linux-kernel - RE: [PATCH v4 5/5] x86/gsseg: use the LKGS instruction if available for load_gs

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Message-ID: <BN6PR1101MB2161DD8F4F587C6FBFCA18EFA82B9@BN6PR1101MB2161.namprd11.prod.outlook.com>
Date:   Wed, 19 Oct 2022 17:45:07 +0000
From:   "Li, Xin3" <xin3.li@...el.com>
To:     "Gross, Jurgen" <jgross@...e.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>
CC:     "tglx@...utronix.de" <tglx@...utronix.de>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "bp@...en8.de" <bp@...en8.de>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        "hpa@...or.com" <hpa@...or.com>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "brgerst@...il.com" <brgerst@...il.com>,
        "Bae, Chang Seok" <chang.seok.bae@...el.com>
Subject: RE: [PATCH v4 5/5] x86/gsseg: use the LKGS instruction if available
 for load_gs_index()

> > +static inline void __init lkgs_init(void) { #ifdef
> > +CONFIG_PARAVIRT_XXL #ifdef CONFIG_X86_64
> > +	if (cpu_feature_enabled(X86_FEATURE_LKGS))
> > +		pv_ops.cpu.load_gs_index = native_lkgs;
> 
> For this to work correctly when running as a Xen PV guest, you need to add
> 
> 	setup_clear_cpu_cap(X86_FEATURE_LKGS);
> 
> to xen_init_capabilities() in arch/x86/xen/enlighten_pv.c, as otherwise the Xen
> specific .load_gs_index vector will be overwritten.

Yeah, we definitely should add it to disable LKGS in a Xen PV guest.

So does it mean that the Xen PV uses a black list during feature detection?
If yes then new features are often required to be masked with an explicit
call to setup_clear_cpu_cap.

Wouldn't a white list be better?
Then the job is more just on the Xen PV side, and it can selectively enable
a new feature, sometimes with Xen PV specific handling code added.

Xin

> 
> 
> Juergen