[PATCH] x86emul: fix FXRSTOR test for most AMD CPUs

[Jan Beulich <jbeulich@xxxxxxxx>]

AMD CPUs that we classify as X86_BUG_FPU_PTRS don't touch the selector/
offset portion of the save image during FXSAVE unless an unmasked
exception is pending. Hence the selector zapping done between the
initial FXSAVE and the emulated FXRSTOR needs to be mirrored onto the
second FXSAVE, output of which gets fed into memcmp() to compare with
the input image.

Reported-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/tools/tests/x86_emulator/test_x86_emulator.c
+++ b/tools/tests/x86_emulator/test_x86_emulator.c
@@ -2577,6 +2577,7 @@ int main(int argc, char **argv)
         regs.ecx = (unsigned long)(res + 0x81);
         rc = x86_emulate(&ctxt, &emulops);
         asm volatile ( "fxsave %0" : "=m" (res[0x100]) :: "memory" );
+        zap_xfpsel(&res[0x100]);
         if ( (rc != X86EMUL_OKAY) ||
              memcmp(res + 0x100, res + 0x80, 0x200) ||
              (regs.eip != (unsigned long)&instr[4]) )