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

[David Laight]

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@xxxxxxxxxx> 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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