Re: [Xen-devel] Initialising MXCSR in PV guests

[Anil Madhavapeddy <anil@xxxxxxxxxx>]

Balraj,

No reply to my query on the Xen list, but I committed the change to
explicitly initialise MXCSR to the latest mirage-platform. Can you confirm
that your errant i7 now successfully boots the kernel that failed before
with this change?

I'm still not entirely sure why the register happens to not be to set on
that one machine though...

cheers
Anil

On Fri, Nov 30, 2012 at 05:14:01PM +0000, Anil Madhavapeddy wrote:
> We're seeing floating point exceptions on some (but not all) machines
> when doing FP operations inside MiniOS.
> 
> It looks like the FPU and SSE control registers are not set to good
> default values (which by default mask various FP exceptions) when MiniOS
> is started as a PV guest. If the call to fpu_init below is not made, the
> division generates a precision error and fails.  
> 
> Do PV guests all need to explicitly initialise MXCSR, or has something
> changed in Xen to trigger this now?  It seems to have been set in the
> past, and only happens on some hosts.
> 
> --- extras/mini-os/kernelorig.c    2012-11-30 15:59:46.308837388 +0000
> +++ extras/mini-os/kernel.c    2012-11-30 16:02:12.532408020 +0000
> @@ -64,10 +64,32 @@
>      }
>  }
>  
> +#define MXCSR_DEFAULT 0x1f80
> +
> +static void fpu_init(void)
> +{
> +  unsigned long val = MXCSR_DEFAULT;
> +  asm volatile ( "fninit; ldmxcsr %0" : : "m" (val) );
> +}
> +
>  /* This should be overridden by the application we are linked against. */
>  __attribute__((weak)) int app_main(start_info_t *si)
>  {
>      printk("Dummy main: start_info=%p\n", si);
> +
> +    double x = 10.0;
> +    double y = 3.0;
> +
> +    /* This division only works when the fpu_init call is made.
> +     * The call sets the fpu and sse control registers to default values,
> +     * which mask various exceptions including the precision exception.
> +     * When the fpu_init call is not made the division below fails with
> +     * a precision exception.
> +     */
> +    double z = x/y;
> +
> +    printk("x = %d, y = %d, z = %d\n", (int) x, (int) y, (int) z);
> +
>      return 0;
>  }
>  
> @@ -98,6 +120,9 @@
>      printk("    cmd_line: %s\n",  
>             si->cmd_line ? (const char *)si->cmd_line : "NULL");
>  
> +    /* Initialize fpu and sse */
> +    fpu_init();
> +
>      /* Set up events. */
>      init_events();
>      
> 
> 
> 
> The offending host is:
> 
> $ uname -a
> Linux Monday 3.6.2 #1 SMP PREEMPT Wed Oct 17 22:22:30 BST 2012 x86_64 
> GNU/Linux
> 
> $ cat /proc/cpuinfo 
> processor     : 0
> vendor_id     : GenuineIntel
> cpu family    : 6
> model  : 58
> model name    : Intel(R) Core(TM) i7-3770 CPU @ 3.40GHz
> stepping      : 9
> microcode     : 0x12
> cpu MHz        : 3392.428
> cache size    : 8192 KB
> physical id   : 0
> siblings      : 8
> core id        : 0
> cpu cores     : 4
> apicid         : 0
> initial apicid        : 0
> fpu    : yes
> fpu_exception : yes
> cpuid level   : 13
> wp     : yes
> flags  : fpu de tsc msr pae mce cx8 apic sep mca cmov pat clflush acpi mmx 
> fxsr sse sse2 ss ht syscall nx lm constant_tsc rep_good nopl nonstop_tsc pni 
> pclmulqdq est ssse3 cx16 sse4_1 sse4_2 x2apic popcnt tsc_deadline_timer aes 
> xsave avx f16c rdrand hypervisor lahf_lm ida arat epb xsaveopt pln pts dtherm 
> fsgsbase erms
> bogomips      : 6784.85
> clflush size  : 64
> cache_alignment       : 64
> address sizes : 36 bits physical, 48 bits virtual
> power management:
> 
> 
> 
> 
> 
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-- 
Anil Madhavapeddy                                 http://anil.recoil.org