Re: [Xen-devel] [PATCH for-4.11] x86: Use spec_ctrl_{enter, exit}_idle() in the S3/S5 path

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 18/04/18 17:14, Jan Beulich wrote:
>>>> On 18.04.18 at 18:02, <andrew.cooper3@xxxxxxxxxx> wrote:
>> On 18/04/18 16:43, Jan Beulich wrote:
>>>>>> On 18.04.18 at 12:43, <andrew.cooper3@xxxxxxxxxx> wrote:
>>>> This avoids opencoding the functionality (and missing one bit of it), and 
>>>> and
>>>> some comments explaining what is going on.
>>> Missing which bit of it? The MSR writes aren't strictly necessary afaict, 
>>> and
>>> functionally clearing bti_ist_info is all that's needed for the entry path 
>>> of
>>> interest, while clearing use_shadow_spec_ctrl is all that's needed for the
>>> exit-to-Xen path. Hence I'm don't see (yet) what bug it is you think this
>>> fixes.
>> It is expected that we don't hand off control to the firmware in cases
>> like this with the mitigations in place.  Here, it doesn't actually
>> matter too much, but there is no point having the mitigations in effect
>> when we're the final CPU and trying to shut down.
> With an improved description the patch can certainly have my R-b; the
> code change itself is clearly fine with me.

How about:

The main purpose of this patch is to avoid opencoding the recovery logic
at the end, but also has the positive side effect of relaxing the
SPEC_CTRL mitigations when working to shut the final CPU down.

~Andrew

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