Re: [Xen-devel] [PATCH 07/10] x86/spec_ctrl: Explicitly set Xen's default MSR_SPEC_CTRL value

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 14/05/18 16:52, Jan Beulich wrote:
>>>> On 14.05.18 at 17:39, <wei.liu2@xxxxxxxxxx> wrote:
>> On Fri, May 11, 2018 at 11:38:11AM +0100, Andrew Cooper wrote:
>>> @@ -417,6 +419,32 @@ void __init init_speculation_mitigations(void)
>>>          setup_clear_cpu_cap(X86_FEATURE_NO_XPTI);
>>>  
>>>      print_details(thunk, caps);
>>> +
>>> +    /*
>>> +     * If MSR_SPEC_CTRL is available, apply Xen's default setting and 
>>> discard
>>> +     * any firmware settings.  For performance reasons on native hardware, 
>>> we
>>> +     * delay applying non-zero settings until after dom0 has been 
>>> constructed.
>>> +     */
>>> +    if ( boot_cpu_has(X86_FEATURE_IBRSB) )
>>> +    {
>>> +        bsp_delay_spec_ctrl = !cpu_has_hypervisor && default_xen_spec_ctrl;
>>> +
>> Why is cpu_has_hypervisor needed here?  This should help nested case as
>> well. And it wouldn't make the setup less secure, right?
> Ah, yes, Andrew, this should indeed be explained in at least one of comment
> or commit message.

I've adjusted this comment to read:

/*

 * If MSR_SPEC_CTRL is available, apply Xen's default setting and discard

 * any firmware settings.  For performance reasons, when safe to do so, we

 * delay applying non-zero settings until after dom0 has been constructed.

 *

 * "when safe to do so" is based on whether we are virtualised.  A native

 * boot won't have any other code running in a position to mount an

 * attack.

 */


and added the same second paragraph to the commit message.

~Andrew

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