On Tue, Sep 03, 2024 at 13:29:28 +0200, Anthony Harivel wrote:
Daniel P. Berrangé, Sep 03, 2024 at 12:08:
On Mon, Sep 02, 2024 at 03:09:42PM +0200, Peter Krempa wrote:
On Thu, Aug 22, 2024 at 17:59:47 +0200, Anthony Harivel wrote:
Add the support in libvirt to activate the RAPL feature in QEMU. I suppose that the 'rapl-helper-socket' is a shared (multiple qemu's use it) resource set up beforehand by the admin. Right?
The qemu-pr-helper could be run as a single instnce, or it could be run per-QEMU instance. The latter would give us better security isolation, for what is a privileged daemon. On the other hand, I wonder about the CPU overhead of having 100's of copies of the process running on a host.
So when I was originally skimming trhough the docs I didn't properly understand that the reason for the helper daemon is that there was a security issue with accessing the measurement counters and thought it was strictly for performance reasons.
If it runs on a single instance, then the socket needs to be chmod/chown to something like qemu / libvirt group with access only to root and group.
Another alternative for a shared instance to be used by multiple qemu instances is that libvirt can open the socket and pass it to qemu, which avoids the potential issue at-least with DAC security labels as the socket can be owned by root:root with mode 600. I'm not sure how the selinux policy for that daemon looks though.
Running one helper instance per-QEMU instance would mean that every instance read 1 MSR / Package every second. The socket is left open (thanks to Daniel suggestion in QEMU review). The impact would be quite low I guess on the housekeeping CPU.
When I designed the daemon with Paolo, the first solution was the main idea but I'm open to any solution that leads to a better adoption of the feature.
Libvirt can obviously manage also a per VM instance, which should be straightforward, but not as simple as this patch. This can theoretically also be added later, e.g. by adding a 'managed' property enabling the libvirt-managed daemon.