[Xen-users] a few questions about superpage support

[Antonin Bas <antoninb@xxxxxxxxxxxxxxx>]

Hi,

I am working on a project that relies on superpages within a guest. Of
course these superpages need to be backed by actual machine pages.

I am using this version of Xen:
(XEN) Xen version 4.2.2_04-0.7.5 (abuild@) (gcc (SUSE Linux) 4.3.4
[gcc-4_3-branch revision 152973]) Fri Jun 14 12:22:34 UTC 2013


HAP is enabled:
...
(XEN) VMX: Supported advanced features:
(XEN)  - APIC MMIO access virtualisation
(XEN)  - APIC TPR shadow
(XEN)  - Extended Page Tables (EPT)
(XEN)  - Virtual-Processor Identifiers (VPID)
(XEN)  - Virtual NMI
(XEN)  - MSR direct-access bitmap
(XEN)  - Unrestricted Guest
(XEN)  - APIC Register Virtualization
(XEN)  - Virtual Interrupt Delivery
(XEN) HVM: ASIDs enabled.
(XEN) HVM: VMX enabled
(XEN) HVM: Hardware Assisted Paging (HAP) detected
(XEN) HVM: HAP page sizes: 4kB, 2MB, 1GB
...

I am  using the boot line option allowsuperpage=1 and the guest config
includes superpages=1


I have a few questions:

1) How can I check that the the VMM is actually backing the 2M
hugepages I request from within the guest with actual machine 2M
hugepages. My first idea was to dump the EPT using 'xl debug D'.
Unfortunately, it seems that the ring buffer for the console is not
large enough and I cannot see all the entries with 'xl dmesg'. Even
when I redirect the console output to /var/log/xen/console (using
XENCONSOLED_TRACE="all" in /etc/sysconfig/xencommons), I am still
missing entries. Is there a way to increase the ring size to see all
the entries?

With a guest memory of 4096, I can observe EPT entries that look like this:
(XEN) gfn: 10f600            mfn: 306c00            order:  9  is_pod: 0
At first I though they meant that guest 2M superpages were indeed
being backed by 2M host machine superpages. I though this was weird
since I could observe these entries even without explicitly requesting
hugepages from within the guest. I set transparent hugepages in the
guest to never (seems to be enabled by default in SUSE) but I could
still observe these 'order: 9' entries, which means I don't actually
know what they represent.

2) 1GB superpage support. When I try to request 1GB in the guest at
boot time, I get the following message from the kernel: "hugepagesz:
Unsupported page size 1024 M", which is not a surprise since the
pdpe1gb cpu flag is not enabled. How can I enable this flag for the
domU vcpus? If this flag can be enabled, will the VMM try to map my
guest 1GB superpages to host physical 1GB hugepage in the EPT?

Thanks,

Antonin

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