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Re: [PATCH v10 08/38] x86/cpufeatures: Add the cpu feature bit for FRED
- To: Thomas Gleixner <tglx@xxxxxxxxxxxxx>, andrew.cooper3@xxxxxxxxxx, Jan Beulich <jbeulich@xxxxxxxx>
- From: Juergen Gross <jgross@xxxxxxxx>
- Date: Fri, 15 Sep 2023 07:27:27 +0200
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- Cc: mingo@xxxxxxxxxx, bp@xxxxxxxxx, dave.hansen@xxxxxxxxxxxxxxx, x86@xxxxxxxxxx, hpa@xxxxxxxxx, luto@xxxxxxxxxx, pbonzini@xxxxxxxxxx, seanjc@xxxxxxxxxx, peterz@xxxxxxxxxxxxx, ravi.v.shankar@xxxxxxxxx, mhiramat@xxxxxxxxxx, jiangshanlai@xxxxxxxxx, Xin Li <xin3.li@xxxxxxxxx>, linux-doc@xxxxxxxxxxxxxxx, linux-kernel@xxxxxxxxxxxxxxx, linux-edac@xxxxxxxxxxxxxxx, linux-hyperv@xxxxxxxxxxxxxxx, kvm@xxxxxxxxxxxxxxx, xen-devel@xxxxxxxxxxxxxxxxxxxx
- Delivery-date: Fri, 15 Sep 2023 05:27:52 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
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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