linux-kernel - Re: [PATCH] reserve end-of-conventional-memory to 1MB, 32-bit, use paravirt

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Message-ID: <20080304185742.GA12193@mailshack.com>
Date:	Tue, 4 Mar 2008 19:57:42 +0100
From:	Alexander van Heukelum <heukelum@...lshack.com>
To:	Jeremy Fitzhardinge <jeremy@...p.org>, Ingo Molnar <mingo@...e.hu>
Cc:	Alexander van Heukelum <heukelum@...tmail.fm>,
	Mark McLoughlin <markmc@...hat.com>,
	Ian Campbell <ijc@...lion.org.uk>,
	"H. Peter Anvin" <hpa@...or.com>, Andi Kleen <ak@...e.de>,
	Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] reserve end-of-conventional-memory to 1MB, 32-bit, use paravirt_enabled

Jeremy Fitzhardinge pointed out that looking at the boot_params
struct to determine if the system is running in a paravirtual
environment is not reliable for the Xen case, currently. He also
points out that there already exists a function to determine if
the system is running in a paravirtual environment. So let's use
that instead. This gets rid of the preprocessor test too.

Signed-off-by: Alexander van Heukelum <heukelum@...tmail.fm>

---

On Tue, Mar 04, 2008 at 09:11:33AM -0800, Jeremy Fitzhardinge wrote:
> Alexander van Heukelum wrote:
> >On Tue, 04 Mar 2008 07:18:48 -0800, "Jeremy Fitzhardinge"
> ><jeremy@...p.org> said:
> >>Yes, is_paravirt() already exists for this purpose.
> >
> >Hi,
> >
> >If it exists, it is well-hidden. A grep for is_paravirt on the testing
> >tree turns up nothing. Did you get the name right?
> >  
> 
> Nope.  paravirt_enabled().

Duh, I should have been able to find that :-/. Thanks for the pointer.

> Just because something is paravirtualized and uses a non-PC 
> hardware_subarch doesn't mean this stuff isn't present.  Even the 
> paravirt_enabled() test isn't accurate, because the environment may 
> still choose to emulate these things (or in the Xen dom0 case, it may 
> directly expose the real hardware).

paravirt_enabled() returns 0 if CONFIG_PARAVIRT is not set, or if a
paravirt-enabled kernel is run on bare hardware: dom0. If that is right,
then it is exactly what is needed.

Greetings,
	Alexander

diff --git a/arch/x86/kernel/setup_32.c b/arch/x86/kernel/setup_32.c
index 20e537b..f595d7b 100644
--- a/arch/x86/kernel/setup_32.c
+++ b/arch/x86/kernel/setup_32.c
@@ -64,6 +64,7 @@
 #include <setup_arch.h>
 #include <bios_ebda.h>
 #include <asm/cacheflush.h>
+#include <asm/processor.h>
 
 /* This value is set up by the early boot code to point to the value
    immediately after the boot time page tables.  It contains a *physical*
@@ -408,12 +409,8 @@ static void __init reserve_ebda_region(void)
 	/* that area is absent. We'll just have to assume */
 	/* that the paravirt case can handle memory setup */
 	/* correctly, without our help. */
-#ifdef CONFIG_PARAVIRT
-	if ((boot_params.hdr.version >= 0x207) &&
-			(boot_params.hdr.hardware_subarch != 0)) {
+	if (paravirt_enabled())
 		return;
-	}
-#endif
 
 	/* end of low (conventional) memory */
 	lowmem = *(unsigned short *)__va(BIOS_LOWMEM_KILOBYTES);

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