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

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Date:   Thu, 20 Oct 2022 06:53:27 +0200
From:   Juergen Gross <jgross@...e.com>
To:     "Li, Xin3" <xin3.li@...el.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.

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.


Juergen


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