Date:	Wed, 9 Jul 2014 10:46:26 +0100
From:	David Vrabel <david.vrabel@...rix.com>
To:	Vitaly Kuznetsov <vkuznets@...hat.com>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	<xen-devel@...ts.xenproject.org>,
	Michael Holzheu <holzheu@...ux.vnet.ibm.com>,
	<linux-kernel@...r.kernel.org>, Vivek Goyal <vgoyal@...hat.com>
Subject: Re: [Xen-devel] [PATCH] mmap_vmcore: skip non-ram pages reported
 by hypervisors

On 09/07/14 10:17, Vitaly Kuznetsov wrote:
> David Vrabel <david.vrabel@...rix.com> writes:
> 
>> On 07/07/14 21:33, Andrew Morton wrote:
>>> On Mon,  7 Jul 2014 17:05:49 +0200 Vitaly Kuznetsov <vkuznets@...hat.com> wrote:
>>>
>>>> we have a special check in read_vmcore() handler to check if the page was
>>>> reported as ram or not by the hypervisor (pfn_is_ram()). However, when
>>>> vmcore is read with mmap() no such check is performed. That can lead to
>>>> unpredictable results, e.g. when running Xen PVHVM guest memcpy() after
>>>> mmap() on /proc/vmcore will hang processing HVMMEM_mmio_dm pages creating
>>>> enormous load in both DomU and Dom0.
>>
>> Does make forward progress though?  Or is it ending up in a repeatedly
>> retrying the same instruction?
> 
> If memcpy is using SSE2 optimization 16-byte 'movdqu' instruction never
> finishes (repeatedly retrying to issue two 8-byte requests to
> qemu-dm). qemu-dm decides that it's hitting 'Neither RAM nor known MMIO
> space' and returns 8 0xff bytes for both of this requests (I was testing
> with qemu-traditional).

Yes, the emulation of instructions with 16-byte operands is a bit
broken.  I should be fixed.

>> Is it failing on a ballooned page in a RAM region? Or is mapping non-RAM
>> regions as well?
> 
> I wasn't using ballooning, it happens that oldmem has several (two in my
> test) pages which are HVMMEM_mmio_dm but qemu-dm considers them being
> neither ram nor mmio.

I think this would also happen with ballooned pages, which are also
not-present in the p2m and thus would show up as HVMMEM_mmio_dm type and
accesses will also be forwarded to qemu (qemu gets everything by default).

>>>> Fix the issue by mapping each non-ram page to the zero page. Keep direct
>>>> path with remap_oldmem_pfn_range() to avoid looping through all pages on
>>>> bare metal.
>>>>
>>>> The issue can also be solved by overriding remap_oldmem_pfn_range() in
>>>> xen-specific code, as remap_oldmem_pfn_range() was been designed for.
>>>> That, however, would involve non-obvious xen code path for all x86 builds
>>>> with CONFIG_XEN_PVHVM=y and would prevent all other hypervisor-specific
>>>> code on x86 arch from doing the same override.
>>
>> The oldmem_pfn_is_ram() is Xen-specific but this problem (ballooned
>> pages) must be common to KVM.  How does KVM handle this?
> 
> Is far as I'm concearned the issue was never hit with KVM. I *think* the
> issue has something to do with the conjunction of 16-byte 'movdqu'
> emulation for io pages in xen hypervisor, 8-byte event channel requests
> and qemu-traditional. But even if it gets fixed on hypervisor side I
> believe fixing the issue kernel-side still worth it as there are
> non-fixed hypervisors out there (e.g. AWS EC2).

I think it would be preferrable to fix this on the hypervisor side so
Xen guests behaves in the same way as KVM guests.

But if this needs to work on non-fixed hypervisors then this patch looks
sensible.  FWIW,

Acked-by: David Vrabel <david.vrabel@...rix.com>

David
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