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

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Message-ID: <826853e4-f7ce-349d-c0c0-74516af6ff8c@suse.com>
Date:   Thu, 20 Oct 2022 06:54:49 +0200
From:   Juergen Gross <jgross@...e.com>
To:     "H. Peter Anvin" <hpa@...or.com>, "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>,
        "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 20:01, H. Peter Anvin wrote:
> On October 19, 2022 10:45:07 AM PDT, "Li, Xin3" <xin3.li@...el.com> 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.
>>
>> Xin
>>
>>>
>>>
>>> Juergen
>>
> 
> Most things don't frob the paravirt list.
> 
> Maybe we should make the paravirt frobbing a separate patch, at it is separable.

Works for me.


Juergen

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