linux-kernel - Re: [PATCH] xen/e820: Don't mark balloon memory as E820

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Date:	Fri, 11 Mar 2011 15:23:36 +0000
From:	Ian Campbell <Ian.Campbell@...citrix.com>
To:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"Jeremy Fitzhardinge" <jeremy@...p.org>,
	Stefano Stabellini <Stefano.Stabellini@...citrix.com>,
	"xen-devel@...ts.xensource.com" <xen-devel@...ts.xensource.com>,
	Konrad Rzeszutek Wilk <konrad@...nel.org>
Subject: Re: [PATCH] xen/e820: Don't mark balloon memory as E820_UNUSABLE
 when running as guest.

On Thu, 2011-03-10 at 20:36 +0000, Konrad Rzeszutek Wilk wrote:
> If we have a guest that asked for:
> 
> memory=1024
> maxmem=20448
> 
> Which means we want 1GB now, and create pagetables so that we can expand
> up to 2GB, we would have this E820 layout:
> 
> [    0.000000] BIOS-provided physical RAM map:
> [    0.000000]  Xen: 0000000000000000 - 00000000000a0000 (usable)
> [    0.000000]  Xen: 00000000000a0000 - 0000000000100000 (reserved)
> [    0.000000]  Xen: 0000000000100000 - 0000000080800000 (usable)
> 
> Due to patch: "xen/setup: Inhibit resource API from using System RAM E820 gaps as PCI mem gaps."
> we would mark the memory past the 1GB mark as unusuable resulting in:
> 
> [    0.000000] BIOS-provided physical RAM map:
> [    0.000000]  Xen: 0000000000000000 - 00000000000a0000 (usable)
> [    0.000000]  Xen: 00000000000a0000 - 0000000000100000 (reserved)
> [    0.000000]  Xen: 0000000000100000 - 0000000040000000 (usable)
> [    0.000000]  Xen: 0000000040000000 - 0000000080800000 (unusable)
> 
> which meant that we could not balloon up anymore. We could
> balloon the guest down. The fix is to run the code introduced
> by the above mentioned patch only for the initial domain.
> 
> We will have to revisit this once we start introducing a modified
> E820 for PCI passthrough so that we can utilize the P2M identity code.
> 
> Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
> ---
>  arch/x86/xen/setup.c |    3 ++-
>  1 files changed, 2 insertions(+), 1 deletions(-)
> 
> diff --git a/arch/x86/xen/setup.c b/arch/x86/xen/setup.c
> index 2a4add9..6e676fa 100644
> --- a/arch/x86/xen/setup.c
> +++ b/arch/x86/xen/setup.c
> @@ -200,7 +200,8 @@ char * __init xen_memory_setup(void)
>  			 * used as potential resource for I/O address (happens
>  			 * when 'allocate_resource' is called).
>  			 */
> -			if (delta && end < 0x100000000UL)
> +			if (delta &&
> +				(xen_initial_domain() && end < 0x100000000UL))

Not a new problem but 0x100000000 will overflow an unsigned long on 32
bit:
  CC      arch/x86/xen/setup.o
arch/x86/xen/setup.c: In function 'xen_memory_setup':
arch/x86/xen/setup.c:254: warning: integer constant is too large for 'unsigned long' type

I think you want ULL? (end is "unsigned long long").

Ian.


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