesses */
         if ( !s )
         {
-            rc = hvm_process_io_intercept(&null_handler, &p);
+            /*
+             * For p2m_ioreq_server pages accessed with read-modify-write
+             * instructions, we provide a read handler to copy the data to
+             * the buffer.
+             */
+            if ( p2mt == p2m_ioreq_server )
+                rc = hvm_process_io_intercept(&mem_handler, &p);
+            else
+                rc = hvm_process_io_intercept(&null_handler, &p);
             vio->io_req.state = STATE_IOREQ_NONE;
         }
         else
-- 
1.9.1


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