x86/HVM: refuse doing string operations in certain situations

We shouldn't do any acceleration for
- "rep movs" when either side is passed through MMIO or when both sides
  are handled by qemu
- "rep ins" and "rep outs" when the memory operand is any kind of MMIO

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/arch/x86/hvm/emulate.c
+++ b/xen/arch/x86/hvm/emulate.c
@@ -686,6 +686,7 @@ static int hvmemul_rep_ins(
     unsigned long addr;
     uint32_t pfec = PFEC_page_present | PFEC_write_access;
     paddr_t gpa;
+    p2m_type_t p2mt;
     int rc;
 
     rc = hvmemul_virtual_to_linear(
@@ -702,6 +703,10 @@ static int hvmemul_rep_ins(
     if ( rc != X86EMUL_OKAY )
         return rc;
 
+    (void) get_gfn_query_unlocked(current->domain, gpa >> PAGE_SHIFT, &p2mt);
+    if ( p2mt == p2m_mmio_direct || p2mt == p2m_mmio_dm )
+        return X86EMUL_UNHANDLEABLE;
+
     return hvmemul_do_pio(src_port, reps, bytes_per_rep, gpa, IOREQ_READ,
                           !!(ctxt->regs->eflags & X86_EFLAGS_DF), NULL);
 }
@@ -719,6 +724,7 @@ static int hvmemul_rep_outs(
     unsigned long addr;
     uint32_t pfec = PFEC_page_present;
     paddr_t gpa;
+    p2m_type_t p2mt;
     int rc;
 
     rc = hvmemul_virtual_to_linear(
@@ -735,6 +741,10 @@ static int hvmemul_rep_outs(
     if ( rc != X86EMUL_OKAY )
         return rc;
 
+    (void) get_gfn_query_unlocked(current->domain, gpa >> PAGE_SHIFT, &p2mt);
+    if ( p2mt == p2m_mmio_direct || p2mt == p2m_mmio_dm )
+        return X86EMUL_UNHANDLEABLE;
+
     return hvmemul_do_pio(dst_port, reps, bytes_per_rep, gpa, IOREQ_WRITE,
                           !!(ctxt->regs->eflags & X86_EFLAGS_DF), NULL);
 }
@@ -787,6 +797,10 @@ static int hvmemul_rep_movs(
     (void) get_gfn_query_unlocked(current->domain, sgpa >> PAGE_SHIFT, &sp2mt);
     (void) get_gfn_query_unlocked(current->domain, dgpa >> PAGE_SHIFT, &dp2mt);
 
+    if ( sp2mt == p2m_mmio_direct || dp2mt == p2m_mmio_direct ||
+         (sp2mt == p2m_mmio_dm && dp2mt == p2m_mmio_dm) )
+        return X86EMUL_UNHANDLEABLE;
+
     if ( sp2mt == p2m_mmio_dm )
         return hvmemul_do_mmio(
             sgpa, reps, bytes_per_rep, dgpa, IOREQ_READ, df, NULL);