x86: refine FPU selector handling code for XSAVEOPT

Some extra tweaks are necessary to deal with the situation of XSAVEOPT
not writing the FPU portion of the save image (due to it detecting that
the register state did not get modified since the last XRSTOR).

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
Tested-by: Ben Guthro <ben.guthro@xxxxxxxxx>

--- a/xen/arch/x86/xstate.c
+++ b/xen/arch/x86/xstate.c
@@ -71,10 +71,28 @@ void xsave(struct vcpu *v, uint64_t mask
 
     if ( word_size <= 0 || !is_pv_32bit_vcpu(v) )
     {
+        typeof(ptr->fpu_sse.fip.sel) fcs = ptr->fpu_sse.fip.sel;
+        typeof(ptr->fpu_sse.fdp.sel) fds = ptr->fpu_sse.fdp.sel;
+
         if ( cpu_has_xsaveopt )
+        {
+            /*
+             * xsaveopt may not write the FPU portion even when the respective
+             * mask bit is set. For the check further down to work we hence
+             * need to put the save image back into the state that it was in
+             * right after the previous xsaveopt.
+             */
+            if ( word_size > 0 &&
+                 (ptr->fpu_sse.x[FPU_WORD_SIZE_OFFSET] == 4 ||
+                  ptr->fpu_sse.x[FPU_WORD_SIZE_OFFSET] == 2) )
+            {
+                ptr->fpu_sse.fip.sel = 0;
+                ptr->fpu_sse.fdp.sel = 0;
+            }
             asm volatile ( ".byte 0x48,0x0f,0xae,0x37"
                            : "=m" (*ptr)
                            : "a" (lmask), "d" (hmask), "D" (ptr) );
+        }
         else
             asm volatile ( ".byte 0x48,0x0f,0xae,0x27"
                            : "=m" (*ptr)
@@ -87,7 +105,14 @@ void xsave(struct vcpu *v, uint64_t mask
               */
              (!(ptr->fpu_sse.fsw & 0x0080) &&
               boot_cpu_data.x86_vendor == X86_VENDOR_AMD) )
+        {
+            if ( cpu_has_xsaveopt && word_size > 0 )
+            {
+                ptr->fpu_sse.fip.sel = fcs;
+                ptr->fpu_sse.fdp.sel = fds;
+            }
             return;
+        }
 
         if ( word_size > 0 &&
              !((ptr->fpu_sse.fip.addr | ptr->fpu_sse.fdp.addr) >> 32) )