On 31/01/2023 11:44, Alistair Francis wrote:
On Sat, Jan 28, 2023 at 12:15 AM Oleksii <oleksii.kuroc...@gmail.com> wrote:
Hi Alistair, Bobby and community,
I would like to ask your help with the following check:
+/*
+ * early_*() can be called from head.S with MMU-off.
+ *
+ * The following requiremets should be honoured for early_*() to
+ * work correctly:
+ * It should use PC-relative addressing for accessing symbols.
+ * To achieve that GCC cmodel=medany should be used.
+ */
+#ifndef __riscv_cmodel_medany
+#error "early_*() can be called from head.S with MMU-off"
+#endif
I have never seen a check like this before.
The check is in the Linux code, see [3].
I don't really understand
what it's looking for, if the linker is unable to call early_*() I
would expect it to throw an error. I'm not sure what this is adding.
When the MMU is off during early boot, you want any C function called to
use PC-relative address rather than absolute address. This is because
the physical address may not match the virtual address.
From my understanding, on RISC-V, the use of PC-relative address is
only guaranteed with medany. So if you were going to change the cmodel
(Andrew suggested you would), then early_*() may end up to be broken.
This check serve as a documentation of the assumption and also help the
developer any change in the model and take the appropriate action to
remediate it.
I think this is safe to remove.
Based on what I wrote above, do you still think this is safe?
Cheers,
Please take a look at the following messages and help me to decide if
the check mentioned above should be in early_printk.c or not:
[1]
https://lore.kernel.org/xen-devel/599792fa-b08c-0b1e-10c1-0451519d9...@xen.org/
[2]
https://lore.kernel.org/xen-devel/0ec33871-96fa-bd9f-eb1b-eb37d3d7c...@xen.org/
[3]
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/riscv/mm/init.c
--
Julien Grall
