Messages in this thread | | | Subject | Re: [RFC][PATCH v2 12/21] x86/pti: Use PTI stack instead of trampoline stack | From | Alexandre Chartre <> | Date | Tue, 17 Nov 2020 09:27:11 +0100 |
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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@oracle.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 > > - > Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK > Registration No: 1397386 (Wales) >
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