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

Alexandre Chartre
2020-11-17T08:27:28Z

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
>
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