Re: [PATCH v2 1/1] qemu: Add support for RAPL MSRs feature

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.