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SubjectRe: [PATCH 04/12] mips: Reserve memory for the kernel image resources
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Hi Serge,

On Fri, Apr 26, 2019 at 03:00:36AM +0300, Serge Semin wrote:
> > 1) Older systems generally had something like an ISA bus which used
> > addresses below the kernel, and bootloaders like YAMON left behind
> > functions that could be called right at the start of RAM. This sort
> > of thing should be accounted for by /memreserve/ in DT or similar
> > platform-specific reservations though rather than generically, and
> > at least Malta & SEAD-3 DTs already have /memreserve/ entries for
> > it. So this part I think is OK. Some other older platforms might
> > need updating, but that's fine.
> >
>
> Regarding ISA. As far as I remember devices on that bus can DMA only to the
> lowest 16MB. So in case if kernel is too big or placed pretty much high,
> they may be left even without reachable memory at all in current
> implementation.

Sure - I'm not too worried about these old buses, platforms can continue
to reserve the memory through DT or otherwise if they need to.

> > 2) trap_init() only allocates memory for the exception vector if using
> > a vectored interrupt mode. In other cases it just uses CAC_BASE
> > which currently gets reserved as part of this region between
> > PHYS_OFFSET & _text.
> >
> > I think this behavior is bogus, and we should instead:
> >
> > - Allocate the exception vector memory using memblock_alloc() for
> > CPUs implementing MIPSr2 or higher (ie. CPUs with a programmable
> > EBase register). If we're not using vectored interrupts then
> > allocating one page will do, and we already have the size
> > calculation for if we are.
> >
> > - Otherwise use CAC_BASE but call memblock_reserve() on the first
> > page.
> >
> > I think we should make that change before this one goes in. I can
> > try to get to it tomorrow, but feel free to beat me to it.
> >
>
> As far as I understood you and the code this should be enough to fix
> the problem:
> diff --git a/arch/mips/kernel/traps.c b/arch/mips/kernel/traps.c
> index 98ca55d62201..f680253e2617 100644
> --- a/arch/mips/kernel/traps.c
> +++ b/arch/mips/kernel/traps.c
> @@ -2326,6 +2326,8 @@ void __init trap_init(void)
> ebase += (read_c0_ebase() & 0x3ffff000);
> }
> }
> +
> + memblock_reserve(ebase, PAGE_SIZE);
> }
>
> if (cpu_has_mmips) {
> ---
>
> Allocation has already been implemented in the if-branch under the
> (cpu_has_veic || cpu_has_vint) condition. So we don't need to change
> there anything.
> In case if vectored interrupts aren't supported the else-clause is
> taken and we need to reserve whatever is set in the exception base
> address variable.
>
> I'll add this patch between 3d and 4th ones if you are ok with it.

I think that would work, but I have other motivations to allocate the
memory in non-vectored cases anyway. I just sent a series that does that
& cleans up a little [1]. If you could take a look that would be great.
With that change made I think this patch will be good to apply.

Thanks,
Paul

[1] https://lore.kernel.org/linux-mips/20190430225216.7164-1-paul.burton@mips.com/T/#t

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