Re-enabling xsave

[coypu%sdf.org@localhost]

Hi,

Due to XSA-52 which was a vulnerability in Xen's implementation of fpu
switching on AMD CPU, they made it possible to disable support for
XSAVE. This resulted in SIGILL for anyone using XSAVE.

This was Xen ~4.1. They have since fixed this issue.

as a stopgap measure, netbsd/xen avoids using XSAVE since, which is a
shame.

OK for reverting this stopgap measure?


XSA-52: https://xenbits.xenproject.org/xsa/advisory-52.html

 Index: identcpu.c
 ===================================================================
 RCS file: /cvsroot/src/sys/arch/x86/x86/identcpu.c,v
 retrieving revision 1.64
 diff -u -r1.64 identcpu.c
 --- identcpu.c	3 Nov 2017 16:21:01 -0000	1.64
 +++ identcpu.c	5 Nov 2017 17:11:12 -0000
 @@ -771,12 +771,7 @@
  	if (descs[2] > 512)
  		x86_fpu_save_size = descs[2];
  
 -#ifdef XEN
 -	/* Don't use xsave, force fxsave with x86_xsave_features = 0. */
 -	x86_fpu_save = FPU_SAVE_FXSAVE;
 -#else
  	x86_xsave_features = (uint64_t)descs[3] << 32 | descs[0];
 -#endif
  }
  
  void