Re: [RFC PATCH 08/35] Add Xen-specific memory managementdefinitions

[Pete Zaitcev]

On Tue, 09 May 2006 00:00:08 -0700, Chris Wright <chrisw@xxxxxxxxxxxx> 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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