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Bug: toolstack allows too low values to be set for shadow_memory
- To: "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- From: Hans van Kranenburg <hans@xxxxxxxxxxx>
- Date: Mon, 25 May 2020 17:51:32 +0200
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- Delivery-date: Mon, 25 May 2020 15:51:54 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
This bug report is a follow-up to the thread "Domu windows 2012 crash"
on the xen-users list. In there we found out that it is possible to set
a value for shadow_memory that is lower than a safe minimum value.
This became apparent after XSA-304, which caused using more of this type
of memory. Having a hardcoded line like shadow_memory = 8 results in
random crashes of the guest, with the following errors in xl dmesg of
the host:
(XEN) Failed to shatter gfn 105245: -12
(XEN) d75v1 EPT violation 0x19c (--x/rw-) gpa 0x00000105245760 mfn
0x285245 type 0
(XEN) d75v1 Walking EPT tables for GFN 105245:
(XEN) d75v1 epte 9c000004105f9007
(XEN) d75v1 epte 9c000002800000f3
(XEN) d75v1 --- GLA 0x7ff98b40d760
(XEN) domain_crash called from vmx.c:3497
(XEN) Domain 75 (vcpu#1) crashed on cpu#4:
(XEN) ----[ Xen-4.11.4-pre x86_64 debug=n Not tainted ]----
(XEN) CPU: 4
(XEN) RIP: 0033:[<00007ff98b40d760>]
(XEN) RFLAGS: 0000000000010216 CONTEXT: hvm guest (d75v1)
(XEN) rax: 0000000000001212 rbx: 000000c714d9da58 rcx: 0000023500001590
(XEN) rdx: 000000c700000001 rsi: 000000c714d9da18 rdi: 000000c714d9db20
(XEN) rbp: 000000c714d9d950 rsp: 000000c714d9d918 r8: 0000023500001470
(XEN) r9: 00000235000014f0 r10: 00007ff99c5c0923 r11: 000000c714d9d970
(XEN) r12: 0000000000000000 r13: 000000c714d9d9d0 r14: 000000c714d9da58
(XEN) r15: 0000000000000006 cr0: 0000000080050031 cr4: 0000000000060678
(XEN) cr3: 00000001d9458002 cr2: 00007ff98b6fa048
(XEN) fsb: 0000000000000000 gsb: 000000c714e9e000 gss: ffffcd015dc40000
(XEN) ds: 002b es: 002b fs: 0053 gs: 002b ss: 002b cs: 0033
Why do users have this shadow_memory line in their guest config file?
Well, I guess it's simply because there are not many proper examples of
how to run Windows under Xen, so whoever is searching ze interwobz for
it will end up on a page like...
https://www.howtoforge.com/how-to-run-fully-virtualized-guests-hvm-with-xen-3.2-on-debian-lenny-x86_64
...and then start copy pasting and adjusting until things work. Or...
already have such a config for 10+ years and just only adjust things
when it breaks after upgrading Xen.
Hans van Kranenburg
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