linux-kernel - Re: [PATCH 12/16] mm/ioremap: Add arch-specific callbacks on ioremap/iounmap calls

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Date:   Tue, 27 Jul 2021 19:12:04 +0100
From:   Will Deacon <will@...nel.org>
To:     Marc Zyngier <maz@...nel.org>
Cc:     linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        qperret@...gle.com, dbrazdil@...gle.com,
        Srivatsa Vaddagiri <vatsa@...eaurora.org>,
        Shanker R Donthineni <sdonthineni@...dia.com>,
        James Morse <james.morse@....com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Alexandru Elisei <alexandru.elisei@....com>,
        kernel-team@...roid.com
Subject: Re: [PATCH 12/16] mm/ioremap: Add arch-specific callbacks on
 ioremap/iounmap calls

On Thu, Jul 15, 2021 at 05:31:55PM +0100, Marc Zyngier wrote:
> Add a pair of hooks (ioremap_page_range_hook/iounmap_page_range_hook)
> that can be implemented by an architecture.
> 
> Signed-off-by: Marc Zyngier <maz@...nel.org>
> ---
>  include/linux/io.h |  3 +++
>  mm/ioremap.c       | 13 ++++++++++++-
>  mm/vmalloc.c       |  8 ++++++++
>  3 files changed, 23 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/io.h b/include/linux/io.h
> index 9595151d800d..0ffc265f114c 100644
> --- a/include/linux/io.h
> +++ b/include/linux/io.h
> @@ -21,6 +21,9 @@ void __ioread32_copy(void *to, const void __iomem *from, size_t count);
>  void __iowrite64_copy(void __iomem *to, const void *from, size_t count);
>  
>  #ifdef CONFIG_MMU
> +void ioremap_page_range_hook(unsigned long addr, unsigned long end,
> +			     phys_addr_t phys_addr, pgprot_t prot);
> +void iounmap_page_range_hook(phys_addr_t phys_addr, size_t size);
>  int ioremap_page_range(unsigned long addr, unsigned long end,
>  		       phys_addr_t phys_addr, pgprot_t prot);
>  #else

Can we avoid these hooks by instead not registering the regions proactively
in the guest and moving that logic to a fault handler which runs off the
back of the injected data abort? From there, we could check if the faulting
IPA is a memory address and register it as MMIO if not.

Dunno, you've spent more time than me thinking about this, but just
wondering if you'd had a crack at doing it that way, as it _seems_ simpler
to my naive brain.

Will