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

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Message-ID: <BN6PR1101MB21614EE46D700C24E7D77E8FA82A9@BN6PR1101MB2161.namprd11.prod.outlook.com>
Date:   Thu, 20 Oct 2022 05:58:12 +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()

> On 19.10.22 19:45, Li, Xin3 wrote:
> >>> +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.
> 
> This is not how it works. Feature detection is generic code, so we'd need to
> tweak that for switching to a whitelist.
>

Yes, a Xen PV guest is basically a Linux system.  However IIRC, the Xen PV
CPUID is para-virtualized, so it's Xen hypervisor's responsibility to decide
features exposed to a Xen PV guest.  No?

> Additionally most features don't require any Xen PV specific handling. This is
> needed for some paravirtualized privileged operations only. So switching to a
> whitelist would add more effort.
> 

LKGS is allowed only in ring 0, thus only Xen hypervisor could use it.

Xin

> 
> Juergen