On 05/05/2020 09:20, Jan Beulich wrote: > If the hardware can handle accesses, we should allow it to do so. This > way we can expose EFRO to HVM guests,
I'm reminded now of the conversation I'm sure we've had before, although I have a feeling it was on IRC. APERF/MPERF (including the EFRO interface on AMD) are free running counters but only in C0. The raw values are not synchronised across threads. A vCPU which gets rescheduled has a 50% chance of finding the one or both values going backwards, and a 100% chance of totally bogus calculation. There is no point exposing APERF/MPERF to guests. It can only be used safely in hypervisor context, on AMD hardware with a CLGI/STGI region, or on Intel hardware in an NMI handler if you trust that a machine check isn't going to ruin your day. VMs have no way of achieving the sampling requirements, and has a fair chance of getting a plausible-but-wrong answer. The only possibility to do this safely is on a VM which is pinned to pCPU for its lifetime, but even I'm unconvinced of the correctness. I don't think we should be exposing this functionality to guests at all, although I might be persuaded if someone wanting to use it in a VM can provide a concrete justification of why the above problems won't get in their way. ~Andrew
