linux-kernel - Re: [PATCH 04/12] mips: Reserve memory for the kernel image resources

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Date:   Tue, 30 Apr 2019 22:58:33 +0000
From:   Paul Burton <paul.burton@...s.com>
To:     Serge Semin <fancer.lancer@...il.com>
CC:     Ralf Baechle <ralf@...ux-mips.org>,
        James Hogan <jhogan@...nel.org>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...e.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Thomas Bogendoerfer <tbogendoerfer@...e.de>,
        Huacai Chen <chenhc@...ote.com>,
        Stefan Agner <stefan@...er.ch>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        Alexandre Belloni <alexandre.belloni@...tlin.com>,
        Juergen Gross <jgross@...e.com>,
        "linux-mips@...r.kernel.org" <linux-mips@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 04/12] mips: Reserve memory for the kernel image resources

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