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

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Date:   Fri, 15 Sep 2023 07:27:27 +0200
From:   Juergen Gross <jgross@...e.com>
To:     Thomas Gleixner <tglx@...utronix.de>, andrew.cooper3@...rix.com,
        Jan Beulich <jbeulich@...e.com>
Cc:     mingo@...hat.com, bp@...en8.de, dave.hansen@...ux.intel.com,
        x86@...nel.org, hpa@...or.com, luto@...nel.org,
        pbonzini@...hat.com, seanjc@...gle.com, peterz@...radead.org,
        ravi.v.shankar@...el.com, mhiramat@...nel.org,
        jiangshanlai@...il.com, Xin Li <xin3.li@...el.com>,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-edac@...r.kernel.org, linux-hyperv@...r.kernel.org,
        kvm@...r.kernel.org, xen-devel@...ts.xenproject.org
Subject: Re: [PATCH v10 08/38] x86/cpufeatures: Add the cpu feature bit for
 FRED

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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