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

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Message-ID: <985e90259d654d7c9eb74398c5d927ee@AcuMS.aculab.com>
Date:   Mon, 16 Nov 2020 21:24:37 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Alexandre Chartre' <alexandre.chartre@...cle.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

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?

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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