[Xen-devel] [PATCH RFC 2/2] x86/HVM: split page straddling emulated accesses in more cases

["Jan Beulich" <JBeulich@xxxxxxxx>]

Assuming consecutive linear addresses map to all RAM or all MMIO is not
correct. Nor is assuming that a page straddling MMIO access will access
the same emulating component for both parts of the access. If a guest
RAM read fails with HVMTRANS_bad_gfn_to_mfn and if the access straddles
a page boundary, issue accesses separately for both parts.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
---
RFC: This clearly wants mirroring to the write path, and perhaps also
     to the fallback code on the RMW path. But I'd like to get a sense
     first on how welcome the general approach is.

--- a/xen/arch/x86/hvm/emulate.c
+++ b/xen/arch/x86/hvm/emulate.c
@@ -1041,6 +1041,48 @@ static inline int hvmemul_linear_mmio_wr
                                       pfec, hvmemul_ctxt, translate);
 }
 
+static int linear_read(unsigned long addr, unsigned int bytes, void *p_data,
+                       uint32_t pfec, struct hvm_emulate_ctxt *hvmemul_ctxt)
+{
+    pagefault_info_t pfinfo;
+    int rc = hvm_copy_from_guest_linear(p_data, addr, bytes, pfec, &pfinfo);
+
+    switch ( rc )
+    {
+        unsigned int offset, part1;
+
+    case HVMTRANS_okay:
+        return X86EMUL_OKAY;
+
+    case HVMTRANS_bad_linear_to_gfn:
+        x86_emul_pagefault(pfinfo.ec, pfinfo.linear, &hvmemul_ctxt->ctxt);
+        return X86EMUL_EXCEPTION;
+
+    case HVMTRANS_bad_gfn_to_mfn:
+        if ( pfec & PFEC_insn_fetch )
+            return X86EMUL_UNHANDLEABLE;
+
+        offset = addr & ~PAGE_MASK;
+        if ( offset + bytes <= PAGE_SIZE )
+            return hvmemul_linear_mmio_read(addr, bytes, p_data, pfec,
+                                            hvmemul_ctxt, 0);
+
+        /* Split the access at the page boundary. */
+        part1 = PAGE_SIZE - offset;
+        rc = linear_read(addr, part1, p_data, pfec, hvmemul_ctxt);
+        if ( rc == X86EMUL_OKAY )
+            rc = linear_read(addr + part1, bytes - part1, p_data + part1,
+                             pfec, hvmemul_ctxt);
+        return rc;
+
+    case HVMTRANS_gfn_paged_out:
+    case HVMTRANS_gfn_shared:
+        return X86EMUL_RETRY;
+    }
+
+    return X86EMUL_UNHANDLEABLE;
+}
+
 static int __hvmemul_read(
     enum x86_segment seg,
     unsigned long offset,
@@ -1049,11 +1091,9 @@ static int __hvmemul_read(
     enum hvm_access_type access_type,
     struct hvm_emulate_ctxt *hvmemul_ctxt)
 {
-    struct vcpu *curr = current;
-    pagefault_info_t pfinfo;
     unsigned long addr, reps = 1;
     uint32_t pfec = PFEC_page_present;
-    struct hvm_vcpu_io *vio = &curr->arch.hvm_vcpu.hvm_io;
+    struct hvm_vcpu_io *vio = &current->arch.hvm_vcpu.hvm_io;
     int rc;
 
     if ( is_x86_system_segment(seg) )
@@ -1073,28 +1113,7 @@ static int __hvmemul_read(
          (vio->mmio_gla == (addr & PAGE_MASK)) )
         return hvmemul_linear_mmio_read(addr, bytes, p_data, pfec, 
hvmemul_ctxt, 1);
 
-    rc = hvm_copy_from_guest_linear(p_data, addr, bytes, pfec, &pfinfo);
-
-    switch ( rc )
-    {
-    case HVMTRANS_okay:
-        break;
-    case HVMTRANS_bad_linear_to_gfn:
-        x86_emul_pagefault(pfinfo.ec, pfinfo.linear, &hvmemul_ctxt->ctxt);
-        return X86EMUL_EXCEPTION;
-    case HVMTRANS_bad_gfn_to_mfn:
-        if ( access_type == hvm_access_insn_fetch )
-            return X86EMUL_UNHANDLEABLE;
-
-        return hvmemul_linear_mmio_read(addr, bytes, p_data, pfec, 
hvmemul_ctxt, 0);
-    case HVMTRANS_gfn_paged_out:
-    case HVMTRANS_gfn_shared:
-        return X86EMUL_RETRY;
-    default:
-        return X86EMUL_UNHANDLEABLE;
-    }
-
-    return X86EMUL_OKAY;
+    return linear_read(addr, bytes, p_data, pfec, hvmemul_ctxt);
 }
 
 static int hvmemul_read(





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