Messages in this thread | | | From | Vitaly Kuznetsov <> | Subject | Re: [Xen-devel] [PATCH] mmap_vmcore: skip non-ram pages reported by hypervisors | Date | Wed, 09 Jul 2014 11:17:13 +0200 |
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David Vrabel <david.vrabel@citrix.com> writes:
> On 07/07/14 21:33, Andrew Morton wrote: >> On Mon, 7 Jul 2014 17:05:49 +0200 Vitaly Kuznetsov <vkuznets@redhat.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).
> > 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.
> >>> 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).
> > David
-- Vitaly
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