[vbox-dev] HGCM calls slow on multicore guest (linux)

Höhn Thomas Hoehn.Thomas at heidenhain.de
Mon Jun 8 14:15:38 GMT 2015

I wrote a VBox host-service for my company (since 2 years) using the HGCM API to do data exchange (1K-64K size) between host/guest. We have chosen HGCM since it's the only host/guest API provided so far and we don't wanna do changes with later VBox versions. (Note: data exchange via network, e.g. host-only iface is no option for our use-case)

The host service on the Linux guest is written as Lib using VBoxGuestLib calls. For the Windows 7 host I build an ExtensionPack (to load HGCM module during runtime, since VBox 4.3.0). I also managed Dll signing on the Win host (separate story) which runs fine for VBox > 4.3.14. The Linux guest runs a PREEMPT RT kernel.

Problem for us now is that we need near "real-time" host/guest communication in future due to a customer request.

With only one core for the Linux VM the guest side host-calls have a reasonable (monotonic) execution time between 40-160us when measured with clock_gettime.

But if I spend >=2 cores for the VM the HGCM calls on the guest take significant more time, e.g. for 2 cores the guest-side host call time then rises to 200us - 1ms. For 4 or 6 cores (on a true 6-core CPU) its even worse. This degrades our host/guest communication speed badly (running in a <20ms task cycle).

On host side the host calls (measured in svcCall) show fixed exec time - sure, as they do not depend on guest VM core number. So timing issue is on guest side only.

I made ftrace measurements on the guest to record the syscalls and I see the sys_ioctl start and end and what happens between but it doesn't lighten me up why it takes so long.

For the HGCM data (mostly ~1K size) I use type VMMDevHGCMParmType_LinAddr_In and LinAddr_Out. In HGCMInternal.cpp there is a lot of copying for this data type. Should I use another, e.g. PageList (with less copy overhead)? What is LinAddr_Lock_In/Out for?

If anyone has an idea why HGCM calls get slower with more cores and possibly how to fix that or how to make the HGCM calls faster I would be grateful.

Thanks for advice,

Thomas Höhn
83301 Traunreut, Deutschland

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