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SubjectRe: [RFC PATCH 08/35] Add Xen-specific memory management definitions
On Tue, 09 May 2006 00:00:08 -0700, Chris Wright <chrisw@sous-sol.org> wrote:

I'm a little concerned with the code below being entirely too smart:

> +static inline unsigned long mfn_to_pfn(unsigned long mfn)
> +{
> +#ifndef CONFIG_XEN_SHADOW_MODE
> + unsigned long pfn;
> +
> + if (xen_feature(XENFEAT_auto_translated_physmap))
> + return mfn;
> +
> + /*
> + * The array access can fail (e.g., device space beyond end of RAM).
> + * In such cases it doesn't matter what we return (we return garbage),
> + * but we must handle the fault without crashing!
> + */
> + asm (
> + "1: movl %1,%0\n"
> + "2:\n"
> + ".section __ex_table,\"a\"\n"
> + " .align 4\n"
> + " .long 1b,2b\n"
> + ".previous"
> + : "=r" (pfn) : "m" (machine_to_phys_mapping[mfn]) );
> +
> + return pfn;
> +#else
> + return mfn;
> +#endif
> +}

I found that if someone tries to use this too early, hypervisor terminates
the domain with a message like this:

(XEN) DOM0: (file=mm.c, line=486) Non-privileged attempt to map I/O space 000fec00

Why can't we use something like this, only a proper number of machine
pages:

diff -urp -X dontdiff linux-2.6.15-1.2054_FC5/include/asm-x86_64/mach-xen/asm/page.h linux-2.6.15-1.2054_FC5.z3/include/asm-x86_64/mach-xen/asm/page.h
--- linux-2.6.15-1.2054_FC5/include/asm-x86_64/mach-xen/asm/page.h 2006-03-17 17:52:38.000000000 -0800
+++ linux-2.6.15-1.2054_FC5.z3/include/asm-x86_64/mach-xen/asm/page.h 2006-05-11 20:36:16.000000000 -0700
@@ -101,26 +101,11 @@ static inline int phys_to_machine_mappin

static inline unsigned long mfn_to_pfn(unsigned long mfn)
{
- unsigned long pfn;
-
if (xen_feature(XENFEAT_auto_translated_physmap))
return mfn;
-
- /*
- * The array access can fail (e.g., device space beyond end of RAM).
- * In such cases it doesn't matter what we return (we return garbage),
- * but we must handle the fault without crashing!
- */
- asm (
- "1: movq %1,%0\n"
- "2:\n"
- ".section __ex_table,\"a\"\n"
- " .align 8\n"
- " .quad 1b,2b\n"
- ".previous"
- : "=r" (pfn) : "m" (machine_to_phys_mapping[mfn]) );
-
- return pfn;
+ if (mfn >= 1048576) /* 4GB _machine_ RAM. XXX How to find out? */
+ return ~0;
+ return machine_to_phys_mapping[mfn];
}

/*
I'm sure you considered this, but decided to be tricky. Why?
No way to find the safe number of machine pages in a guest?

-- Pete
-
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