linux-kernel - Re: [PATCH 1/2] xen/mmu: Add workaround "x86-64, mm: Put early page table high"

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Date:	Thu, 5 May 2011 10:01:24 -0400
From:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:	Stefano Stabellini <stefano.stabellini@...citrix.com>
Cc:	Daniel Kiper <dkiper@...-space.pl>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"yinghai@...nel.org" <yinghai@...nel.org>,
	"hpa@...or.com" <hpa@...or.com>,
	"xen-devel@...ts.xensource.com" <xen-devel@...ts.xensource.com>
Subject: Re: [PATCH 1/2] xen/mmu: Add workaround "x86-64, mm: Put early page
 table high"

On Thu, May 05, 2011 at 01:55:22PM +0100, Stefano Stabellini wrote:
> On Wed, 4 May 2011, Daniel Kiper wrote:
> > On Tue, May 03, 2011 at 09:51:41PM +0200, Daniel Kiper wrote:
> > > On Tue, May 03, 2011 at 11:12:06AM -0400, Konrad Rzeszutek Wilk wrote:
> > > > On Tue, May 03, 2011 at 02:55:27AM +0200, Daniel Kiper wrote:
> > > > > On Mon, May 02, 2011 at 01:22:21PM -0400, Konrad Rzeszutek Wilk wrote:
> > 
> > [...]
> > 
> > > > > I think that (Stefano please confirm or not) this patch was prepared
> > > > > as workaround for similar issues. However, I do not like this patch
> > > > > because on systems with small amount of memory it leaves huge (to some
> > > > > extent) hole between max_low_pfn and 4G. Additionally, it affects
> > > > > memory hotplug a bit because it allocates memory starting from current
> > > > > max_mfn. It also breaks memory hotplug on i386 (maybe also others
> > > > > thinks, however, I could not confirm that). If it stay for some
> > > > > reason it should be amended in follwing way:
> > > > >
> > > > > #ifdef CONFIG_X86_32
> > > > >         xen_extra_mem_start = mem_end;
> > > > > #else
> > > > >         xen_extra_mem_start = max((1ULL << 32), mem_end);
> > > > > #endif
> > > > >
> > > > > Regarding comment for this patch it should be mentioned that without this
> > > > > patch e820_end_of_low_ram_pfn() is not broken. It is not called simply.
> > > > >
> > > > > Last but least. I found that memory sizes below and including exactly 1 GiB and
> > > > > exactly 2 GiB, 3 GiB (maybe higher, i.e. 4 GiB, 5 GiB, ...; I was not able to test
> > > > > them because I do not have sufficient memory) are magic. It means that if memory
> > > > > is set with those sizes everything is working good (without 4b239f458c229de044d6905c2b0f9fe16ed9e01e
> > > > > and 24bdb0b62cc82120924762ae6bc85afc8c3f2b26 applied). It means that domU
> > > > > should be tested with sizes which are not power of two nor multiple of that.
> > > >
> > > > Hmm, I thought I did test 1500M.
> > >
> > > It does not work on my machine (24bdb0b62cc82120924762ae6bc85afc8c3f2b26
> > > removed and 4b239f458c229de044d6905c2b0f9fe16ed9e01e applied).
> > 
> > It does not work on my machine (x86_64) with Linux Kernel Ver. 2.6.39-rc6 without
> > git commit 24bdb0b62cc82120924762ae6bc85afc8c3f2b26 (xen: do not create the extra
> > e820 region at an addr lower than 4G). As I said ealier bug introduced by git
> > commit 4b239f458c229de044d6905c2b0f9fe16ed9e01e (x86-64, mm: Put early page table
> > high) is probably hidden (repaird/workarounded ???) by git commit
> > 24bdb0b62cc82120924762ae6bc85afc8c3f2b26 (xen: do not create the extra
> > e820 region at an addr lower than 4G).
> > 
> > Konrad, Stefano could you confirm that ??? If it is true
> > how could I help you in removing this bug ???
> 
> The reason why "xen: do not create the extra e820 region at an addr
> lower than 4G" is needed is the following:
> 
> starting the extra memory region at an address lower than 4G is
> dangerous because if the extra memory region spans the 4G boundary then
> e820_end_of_ram_pfn will get confused and return 0x100000000, therefore
> init_memory_mapping will setup 1:1 mapping for 0-0x100000000, including
> the whole 3G-4G range.
> Of course this is wrong because among other things will cause the kernel
> to remap the IOAPIC MMIO registers that should go through the fixmap
> instead.
> I don't think "x86-64, mm: Put early page table high" is related to the
> bug that this commit is trying to solve.
> 
> Could you please explain in details what problems do this patch create?
> 
> In any case you are right about the fact that the change is not needed
> on X86_32 so if it has any bad side effects on X86_32 we can always do

Why not? Can't you boot a 32-bit Dom0 on those machines?

> the following, like you suggested:
> 
> diff --git a/arch/x86/xen/setup.c b/arch/x86/xen/setup.c
> index 90bac0a..721f576 100644
> --- a/arch/x86/xen/setup.c
> +++ b/arch/x86/xen/setup.c
> @@ -227,7 +227,11 @@ char * __init xen_memory_setup(void)
>  
>  	memcpy(map_raw, map, sizeof(map));
>  	e820.nr_map = 0;
> +#ifdef CONFIG_X86_32
> +	xen_extra_mem_start = mem_end;
> +#else
>  	xen_extra_mem_start = max((1ULL << 32), mem_end);
> +#endif
>  	for (i = 0; i < memmap.nr_entries; i++) {
>  		unsigned long long end;
>  
> --
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