linux-kernel - Re: [Xen-devel] ce56a86e2a ("x86/mm: Limit mmap() of /dev/mem to valid physical addresses"): kernel BUG at arch/x86/mm/physaddr.c:79!

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Date:   Fri, 27 Oct 2017 01:07:41 -0600
From:   "Jan Beulich" <JBeulich@...e.com>
To:     "Sander Eikelenboom" <linux@...elenboom.it>
Cc:     "LKP" <lkp@...org>, "Craig Bergstrom" <craigb@...gle.com>,
        "Fengguang Wu" <fengguang.wu@...el.com>,
        "Ingo Molnar" <mingo@...nel.org>,
        "Linus Torvalds" <torvalds@...ux-foundation.org>,
        <wfg@...ux.intel.com>, "Xen-devel" <xen-devel@...ts.xen.org>,
        "Boris Ostrovsky" <boris.ostrovsky@...cle.com>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [Xen-devel] ce56a86e2a ("x86/mm: Limit mmap() of /dev/mem
 to valid physical addresses"): kernel BUG at
 arch/x86/mm/physaddr.c:79!

>>> On 26.10.17 at 21:29, <linux@...elenboom.it> wrote:
> On 26/10/17 19:49, Craig Bergstrom wrote:
>> Sander, thanks for the details, they've been very useful.
>> 
>> I suspect that your host system's mem=2048M parameter is causing the
>> problem.  Any chance you can confirm by removing the parameter and
>> running the guest code path?
> 
> I removed it, but kept the hypervisor limiting dom0 memory to 2046M intact 
> (in grub using the xen bootcmd: 
> "multiboot       /xen-4.10.gz  dom0_mem=2048M,max:2048M ....."
> 
> Unfortunately that doesn't change anything, the guest still fails to start 
> with the same errors.
> 
>> More specifically, since you're telling the kernel that it's high
>> memory address is at 2048M and your device is at 0xfe1fe000 (~4G), the
>> new mmap() limits are preventing you from mapping addresses that are
>> explicitly disallowed by the parameter.
>> 
> 
> Which would probably mean the current patch prohibits hard limiting the dom0 
> memory to a certain value (below 4G)
> at least in combination with PCI-passthrough. So the only thing left would 
> be to have no hard memory restriction on dom0
> and rely on auto-ballooning, but I'm not a great fan of that.

Plus - how would things work with any RAM size if the PCI BAR was
a 64-bit one, pointing somewhere high up beyond 4Gb?

Jan