linux-kernel - Re: [PATCH v6 5/6] Optionally flush L1D on context switch

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Message-ID: <87pnb6zr81.fsf@nanos.tec.linutronix.de>
Date:   Thu, 14 May 2020 13:33:02 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     "Singh\, Balbir" <sblbir@...zon.com>,
        "linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>
Cc:     "keescook\@chromium.org" <keescook@...omium.org>,
        "thomas.lendacky\@amd.com" <thomas.lendacky@....com>,
        "tony.luck\@intel.com" <tony.luck@...el.com>,
        "benh\@kernel.crashing.org" <benh@...nel.crashing.org>,
        "jpoimboe\@redhat.com" <jpoimboe@...hat.com>,
        "x86\@kernel.org" <x86@...nel.org>,
        "dave.hansen\@intel.com" <dave.hansen@...el.com>
Subject: Re:  [PATCH v6 5/6] Optionally flush L1D on context switch

Balbir,

"Singh, Balbir" <sblbir@...zon.com> writes:
> On Wed, 2020-05-13 at 18:16 +0200, Thomas Gleixner wrote:
>> Balbir Singh <sblbir@...zon.com> writes:
>> But looking at this deeper (yes I should have noticed earlier):
>> 
>>     Why do we need yet another PRCTL?
>> 
>> We already have PR_SET_SPECULATION_CTRL/PR_GET_SPECULATION_CTRL. That
>> L1D flush thingy fits into this category, right?
>
> It does, I thought about it for a while when I was changing the code and
> left it aside because, looking at the definition
>
> 1    PR_SPEC_ENABLE         The speculation feature is enabled,
> mitigation is disabled.
> 2    PR_SPEC_DISABLE        The speculation feature is disabled,
> mitigation is enabled.
>
> With L1D flush, there is no overriding of the feature as such (as in
> enable when the mitigation is disabled and vice-versa). I am happy to
> reconsider my initial thought though.

L1D is always enabled as L1D will be a source of trouble forever :)

Thanks,

        tglx