linux-kernel - Re: [PATCH v05 54/72] include/uapi/xen/privcmd.h: fix compilation in userspace

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Date:   Tue, 23 Aug 2016 11:13:52 -0700 (PDT)
From:   Stefano Stabellini <sstabellini@...nel.org>
To:     Russell King - ARM Linux <linux@...linux.org.uk>
cc:     Mikko Rapeli <mikko.rapeli@....fi>, linux-kernel@...r.kernel.org,
        David Vrabel <david.vrabel@...rix.com>,
        Stefano Stabellini <sstabellini@...nel.org>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Juergen Gross <jgross@...e.com>,
        xen-devel@...ts.xenproject.org,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v05 54/72] include/uapi/xen/privcmd.h: fix compilation
 in userspace

On Tue, 23 Aug 2016, Russell King - ARM Linux wrote:
> On Mon, Aug 22, 2016 at 08:33:11PM +0200, Mikko Rapeli wrote:
> > xen/interface/xen.h is not exported from kernel headers so remove the
> > dependency and provide needed defines for domid_t and xen_pfn_t if they
> > are not already defined by some other e.g. Xen specific headers.
> 
> I'm confused.  How did we end up with a 64-bit PFN number on ARM?  It's
> insane - especially as the kernel uses "unsigned long" almost everywhere
> for PFNs - we can't have physical addresses more than 44 bits (32 bit
> pfn + 4k page size).

That's because xen_pfn_t is the type used to store pfns in structures
passed to Xen via hypercalls. The Xen hypercall ABI is shared between
ARM and ARM64. On x86_32 and x86_64 we have different sizes for pfn
types in the hypercall ABI and it caused quite a bit of trouble in the
past as it is possible to run 32bit domains on a 64bit hypervisor.
Having a common type for pfns on ARM and ARM64 helped a lot in that
respect.


> > Suggested by Andrew Cooper <andrew.cooper3@...rix.com> on lkml message
> > <5569F9C9.8000607@...rix.com>.
> > 
> > The ifdef for ARM is ugly but did not find better solutions for it.
> 
> #ifdef __arm__
> 
> maybe?  Even if not, the unsightly parens are not necessary.

Yes, I think it should be:

#if defined(__arm__) || defined(__aarch64__)


> diff --git a/arch/arm/include/asm/xen/interface.h b/arch/arm/include/asm/xen/interface.h
> index 75d5968..6898ee1 100644
> --- a/arch/arm/include/asm/xen/interface.h
> +++ b/arch/arm/include/asm/xen/interface.h
> @@ -38,7 +38,7 @@
>   * fine since it simply wouldn't be able to create any sure pfns in
>   * the first place.
>   */
> -typedef uint64_t xen_pfn_t;
> +typedef __u64 xen_pfn_t;
>  #define PRI_xen_pfn "llx"
>  typedef uint64_t xen_ulong_t;
>  #define PRI_xen_ulong "llx"

Why this change?