[Xen-devel] [PATCH v4 3/4] xen/hvm: introduce a flags field in the CPU save record

[Roger Pau Monne <roger.pau@xxxxxxxxxx>]

Introduce a new flags field and use bit 0 to signal if the FPU has been
initialised or not. Previously Xen always wrongly assumed the FPU was
initialised on restore.

Signed-off-by: Roger Pau Monnà <roger.pau@xxxxxxxxxx>
Cc: Jan Beulich <jbeulich@xxxxxxxx>
Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
Changes since v3:
 - Don't add a comment in the compat structure regaring the fpu_initialised
   field.
 - Rename fpu_initialised to flags and use it as a bit field. Bit 0 will be
   used to signal whether the fpu is initialised.
 - Only save the fpu context if it's initialised.
 - Only restore the fpu context from the save record if the fpu is
   initialised.
 - Check that unused bits in the flags field are 0.

Changes since v1:
 - Don't add yet another compat structure, new fields should always be added
   to the end of the existing structure and offsetof should be used to
   compare sizes.
 - Leave the previous compat structure as-is, since the field was not added
   to the end we cannot remove it and use offsetof in this case.
 - Set xstate_bv based on fpu_initialised value instead of unconditionally
   setting it to XSTATE_FP_SSE.
---
 xen/arch/x86/hvm/hvm.c                 | 20 +++++++++++++-------
 xen/include/public/arch-x86/hvm/save.h | 27 ++++++++++++++++++++-------
 2 files changed, 33 insertions(+), 14 deletions(-)

diff --git a/xen/arch/x86/hvm/hvm.c b/xen/arch/x86/hvm/hvm.c
index 141a130..d966074 100644
--- a/xen/arch/x86/hvm/hvm.c
+++ b/xen/arch/x86/hvm/hvm.c
@@ -1798,8 +1798,7 @@ static int hvm_save_cpu_ctxt(struct domain *d, 
hvm_domain_context_t *h)
 
         if ( v->fpu_initialised )
             memcpy(ctxt.fpu_regs, v->arch.fpu_ctxt, sizeof(ctxt.fpu_regs));
-        else 
-            memset(ctxt.fpu_regs, 0, sizeof(ctxt.fpu_regs));
+        ctxt.flags = v->fpu_initialised ? XEN_X86_FPU_INITIALISED : 0;
 
         ctxt.rax = v->arch.user_regs.eax;
         ctxt.rbx = v->arch.user_regs.ebx;
@@ -1979,7 +1978,7 @@ static int hvm_load_cpu_ctxt(struct domain *d, 
hvm_domain_context_t *h)
         return -EINVAL;
     }
 
-    if ( hvm_load_entry(CPU, h, &ctxt) != 0 ) 
+    if ( hvm_load_entry_zeroextend(CPU, h, &ctxt) != 0 )
         return -EINVAL;
 
     /* Sanity check some control registers. */
@@ -2007,6 +2006,13 @@ static int hvm_load_cpu_ctxt(struct domain *d, 
hvm_domain_context_t *h)
         return -EINVAL;
     }
 
+    if ( (ctxt.flags & ~XEN_X86_FPU_INITIALISED) != 0 )
+    {
+        gprintk(XENLOG_ERR, "bad flags value in CPU context: %#x\n",
+                ctxt.flags);
+        return -EINVAL;
+    }
+
     /* Older Xen versions used to save the segment arbytes directly 
      * from the VMCS on Intel hosts.  Detect this and rearrange them
      * into the struct segment_register format. */
@@ -2085,16 +2091,17 @@ static int hvm_load_cpu_ctxt(struct domain *d, 
hvm_domain_context_t *h)
     seg.attr.bytes = ctxt.ldtr_arbytes;
     hvm_set_segment_register(v, x86_seg_ldtr, &seg);
 
+    v->fpu_initialised = !!(ctxt.flags & XEN_X86_FPU_INITIALISED);
     /* In case xsave-absent save file is restored on a xsave-capable host */
-    if ( cpu_has_xsave && !xsave_enabled(v) )
+    if ( cpu_has_xsave && !xsave_enabled(v) && v->fpu_initialised )
     {
         struct xsave_struct *xsave_area = v->arch.xsave_area;
 
         memcpy(v->arch.xsave_area, ctxt.fpu_regs, sizeof(ctxt.fpu_regs));
         xsave_area->xsave_hdr.xstate_bv = XSTATE_FP_SSE;
     }
-    else
-        memcpy(v->arch.fpu_ctxt, ctxt.fpu_regs, sizeof(ctxt.fpu_regs));
+    else if ( v->fpu_initialised )
+            memcpy(v->arch.fpu_ctxt, ctxt.fpu_regs, sizeof(ctxt.fpu_regs));
 
     v->arch.user_regs.eax = ctxt.rax;
     v->arch.user_regs.ebx = ctxt.rbx;
@@ -2122,7 +2129,6 @@ static int hvm_load_cpu_ctxt(struct domain *d, 
hvm_domain_context_t *h)
     v->arch.debugreg[7] = ctxt.dr7;
 
     v->arch.vgc_flags = VGCF_online;
-    v->fpu_initialised = 1;
 
     /* Auxiliary processors should be woken immediately. */
     v->is_initialised = 1;
diff --git a/xen/include/public/arch-x86/hvm/save.h 
b/xen/include/public/arch-x86/hvm/save.h
index 29d513c..b6b1bf8 100644
--- a/xen/include/public/arch-x86/hvm/save.h
+++ b/xen/include/public/arch-x86/hvm/save.h
@@ -47,7 +47,9 @@ DECLARE_HVM_SAVE_TYPE(HEADER, 1, struct hvm_save_header);
 /*
  * Processor
  *
- * Compat: Pre-3.4 didn't have msr_tsc_aux
+ * Compat:
+ *     - Pre-3.4 didn't have msr_tsc_aux
+ *     - Pre-4.7 didn't have fpu_initialised
  */
 
 struct hvm_hw_cpu {
@@ -157,6 +159,10 @@ struct hvm_hw_cpu {
     };
     /* error code for pending event */
     uint32_t error_code;
+
+#define _XEN_X86_FPU_INITIALISED        0
+#define XEN_X86_FPU_INITIALISED         (1U<<_XEN_X86_FPU_INITIALISED)
+    uint32_t flags;
 };
 
 struct hvm_hw_cpu_compat {
@@ -275,12 +281,19 @@ static inline int _hvm_hw_fix_cpu(void *h, uint32_t size) 
{
         struct hvm_hw_cpu_compat cmp;
     } *ucpu = (union hvm_hw_cpu_union *)h;
 
-    /* If we copy from the end backwards, we should
-     * be able to do the modification in-place */
-    ucpu->nat.error_code = ucpu->cmp.error_code;
-    ucpu->nat.pending_event = ucpu->cmp.pending_event;
-    ucpu->nat.tsc = ucpu->cmp.tsc;
-    ucpu->nat.msr_tsc_aux = 0;
+    if ( size == sizeof(struct hvm_hw_cpu_compat) )
+    {
+        /*
+         * If we copy from the end backwards, we should
+         * be able to do the modification in-place.
+         */
+        ucpu->nat.error_code = ucpu->cmp.error_code;
+        ucpu->nat.pending_event = ucpu->cmp.pending_event;
+        ucpu->nat.tsc = ucpu->cmp.tsc;
+        ucpu->nat.msr_tsc_aux = 0;
+    }
+    /* Mimic the old behaviour by unconditionally setting fpu_initialised. */
+    ucpu->nat.flags = XEN_X86_FPU_INITIALISED;
 
     return 0;
 }
-- 
1.9.5 (Apple Git-50.3)


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