Re: [PATCH v10 08/38] x86/cpufeatures: Add the cpu feature bit for FRED

Juergen Gross
2023-09-15T05:27:34Z

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On 15.09.23 03:07, Thomas Gleixner wrote:
> On Thu, Sep 14 2023 at 14:15, andrew wrote:
>> PV guests are never going to see FRED (or LKGS for that matter) because
>> it advertises too much stuff which simply traps because the kernel is in
>> CPL3.
>>
>> That said, the 64bit PV ABI is a whole lot closer to FRED than it is to
>> IDT delivery.  (Almost as if we decided 15 years ago that giving the PV
>> guest kernel a good stack and GSbase was the right thing to do...)
>
> No argument about that.
>
>> In some copious free time, I think we ought to provide a
>> minorly-paravirt FRED to PV guests because there are still some
>> improvements available as low hanging fruit.
>>
>> My plan was to have a PV hypervisor leaf advertising paravirt versions
>> of hardware features, so a guest could see "I don't have architectural
>> FRED, but I do have paravirt-FRED which is as similar as we can
>> reasonably make it".  The same goes for a whole bunch of other features.
>
> *GROAN*
>
> I told you before that we want less paravirt nonsense and not more.

I agree.

We will still have to support the PV stuff for non-FRED hypervisors even with
pv-FRED being available on new Xen. So adding pv-FRED would just add more PV
interfaces without the ability to remove old stuff.

Juergen

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