linux-kernel - Re: [RFC][PATCH v2 12/21] x86/pti: Use PTI stack instead of trampoline stack

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Message-ID: <87zh3dnivt.fsf@nanos.tec.linutronix.de>
Date:   Thu, 19 Nov 2020 22:20:38 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Alexandre Chartre <alexandre.chartre@...cle.com>,
        Andy Lutomirski <luto@...nel.org>
Cc:     Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>, X86 ML <x86@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Tom Lendacky <thomas.lendacky@....com>,
        Joerg Roedel <jroedel@...e.de>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        jan.setjeeilers@...cle.com, Junaid Shahid <junaids@...gle.com>,
        oweisse@...gle.com, Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        Alexander Graf <graf@...zon.de>, mgross@...ux.intel.com,
        kuzuno@...il.com
Subject: Re: [RFC][PATCH v2 12/21] x86/pti: Use PTI stack instead of trampoline stack

On Thu, Nov 19 2020 at 20:55, Alexandre Chartre wrote:
> On 11/19/20 8:10 PM, Thomas Gleixner wrote:
> Sorry I mixed up a few thing; I got confused with my own code which is not a
> good sign...
>
> It's not sleeping with the user page-table which, as you mentioned, doesn't
> make sense, it's sleeping with the kernel page-table but with the PTI stack.
>
> Basically, it is:
>    - entering C code with (user page-table, PTI stack);
>    - then it switches to the kernel page-table so we have (kernel page-table, PTI stack);
>    - and then it switches to the kernel stack so we have (kernel page-table, kernel stack).
>
> As this is all C code, some of which is executed with the PTI stack, we need the PTI stack
> to be per-task so that the stack is preserved, in case that C code does a sleep/schedule
> (no matter if this happens when using the PTI stack or the kernel stack).

That makes some more sense, but I'm not convinced that this dual stack
is really a good thing.

Thanks,

        tglx