linux-kernel - Re: [Xen-devel] 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 13:34:48 +0200
From:	Daniel Kiper <dkiper@...-space.pl>
To:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
Cc:	Daniel Kiper <dkiper@...-space.pl>, linux-kernel@...r.kernel.org,
	stefano.stabellini@...citrix.com, yinghai@...nel.org,
	hpa@...or.com, xen-devel@...ts.xensource.com
Subject: Re: [Xen-devel] Re: [PATCH 1/2] xen/mmu: Add workaround "x86-64, mm: Put early page table high"

On Wed, May 04, 2011 at 03:33:53PM -0400, Konrad Rzeszutek Wilk wrote:
> On Wed, May 04, 2011 at 08:59:03PM +0200, 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
> 
> It was actually to fix SandyBridge boxes. Their last E820 reserved
> region was around fed40000 and then the RAM region started at
> 100000000. Which meant that we misinterpreted the gap (starting at fed40 mfn)
> as the start of RAM.

Thanks.

> > > > > 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.
> 
> Hmm. What is max_pfn set to?
> Can you send the full dmesg of your guest?

Look into attachments. Both dmesgs are from plain 2.6.39-rc6.
Guests had allocated 2 GiB of memory.

> > > > > 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).
> 
> There are a couple of things that have been going to fix "x86-64, mm: Put
> early page table high" and also .. "cleanup highmem" (something) - which
> has been plaguing us since 2.6.32 (and was the one you hit long time ago).
> 
> Anyhow, regarding the setting xen_extra_mem_start to 4GB or higher should
> be reworked. Not sure yet how. 

OK. As I can see it is __VERY__ difficult problem. I will wait for
proper solution. However, if I could help you in any way
please drop me a line.

Daniel

View attachment "dmesg.i386" of type "text/plain" (8003 bytes)

View attachment "dmesg.x86_64" of type "text/plain" (8094 bytes)