linux-kernel - Re: [PATCH 06/11] xen/setup: Skip over 1st gap after System RAM.

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Message-ID: <1296572896.13091.240.camel@zakaz.uk.xensource.com>
Date:	Tue, 1 Feb 2011 15:08:16 +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>,
	"Xen-devel@...ts.xensource.com" <Xen-devel@...ts.xensource.com>,
	"konrad@...nel.org" <konrad@...nel.org>,
	"jeremy@...p.org" <jeremy@...p.org>,
	"hpa@...or.com" <hpa@...or.com>,
	Stefano Stabellini <Stefano.Stabellini@...citrix.com>
Subject: Re: [PATCH 06/11] xen/setup: Skip over 1st gap after System RAM.

On Mon, 2011-01-31 at 22:44 +0000, Konrad Rzeszutek Wilk wrote:
> If the kernel is booted with dom0_mem=max:512MB and the
> machine has more than 512MB of RAM, the E820 we get is:
> 
> Xen: 0000000000100000 - 0000000020000000 (usable)
> Xen: 00000000b7ee0000 - 00000000b7ee3000 (ACPI NVS)
> 
> while in actuality it is:
> 
> (XEN)  0000000000100000 - 00000000b7ee0000 (usable)
> (XEN)  00000000b7ee0000 - 00000000b7ee3000 (ACPI NVS)
> 
> Based on that, we would determine that the "gap" between
> 0x20000 -> 0xb7ee0 is not System RAM and try to assign it to
> 1-1 mapping. This meant that later on when we setup the page tables
> we would try to assign those regions to DOMID_IO and the
> Xen hypervisor would fail such operation. This patch
> guards against that and sets the "gap" to be after the first
> non-RAM E820 region.

This seems dodgy to me and makes assumptions about the sanity of the
BIOS provided e820 maps. e.g. it's not impossible that there are systems
out there with 2 or more little holes under 1M etc.

The truncation (from 0xb7ee0000 to 0x20000000 in this case) happens in
the dom0 kernel not the hypervisor right? So we can at least know that
we've done it.

Can we do the identity setup before that truncation happens? If not can
can we not remember the untruncated map too and refer to it as
necessary. One way of doing that might be to insert an e820 region
covering the truncated region to identify it as such (perhaps
E820_UNUSABLE?) or maybe integrating e.g. with the memblock reservations
(or whatever the early enough allocator is).

The scheme we have is that all pre-ballooned memory goes at the end of
the e820 right, as opposed to allowing it to first fill truncated
regions such as this? 

Ian.

> 
> Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
> ---
>  arch/x86/xen/setup.c |   20 ++++++++++++++++++--
>  1 files changed, 18 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/xen/setup.c b/arch/x86/xen/setup.c
> index c2a5b5f..5b2ae49 100644
> --- a/arch/x86/xen/setup.c
> +++ b/arch/x86/xen/setup.c
> @@ -147,6 +147,7 @@ static unsigned long __init xen_set_identity(const struct e820map *e820)
>  {
>  	phys_addr_t last = xen_initial_domain() ? 0 : ISA_END_ADDRESS;
>  	phys_addr_t start_pci = last;
> +	phys_addr_t ram_end = last;
>  	int i;
>  	unsigned long identity = 0;
>  
> @@ -168,11 +169,26 @@ static unsigned long __init xen_set_identity(const struct e820map *e820)
>  			if (start > start_pci)
>  				identity += set_phys_range_identity(
>  					PFN_UP(start_pci), PFN_DOWN(start));
> -			start_pci = end;
>  			/* Without saving 'last' we would gooble RAM too. */
> -			last = end;
> +			start_pci = last = ram_end = end;
>  			continue;
>  		}
> +		/* Gap found right after the 1st RAM region. Skip over it.
> +		 * Why? That is b/c if we pass in dom0_mem=max:512MB and
> +		 * have in reality 1GB, the E820 is clipped at 512MB.
> +		 * In xen_set_pte_init we end up calling xen_set_domain_pte
> +		 * which asks Xen hypervisor to alter the ownership of the MFN
> +		 * to DOMID_IO. We would try to set that on PFNs from 512MB
> +		 * up to the next System RAM region (likely from 0x20000->
> +		 * 0x100000). But changing the ownership on "real" RAM regions
> +		 * will infuriate Xen hypervisor and we will fail (WARN).
> +		 * So instead of trying to set IDENTITY mapping on the gap
> +		 * between the System RAM and the first non-RAM E820 region
> +		 * we start at the non-RAM E820 region.*/
> +		if (ram_end && start >= ram_end) {
> +			start_pci = start;
> +			ram_end = 0;
> +		}
>  		start_pci = min(start, start_pci);
>  		last = end;
>  	}


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