On Tue, 14 Jan 2025 08:38:48 +0000, Ahmad Fatoum a.fatoum@pengutronix.de wrote:
Hi Pierre,
On 08.01.25 16:52, Pierre-Clément Tosi wrote:
Hi Caleb,
On Wed, Jan 08, 2025 at 04:19:07PM +0100, Caleb Connolly wrote:
Hi Marc,
Thanks for your comments.
On 08/01/2025 16:05, Marc Zyngier wrote:
On Wed, 08 Jan 2025 14:22:24 +0000, Caleb Connolly caleb.connolly@linaro.org wrote:
This seems to cause crashes on a bunch of Qualcomm platforms. It's safer to just update the live table and flush it.
You may want to provide a bit more information, because that's not much to go on, really.
Best I have is "the board hangs and then resets when trying to switch pagetables", I didn't manage to narrow it down exactly, but since it seems to work to update the tables without switching that feels like a better approach at least for now.
In hindsight, this mmu_map_region() function is not great in a lot of ways, I'm still getting my head around the MMU and I expect this will keep being improved.
Yes, [1] should not have used map_range() on live PTs as it assumes that
- the PTs it modifies aren't live so doesn't perform Break Before Make;
Quoting ARM ARM (L.a) D8.17.1 "Using break-before-make when updating translation table entries":
If multiple execution threads use the same translation tables, then when a translation table entry is modified in one or more of the following ways, the architecture requires software to use a break-before-make sequence: [...]
So I am wondering: Does the break-before-make requirement even apply to a single-threaded software like U-Boot?
Yes, this equally applies, as the page-table walker is a separate agent in the system, and can *speculatively* walk page tables any time it wants. These walks are allowed to be cached as TLBs, and this can result in a single VA translating to multiple PAs.
Don't do that.
M.