Re: [PATCH 1/2] x86/xen: Set EFER.NX and EFER.SCE in PVH guests

[David Vrabel <david.vrabel@citrix.com>]

On 05/09/14 15:34, Jan Beulich wrote:
>>>> On 05.09.14 at 16:11, <david.vrabel@citrix.com> wrote:
>> --- a/arch/x86/xen/xen-head.S
>> +++ b/arch/x86/xen/xen-head.S
>> @@ -47,6 +47,36 @@ ENTRY(startup_xen)
>>  
>>  	__FINIT
>>  
>> +#ifdef CONFIG_XEN_PVH
>> +
>> +/**
>> + * xen_pvh_cpu_early_init() - early PVH VCPU initialization
>> + * @cpu: this cpu number (%rdi)
>> + *
>> + * Note: This is called as a function on the boot CPU and as the secondary
>> + * CPU entry point.
>> + */
>> +ENTRY(xen_pvh_cpu_early_init)
>> +	/* Gather features to see if NX implemented. */
>> +	mov     $0x80000001, %eax
>> +	cpuid
>> +	mov     %edx,%esi
>> +
>> +	mov     $MSR_EFER, %ecx
>> +	rdmsr
>> +	bts     $_EFER_SCE, %eax
>> +
>> +	bt      $20,%esi
>> +	jnc     1f      /* No NX, skip setting it */
>> +	bts     $_EFER_NX, %eax
>> +1:	wrmsr
>> +
>> +	cmp     $0,%rdi /* non-zero => secondary cpu */
> 
> Isn't Linux is specifically moving away from this assumption?

I don't think this is relevant.  Is Xen ever going boot with a non-zero
VCPU?

>> +	jne     cpu_bringup_and_idle
>> +	ret
>> +
>> +#endif /* CONFIG_XEN_PVH */
> 
> Also, does all this really need to be done in assembly?

There's no usable stack until EFER.NX is set.  I couldn't think of a way
to write this in C that would guarantee no stack use, so I didn't
suggest it.

Can you think of a way?

David