On Thu, Jan 23, 2025 at 09:31:31AM +0100, Peter Zijlstra wrote:

On Wed, Jan 22, 2025 at 02:36:25PM -0800, Josh Poimboeuf wrote:

quoted

On Wed, Jan 22, 2025 at 02:57:00PM +0100, Peter Zijlstra wrote:

quoted

On Tue, Jan 21, 2025 at 06:31:21PM -0800, Josh Poimboeuf wrote:

But if the NMI happens late in the exit-to-user path, with IRQs
disabled, right before nr_entries gets cleared, the cache won't get
used in the task work.

However I think we can clear it on entry-from-user.

Return to user runs with interrupts disabled, if an NMI hits that, it
will have to set TIF_NOTIFY_RESUME again and queue the IRQ work thing.
That self-IPI will hit the moment we do IRET (which is what re-enables
interrupts) and we're going back into the kernel.

Anyway, I suppose that is a long way of saying that you should be able
to do this on return to user.

Indeed, I knew that but somehow overlooked the fact that the IRQ would
clear the cookie so the cache wouldn't be usable anyway.

-- 
Josh