linux-kernel - Re: [PATCH v2 4/4] lib, pci: unify generic pci

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Message-Id: <619ea619-29e4-42fb-9b27-1d1a32e0ee66@app.fastmail.com>
Date:   Fri, 01 Dec 2023 16:26:05 +0100
From:   "Arnd Bergmann" <arnd@...db.de>
To:     "Philipp Stanner" <pstanner@...hat.com>,
        "Bjorn Helgaas" <bhelgaas@...gle.com>,
        "Andrew Morton" <akpm@...ux-foundation.org>,
        "Dan Williams" <dan.j.williams@...el.com>,
        "Jonathan Cameron" <Jonathan.Cameron@...wei.com>,
        "Jakub Kicinski" <kuba@...nel.org>,
        "Dave Jiang" <dave.jiang@...el.com>,
        "Uladzislau Koshchanka" <koshchanka@...il.com>,
        "Neil Brown" <neilb@...e.de>,
        "Niklas Schnelle" <schnelle@...ux.ibm.com>,
        "John Sanpe" <sanpeqf@...il.com>,
        "Kent Overstreet" <kent.overstreet@...il.com>,
        "Masami Hiramatsu" <mhiramat@...nel.org>,
        "Kees Cook" <keescook@...omium.org>,
        "David Gow" <davidgow@...gle.com>,
        "Yury Norov" <yury.norov@...il.com>,
        "wuqiang.matt" <wuqiang.matt@...edance.com>,
        "Jason Baron" <jbaron@...mai.com>,
        "Kefeng Wang" <wangkefeng.wang@...wei.com>,
        "Ben Dooks" <ben.dooks@...ethink.co.uk>,
        "Danilo Krummrich" <dakr@...hat.com>
Cc:     linux-kernel@...r.kernel.org, linux-pci@...r.kernel.org,
        Linux-Arch <linux-arch@...r.kernel.org>,
        "Arnd Bergmann" <arnd@...nel.org>
Subject: Re: [PATCH v2 4/4] lib, pci: unify generic pci_iounmap()

On Fri, Dec 1, 2023, at 13:16, Philipp Stanner wrote:
> The implementation of pci_iounmap() is currently scattered over two
> files, drivers/pci/iounmap.c and lib/iomap.c. Additionally,
> architectures can define their own version.
>
> Besides one unified version being desirable in the first place, the old
> version in drivers/pci/iounmap.c contained a bug and could leak memory
> mappings. The bug was that #ifdef ARCH_HAS_GENERIC_IOPORT_MAP should not
> have guarded iounmap(p); in addition to the preceding code.
>
> To have only one version, it's necessary to create a helper function,
> iomem_is_ioport(), that tells pci_iounmap() whether the passed address
> points to an ioport or normal memory.
>
> iomem_is_ioport() can be provided through three different ways:
>   1. The architecture itself provides it.
>   2. As a default version in include/asm-generic/io.h for those
>      architectures that don't use CONFIG_GENERIC_IOMAP, but also don't
>      provide their own version of iomem_is_ioport().
>   3. As a default version in lib/iomap.c for those architectures that
>      define and use CONFIG_GENERIC_IOMAP (currently, only x86 really
>      uses the functions in lib/iomap.c)

I would count 3 as a special case of 1 here.

> Create a unified version of pci_iounmap() in drivers/pci/iomap.c.
> Provide the function iomem_is_ioport() in include/asm-generic/io.h and
> lib/iomap.c.
>
> Remove the CONFIG_GENERIC_IOMAP guard around
> ARCH_WANTS_GENERIC_PCI_IOUNMAP so that configs that set
> CONFIG_GENERIC_PCI_IOMAP without CONFIG_GENERIC_IOMAP still get the
> function.
>
> Fixes: 316e8d79a095 ("pci_iounmap'2: Electric Boogaloo: try to make 
> sense of it all")
> Suggested-by: Arnd Bergmann <arnd@...nel.org>
> Signed-off-by: Philipp Stanner <pstanner@...hat.com>

Looks good overall. It would be nice to go further than this
and replace all the custom pci_iounmap() variants with custom
iomem_is_ioport() implementations, but that can be a follow-up
along with removing the incorrect or useless 'select GENERIC_IOMAP'
parts.

>  		return;
> -	iounmap(p);
> +	}
>  #endif
> +	iounmap(addr);
>  }

I think the bugfix should be a separate patch so we can backport
it to stable kernels.

> +#ifndef CONFIG_GENERIC_IOMAP
> +static inline bool iomem_is_ioport(void __iomem *addr)
> +{
> +	unsigned long port = (unsigned long __force)addr;
> +
> +	// TODO: do we have to take IO_SPACE_LIMIT and PCI_IOBASE into account
> +	// similar as in ioport_map() ?
> +
> +	if (port > MMIO_UPPER_LIMIT)
> +		return false;
> +
> +	return true;
> +}

This has to have the exact logic that was present in the
old pci_iounmap(). For the default version that is currently
in lib/pci_iomap.c, this means something along the linens of

static inline bool struct iomem_is_ioport(void __iomem *p)
{
#ifdef CONFIG_HAS_IOPORT
        uintptr_t start = (uintptr_t) PCI_IOBASE;
        uintptr_t addr = (uintptr_t) p;

        if (addr >= start && addr < start + IO_SPACE_LIMIT)
                return true;
#endif
        return false;
}

> +#else /* CONFIG_GENERIC_IOMAP. Version from lib/iomap.c will be used. 
> */
> +bool iomem_is_ioport(void __iomem *addr);
> +#define ARCH_WANTS_GENERIC_IOMEM_IS_IOPORT

I'm not sure what this macro is for, since it appears to
do the opposite of what its name suggests: rather than
provide the generic version of iomem_is_ioport(), it
skips that and provides a custom one to go with lib/iomap.c

     Arnd