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[Xen-devel] [PATCH V3 1/3] xen/mem_access: Support for memory-content hiding



This patch adds support for memory-content hiding, by modifying the
value returned by emulated instructions that read certain memory
addresses that contain sensitive data. The patch only applies to
cases where MEM_ACCESS_EMULATE or MEM_ACCESS_EMULATE_NOWRITE have
been set to a vm_event response.

Signed-off-by: Razvan Cojocaru <rcojocaru@xxxxxxxxxxxxxxx>

---
Changes since V2:
 - Now memset()-ing all read buffers to 0 before writing the response
   data (don't leak HV stack contents).
 - Safe_bytes is now unsigned int everywhere instead of unsigned long.
 - Removed the custom hvmemul_rep_stos() handler.
 - EMUL_KIND_NORMAL became the default case in
   hvm_mem_access_emulate_one().
 - Replaced a 'break' with a more appropriate 'continue' in
   vm_event_enable().
 - The rep outs case now acts consistent with the read case
   (no longer ignoring *reps).
 - Removed the individual set_context static handlers in favour of
   a flag in struct hvmemul_ctxt.
 - Corrected patch title (it's mem_access and vm_event, there's no
   such thing as vm_access).
---
 tools/tests/xen-access/xen-access.c |    2 +-
 xen/arch/x86/domain.c               |    1 +
 xen/arch/x86/hvm/emulate.c          |  132 ++++++++++++++++++++++++++++++++++-
 xen/arch/x86/hvm/event.c            |   50 ++++++-------
 xen/arch/x86/mm/p2m.c               |   90 +++++++++++++-----------
 xen/common/vm_event.c               |   23 ++++++
 xen/include/asm-x86/domain.h        |    2 +
 xen/include/asm-x86/hvm/emulate.h   |   10 ++-
 xen/include/public/vm_event.h       |   21 +++++-
 9 files changed, 259 insertions(+), 72 deletions(-)

diff --git a/tools/tests/xen-access/xen-access.c 
b/tools/tests/xen-access/xen-access.c
index 12ab921..e6ca9ba 100644
--- a/tools/tests/xen-access/xen-access.c
+++ b/tools/tests/xen-access/xen-access.c
@@ -530,7 +530,7 @@ int main(int argc, char *argv[])
                 break;
             case VM_EVENT_REASON_SOFTWARE_BREAKPOINT:
                 printf("Breakpoint: rip=%016"PRIx64", gfn=%"PRIx64" (vcpu 
%d)\n",
-                       req.regs.x86.rip,
+                       req.data.regs.x86.rip,
                        req.u.software_breakpoint.gfn,
                        req.vcpu_id);
 
diff --git a/xen/arch/x86/domain.c b/xen/arch/x86/domain.c
index a8fe046..cbbc354 100644
--- a/xen/arch/x86/domain.c
+++ b/xen/arch/x86/domain.c
@@ -269,6 +269,7 @@ struct vcpu *alloc_vcpu_struct(void)
 
 void free_vcpu_struct(struct vcpu *v)
 {
+    xfree(v->arch.vm_event.emul_read_data);
     free_xenheap_page(v);
 }
 
diff --git a/xen/arch/x86/hvm/emulate.c b/xen/arch/x86/hvm/emulate.c
index fe5661d..3678e29 100644
--- a/xen/arch/x86/hvm/emulate.c
+++ b/xen/arch/x86/hvm/emulate.c
@@ -653,6 +653,28 @@ static int hvmemul_read(
     unsigned int bytes,
     struct x86_emulate_ctxt *ctxt)
 {
+    struct hvm_emulate_ctxt *hvmemul_ctxt =
+        container_of(ctxt, struct hvm_emulate_ctxt, ctxt);
+
+    if ( unlikely(hvmemul_ctxt->set_context) )
+    {
+        struct vcpu *curr = current;
+        unsigned int len;
+
+        if ( !curr->arch.vm_event.emul_read_data )
+            return X86EMUL_UNHANDLEABLE;
+
+        len = min_t(unsigned int,
+                    bytes, curr->arch.vm_event.emul_read_data->size);
+
+        if ( len ) {
+            memset(p_data, 0, bytes);
+            memcpy(p_data, curr->arch.vm_event.emul_read_data->data, len);
+        }
+
+        return X86EMUL_OKAY;
+    }
+
     return __hvmemul_read(
         seg, offset, p_data, bytes, hvm_access_read,
         container_of(ctxt, struct hvm_emulate_ctxt, ctxt));
@@ -893,6 +915,24 @@ static int hvmemul_cmpxchg(
     unsigned int bytes,
     struct x86_emulate_ctxt *ctxt)
 {
+    struct hvm_emulate_ctxt *hvmemul_ctxt =
+        container_of(ctxt, struct hvm_emulate_ctxt, ctxt);
+
+    if ( unlikely(hvmemul_ctxt->set_context) )
+    {
+        struct vcpu *curr = current;
+
+        if ( curr->arch.vm_event.emul_read_data )
+        {
+            unsigned int safe_bytes = min_t(unsigned int, bytes,
+                curr->arch.vm_event.emul_read_data->size);
+
+            memset(p_new, 0, bytes);
+            memcpy(p_new, curr->arch.vm_event.emul_read_data->data,
+                   safe_bytes);
+        }
+    }
+
     /* Fix this in case the guest is really relying on r-m-w atomicity. */
     return hvmemul_write(seg, offset, p_new, bytes, ctxt);
 }
@@ -935,6 +975,41 @@ static int hvmemul_rep_ins(
                                !!(ctxt->regs->eflags & X86_EFLAGS_DF), gpa);
 }
 
+static int hvmemul_rep_outs_set_context(
+    enum x86_segment src_seg,
+    unsigned long src_offset,
+    uint16_t dst_port,
+    unsigned int bytes_per_rep,
+    unsigned long *reps,
+    struct x86_emulate_ctxt *ctxt)
+{
+    unsigned int bytes = *reps * bytes_per_rep;
+    struct vcpu *curr = current;
+    unsigned int safe_bytes;
+    char *buf = NULL;
+    int rc;
+
+    if ( !curr->arch.vm_event.emul_read_data )
+        return X86EMUL_UNHANDLEABLE;
+
+    buf = xmalloc_bytes(bytes);
+
+    if ( buf == NULL )
+        return X86EMUL_UNHANDLEABLE;
+
+    memset(buf, 0, bytes);
+
+    safe_bytes = min_t(unsigned int, bytes,
+                       curr->arch.vm_event.emul_read_data->size);
+
+    memcpy(buf, curr->arch.vm_event.emul_read_data->data, safe_bytes);
+    rc = hvmemul_do_pio_buffer(dst_port, bytes, IOREQ_WRITE, buf);
+
+    xfree(buf);
+
+    return rc;
+}
+
 static int hvmemul_rep_outs(
     enum x86_segment src_seg,
     unsigned long src_offset,
@@ -951,6 +1026,10 @@ static int hvmemul_rep_outs(
     p2m_type_t p2mt;
     int rc;
 
+    if ( unlikely(hvmemul_ctxt->set_context) )
+        return hvmemul_rep_outs_set_context(src_seg, src_offset, dst_port,
+                                            bytes_per_rep, reps, ctxt);
+
     rc = hvmemul_virtual_to_linear(
         src_seg, src_offset, bytes_per_rep, reps, hvm_access_read,
         hvmemul_ctxt, &addr);
@@ -1063,7 +1142,20 @@ static int hvmemul_rep_movs(
      */
     rc = hvm_copy_from_guest_phys(buf, sgpa, bytes);
     if ( rc == HVMCOPY_okay )
+    {
+        struct vcpu *curr = current;
+
+        if ( unlikely(hvmemul_ctxt->set_context) &&
+             curr->arch.vm_event.emul_read_data )
+        {
+            unsigned int safe_bytes = min_t(unsigned int, bytes,
+                curr->arch.vm_event.emul_read_data->size);
+
+            memcpy(buf, curr->arch.vm_event.emul_read_data->data, safe_bytes);
+        }
+
         rc = hvm_copy_to_guest_phys(dgpa, buf, bytes);
+    }
 
     xfree(buf);
 
@@ -1222,7 +1314,27 @@ static int hvmemul_read_io(
     unsigned long *val,
     struct x86_emulate_ctxt *ctxt)
 {
+    struct hvm_emulate_ctxt *hvmemul_ctxt =
+        container_of(ctxt, struct hvm_emulate_ctxt, ctxt);
+
     *val = 0;
+
+    if ( unlikely(hvmemul_ctxt->set_context) )
+    {
+        struct vcpu *curr = current;
+        unsigned int safe_bytes;
+
+        if ( !curr->arch.vm_event.emul_read_data )
+            return X86EMUL_UNHANDLEABLE;
+
+        safe_bytes = min_t(unsigned int, bytes,
+            curr->arch.vm_event.emul_read_data->size);
+
+        memcpy(val, curr->arch.vm_event.emul_read_data->data, safe_bytes);
+
+        return X86EMUL_OKAY;
+    }
+
     return hvmemul_do_pio_buffer(port, bytes, IOREQ_READ, val);
 }
 
@@ -1599,6 +1711,7 @@ static int _hvm_emulate_one(struct hvm_emulate_ctxt 
*hvmemul_ctxt,
 int hvm_emulate_one(
     struct hvm_emulate_ctxt *hvmemul_ctxt)
 {
+    hvmemul_ctxt->set_context = 0;
     return _hvm_emulate_one(hvmemul_ctxt, &hvm_emulate_ops);
 }
 
@@ -1608,7 +1721,14 @@ int hvm_emulate_one_no_write(
     return _hvm_emulate_one(hvmemul_ctxt, &hvm_emulate_ops_no_write);
 }
 
-void hvm_mem_access_emulate_one(bool_t nowrite, unsigned int trapnr,
+static int hvm_emulate_one_set_context(
+    struct hvm_emulate_ctxt *hvmemul_ctxt)
+{
+    hvmemul_ctxt->set_context = 1;
+    return _hvm_emulate_one(hvmemul_ctxt, &hvm_emulate_ops_no_write);
+}
+
+void hvm_mem_access_emulate_one(enum emul_kind kind, unsigned int trapnr,
     unsigned int errcode)
 {
     struct hvm_emulate_ctxt ctx = {{ 0 }};
@@ -1616,10 +1736,16 @@ void hvm_mem_access_emulate_one(bool_t nowrite, 
unsigned int trapnr,
 
     hvm_emulate_prepare(&ctx, guest_cpu_user_regs());
 
-    if ( nowrite )
+    switch ( kind ) {
+    case EMUL_KIND_NOWRITE:
         rc = hvm_emulate_one_no_write(&ctx);
-    else
+        break;
+    case EMUL_KIND_SET_CONTEXT:
+        rc = hvm_emulate_one_set_context(&ctx);
+        break;
+    default:
         rc = hvm_emulate_one(&ctx);
+    }
 
     switch ( rc )
     {
diff --git a/xen/arch/x86/hvm/event.c b/xen/arch/x86/hvm/event.c
index 53b9ca4..5341937 100644
--- a/xen/arch/x86/hvm/event.c
+++ b/xen/arch/x86/hvm/event.c
@@ -30,31 +30,31 @@ static void hvm_event_fill_regs(vm_event_request_t *req)
     const struct cpu_user_regs *regs = guest_cpu_user_regs();
     const struct vcpu *curr = current;
 
-    req->regs.x86.rax = regs->eax;
-    req->regs.x86.rcx = regs->ecx;
-    req->regs.x86.rdx = regs->edx;
-    req->regs.x86.rbx = regs->ebx;
-    req->regs.x86.rsp = regs->esp;
-    req->regs.x86.rbp = regs->ebp;
-    req->regs.x86.rsi = regs->esi;
-    req->regs.x86.rdi = regs->edi;
-
-    req->regs.x86.r8  = regs->r8;
-    req->regs.x86.r9  = regs->r9;
-    req->regs.x86.r10 = regs->r10;
-    req->regs.x86.r11 = regs->r11;
-    req->regs.x86.r12 = regs->r12;
-    req->regs.x86.r13 = regs->r13;
-    req->regs.x86.r14 = regs->r14;
-    req->regs.x86.r15 = regs->r15;
-
-    req->regs.x86.rflags = regs->eflags;
-    req->regs.x86.rip    = regs->eip;
-
-    req->regs.x86.msr_efer = curr->arch.hvm_vcpu.guest_efer;
-    req->regs.x86.cr0 = curr->arch.hvm_vcpu.guest_cr[0];
-    req->regs.x86.cr3 = curr->arch.hvm_vcpu.guest_cr[3];
-    req->regs.x86.cr4 = curr->arch.hvm_vcpu.guest_cr[4];
+    req->data.regs.x86.rax = regs->eax;
+    req->data.regs.x86.rcx = regs->ecx;
+    req->data.regs.x86.rdx = regs->edx;
+    req->data.regs.x86.rbx = regs->ebx;
+    req->data.regs.x86.rsp = regs->esp;
+    req->data.regs.x86.rbp = regs->ebp;
+    req->data.regs.x86.rsi = regs->esi;
+    req->data.regs.x86.rdi = regs->edi;
+
+    req->data.regs.x86.r8  = regs->r8;
+    req->data.regs.x86.r9  = regs->r9;
+    req->data.regs.x86.r10 = regs->r10;
+    req->data.regs.x86.r11 = regs->r11;
+    req->data.regs.x86.r12 = regs->r12;
+    req->data.regs.x86.r13 = regs->r13;
+    req->data.regs.x86.r14 = regs->r14;
+    req->data.regs.x86.r15 = regs->r15;
+
+    req->data.regs.x86.rflags = regs->eflags;
+    req->data.regs.x86.rip    = regs->eip;
+
+    req->data.regs.x86.msr_efer = curr->arch.hvm_vcpu.guest_efer;
+    req->data.regs.x86.cr0 = curr->arch.hvm_vcpu.guest_cr[0];
+    req->data.regs.x86.cr3 = curr->arch.hvm_vcpu.guest_cr[3];
+    req->data.regs.x86.cr4 = curr->arch.hvm_vcpu.guest_cr[4];
 }
 
 static int hvm_event_traps(uint8_t sync, vm_event_request_t *req)
diff --git a/xen/arch/x86/mm/p2m.c b/xen/arch/x86/mm/p2m.c
index 6b39733..5d40d2c 100644
--- a/xen/arch/x86/mm/p2m.c
+++ b/xen/arch/x86/mm/p2m.c
@@ -1369,49 +1369,49 @@ static void p2m_vm_event_fill_regs(vm_event_request_t 
*req)
     /* Architecture-specific vmcs/vmcb bits */
     hvm_funcs.save_cpu_ctxt(curr, &ctxt);
 
-    req->regs.x86.rax = regs->eax;
-    req->regs.x86.rcx = regs->ecx;
-    req->regs.x86.rdx = regs->edx;
-    req->regs.x86.rbx = regs->ebx;
-    req->regs.x86.rsp = regs->esp;
-    req->regs.x86.rbp = regs->ebp;
-    req->regs.x86.rsi = regs->esi;
-    req->regs.x86.rdi = regs->edi;
-
-    req->regs.x86.r8  = regs->r8;
-    req->regs.x86.r9  = regs->r9;
-    req->regs.x86.r10 = regs->r10;
-    req->regs.x86.r11 = regs->r11;
-    req->regs.x86.r12 = regs->r12;
-    req->regs.x86.r13 = regs->r13;
-    req->regs.x86.r14 = regs->r14;
-    req->regs.x86.r15 = regs->r15;
-
-    req->regs.x86.rflags = regs->eflags;
-    req->regs.x86.rip    = regs->eip;
-
-    req->regs.x86.dr7 = curr->arch.debugreg[7];
-    req->regs.x86.cr0 = ctxt.cr0;
-    req->regs.x86.cr2 = ctxt.cr2;
-    req->regs.x86.cr3 = ctxt.cr3;
-    req->regs.x86.cr4 = ctxt.cr4;
-
-    req->regs.x86.sysenter_cs = ctxt.sysenter_cs;
-    req->regs.x86.sysenter_esp = ctxt.sysenter_esp;
-    req->regs.x86.sysenter_eip = ctxt.sysenter_eip;
-
-    req->regs.x86.msr_efer = ctxt.msr_efer;
-    req->regs.x86.msr_star = ctxt.msr_star;
-    req->regs.x86.msr_lstar = ctxt.msr_lstar;
+    req->data.regs.x86.rax = regs->eax;
+    req->data.regs.x86.rcx = regs->ecx;
+    req->data.regs.x86.rdx = regs->edx;
+    req->data.regs.x86.rbx = regs->ebx;
+    req->data.regs.x86.rsp = regs->esp;
+    req->data.regs.x86.rbp = regs->ebp;
+    req->data.regs.x86.rsi = regs->esi;
+    req->data.regs.x86.rdi = regs->edi;
+
+    req->data.regs.x86.r8  = regs->r8;
+    req->data.regs.x86.r9  = regs->r9;
+    req->data.regs.x86.r10 = regs->r10;
+    req->data.regs.x86.r11 = regs->r11;
+    req->data.regs.x86.r12 = regs->r12;
+    req->data.regs.x86.r13 = regs->r13;
+    req->data.regs.x86.r14 = regs->r14;
+    req->data.regs.x86.r15 = regs->r15;
+
+    req->data.regs.x86.rflags = regs->eflags;
+    req->data.regs.x86.rip    = regs->eip;
+
+    req->data.regs.x86.dr7 = curr->arch.debugreg[7];
+    req->data.regs.x86.cr0 = ctxt.cr0;
+    req->data.regs.x86.cr2 = ctxt.cr2;
+    req->data.regs.x86.cr3 = ctxt.cr3;
+    req->data.regs.x86.cr4 = ctxt.cr4;
+
+    req->data.regs.x86.sysenter_cs = ctxt.sysenter_cs;
+    req->data.regs.x86.sysenter_esp = ctxt.sysenter_esp;
+    req->data.regs.x86.sysenter_eip = ctxt.sysenter_eip;
+
+    req->data.regs.x86.msr_efer = ctxt.msr_efer;
+    req->data.regs.x86.msr_star = ctxt.msr_star;
+    req->data.regs.x86.msr_lstar = ctxt.msr_lstar;
 
     hvm_get_segment_register(curr, x86_seg_fs, &seg);
-    req->regs.x86.fs_base = seg.base;
+    req->data.regs.x86.fs_base = seg.base;
 
     hvm_get_segment_register(curr, x86_seg_gs, &seg);
-    req->regs.x86.gs_base = seg.base;
+    req->data.regs.x86.gs_base = seg.base;
 
     hvm_get_segment_register(curr, x86_seg_cs, &seg);
-    req->regs.x86.cs_arbytes = seg.attr.bytes;
+    req->data.regs.x86.cs_arbytes = seg.attr.bytes;
 }
 
 void p2m_mem_access_emulate_check(struct vcpu *v,
@@ -1466,6 +1466,10 @@ void p2m_mem_access_emulate_check(struct vcpu *v,
         }
 
         v->arch.vm_event.emulate_flags = violation ? rsp->flags : 0;
+
+        if ( rsp->flags & MEM_ACCESS_SET_EMUL_READ_DATA &&
+             v->arch.vm_event.emul_read_data )
+            *v->arch.vm_event.emul_read_data = rsp->data.emul_read_data;
     }
 }
 
@@ -1552,9 +1556,15 @@ bool_t p2m_mem_access_check(paddr_t gpa, unsigned long 
gla,
 
     if ( v->arch.vm_event.emulate_flags )
     {
-        hvm_mem_access_emulate_one((v->arch.vm_event.emulate_flags &
-                                    MEM_ACCESS_EMULATE_NOWRITE) != 0,
-                                   TRAP_invalid_op, HVM_DELIVER_NO_ERROR_CODE);
+        enum emul_kind kind = EMUL_KIND_NORMAL;
+
+        if ( v->arch.vm_event.emulate_flags & MEM_ACCESS_SET_EMUL_READ_DATA )
+            kind = EMUL_KIND_SET_CONTEXT;
+        else if ( v->arch.vm_event.emulate_flags & MEM_ACCESS_EMULATE_NOWRITE )
+            kind = EMUL_KIND_NOWRITE;
+
+        hvm_mem_access_emulate_one(kind, TRAP_invalid_op,
+                                   HVM_DELIVER_NO_ERROR_CODE);
 
         v->arch.vm_event.emulate_flags = 0;
         return 1;
diff --git a/xen/common/vm_event.c b/xen/common/vm_event.c
index 120a78a..11438da 100644
--- a/xen/common/vm_event.c
+++ b/xen/common/vm_event.c
@@ -48,6 +48,7 @@ static int vm_event_enable(
 {
     int rc;
     unsigned long ring_gfn = d->arch.hvm_domain.params[param];
+    struct vcpu *v;
 
     /* Only one helper at a time. If the helper crashed,
      * the ring is in an undefined state and so is the guest.
@@ -63,6 +64,21 @@ static int vm_event_enable(
     vm_event_ring_lock_init(ved);
     vm_event_ring_lock(ved);
 
+    for_each_vcpu( d, v )
+    {
+        if ( v->arch.vm_event.emul_read_data )
+            continue;
+
+        v->arch.vm_event.emul_read_data =
+            xmalloc(struct vm_event_emul_read_data);
+
+        if ( !v->arch.vm_event.emul_read_data )
+        {
+            rc = -ENOMEM;
+            goto err;
+        }
+    }
+
     rc = prepare_ring_for_helper(d, ring_gfn, &ved->ring_pg_struct,
                                     &ved->ring_page);
     if ( rc < 0 )
@@ -225,6 +241,13 @@ static int vm_event_disable(struct domain *d, struct 
vm_event_domain *ved)
 
         destroy_ring_for_helper(&ved->ring_page,
                                 ved->ring_pg_struct);
+
+        for_each_vcpu( d, v )
+        {
+            xfree(v->arch.vm_event.emul_read_data);
+            v->arch.vm_event.emul_read_data = NULL;
+        }
+
         vm_event_ring_unlock(ved);
     }
 
diff --git a/xen/include/asm-x86/domain.h b/xen/include/asm-x86/domain.h
index 96bde65..7908844 100644
--- a/xen/include/asm-x86/domain.h
+++ b/xen/include/asm-x86/domain.h
@@ -10,6 +10,7 @@
 #include <asm/mce.h>
 #include <public/vcpu.h>
 #include <public/hvm/hvm_info_table.h>
+#include <public/vm_event.h>
 
 #define has_32bit_shinfo(d)    ((d)->arch.has_32bit_shinfo)
 #define is_pv_32bit_domain(d)  ((d)->arch.is_32bit_pv)
@@ -508,6 +509,7 @@ struct arch_vcpu
         uint32_t emulate_flags;
         unsigned long gpa;
         unsigned long eip;
+        struct vm_event_emul_read_data *emul_read_data;
     } vm_event;
 
 };
diff --git a/xen/include/asm-x86/hvm/emulate.h 
b/xen/include/asm-x86/hvm/emulate.h
index 594be38..49134b5 100644
--- a/xen/include/asm-x86/hvm/emulate.h
+++ b/xen/include/asm-x86/hvm/emulate.h
@@ -32,13 +32,21 @@ struct hvm_emulate_ctxt {
     struct hvm_trap trap;
 
     uint32_t intr_shadow;
+
+    bool_t set_context;
+};
+
+enum emul_kind {
+    EMUL_KIND_NORMAL,
+    EMUL_KIND_NOWRITE,
+    EMUL_KIND_SET_CONTEXT
 };
 
 int hvm_emulate_one(
     struct hvm_emulate_ctxt *hvmemul_ctxt);
 int hvm_emulate_one_no_write(
     struct hvm_emulate_ctxt *hvmemul_ctxt);
-void hvm_mem_access_emulate_one(bool_t nowrite,
+void hvm_mem_access_emulate_one(enum emul_kind kind,
     unsigned int trapnr,
     unsigned int errcode);
 void hvm_emulate_prepare(
diff --git a/xen/include/public/vm_event.h b/xen/include/public/vm_event.h
index 577e971..3223bb4 100644
--- a/xen/include/public/vm_event.h
+++ b/xen/include/public/vm_event.h
@@ -149,6 +149,13 @@ struct vm_event_regs_x86 {
  * potentially having side effects (like memory mapped or port I/O) disabled.
  */
 #define MEM_ACCESS_EMULATE_NOWRITE      (1 << 7)
+/*
+ * Data is being sent back to the hypervisor in the event response, to be
+ * returned by the read function when emulating an instruction.
+ * This flag is only useful when combined with MEM_ACCESS_EMULATE or
+ * MEM_ACCESS_EMULATE_NOWRITE.
+ */
+#define MEM_ACCESS_SET_EMUL_READ_DATA   (1 << 8)
 
 struct vm_event_mem_access {
     uint64_t gfn;
@@ -189,6 +196,12 @@ struct vm_event_sharing {
     uint32_t _pad;
 };
 
+struct vm_event_emul_read_data {
+    uint32_t size;
+    /* The struct is used in a union with vm_event_regs_x86. */
+    uint8_t  data[sizeof(struct vm_event_regs_x86) - sizeof(uint32_t)];
+};
+
 typedef struct vm_event_st {
     uint32_t version;   /* VM_EVENT_INTERFACE_VERSION */
     uint32_t flags;     /* VM_EVENT_FLAG_* */
@@ -206,8 +219,12 @@ typedef struct vm_event_st {
     } u;
 
     union {
-        struct vm_event_regs_x86 x86;
-    } regs;
+        union {
+            struct vm_event_regs_x86 x86;
+        } regs;
+
+        struct vm_event_emul_read_data emul_read_data;
+    } data;
 } vm_event_request_t, vm_event_response_t;
 
 DEFINE_RING_TYPES(vm_event, vm_event_request_t, vm_event_response_t);
-- 
1.7.9.5


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