Re: [PATCH v7 1/2] xen/riscv: introduce early_printk basic stuff

[Julien Grall <julien@xxxxxxx>]

On 31/01/2023 11:44, Alistair Francis wrote:
> On Sat, Jan 28, 2023 at 12:15 AM Oleksii <oleksii.kurochko@xxxxxxxxx> 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-10c1-0451519d9e4a@xxxxxxx/
> > [2]
> > https://lore.kernel.org/xen-devel/0ec33871-96fa-bd9f-eb1b-eb37d3d7c982@xxxxxxx/
[3] 
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/riscv/mm/init.c

--
Julien Grall