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

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Message-ID: <e6e168db-5d16-66a6-e817-6d23ea0bf1eb@oracle.com>
Date:   Tue, 17 Nov 2020 09:27:11 +0100
From:   Alexandre Chartre <alexandre.chartre@...cle.com>
To:     David Laight <David.Laight@...LAB.COM>,
        Andy Lutomirski <luto@...nel.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        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" <jan.setjeeilers@...cle.com>,
        Junaid Shahid <junaids@...gle.com>,
        "oweisse@...gle.com" <oweisse@...gle.com>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        Alexander Graf <graf@...zon.de>,
        "mgross@...ux.intel.com" <mgross@...ux.intel.com>,
        "kuzuno@...il.com" <kuzuno@...il.com>
Subject: Re: [RFC][PATCH v2 12/21] x86/pti: Use PTI stack instead of
 trampoline stack


On 11/16/20 10:24 PM, David Laight wrote:
> From: Alexandre Chartre
>> Sent: 16 November 2020 18:10
>>
>> On 11/16/20 5:57 PM, Andy Lutomirski wrote:
>>> On Mon, Nov 16, 2020 at 6:47 AM Alexandre Chartre
>>> <alexandre.chartre@...cle.com> wrote:
>>>>
>>>> When entering the kernel from userland, use the per-task PTI stack
>>>> instead of the per-cpu trampoline stack. Like the trampoline stack,
>>>> the PTI stack is mapped both in the kernel and in the user page-table.
>>>> Using a per-task stack which is mapped into the kernel and the user
>>>> page-table instead of a per-cpu stack will allow executing more code
>>>> before switching to the kernel stack and to the kernel page-table.
>>>
>>> Why?
>>
>> When executing more code in the kernel, we are likely to reach a point
>> where we need to sleep while we are using the user page-table, so we need
>> to be using a per-thread stack.
> 
> Isn't that going to allocate a lot more kernel memory?

That's one of my concern, hence this RFC. The current code is doubling the
task stack (this was an easy solution), so that's +8KB per task. See my
reply to Boris, it has a bit more details.

alex.


> ISTR some thoughts about using dynamically allocated kernel
> stacks when (at least some) wakeups are done by directly
> restarting the system call - so that the sleeping thread
> doesn't even need a kernel stack.
> (I can't remember if that was linux or one of the BSDs)
> 
> 	David
> 
> -
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