Tony Luck <[email protected]> writes: > On Wed, Jan 9, 2019 at 5:00 AM Borislav Petkov <[email protected]> wrote: >> >> On Wed, Jan 09, 2019 at 01:09:31PM +0100, Vitaly Kuznetsov wrote: >> > Hm, why is that? In theory, hypervisors can pass through or emulate the >> > required MSRs... >> >> ...and when the theory becomes reality we'll remove the check. > > In practice that may be a long time coming. We don't have many CLOSIDs, or > bits in a cache mask, at the h/w level. If you start trying to > subdivide those resources to pass a subset to a guest, then you'll > quickly find that you have no flexibility in the guest to do anything > useful. It would only work if you limited to two, or perhaps three > guests.
Running a single guest on a physical CPU is a very common scenario. In fact, sharing cores is very rare for public clouds: e.g. all worthy instance types on AWS/Azure give you dedicated cores and I don't see why hypervisor can't pass through resctl features. The other thing is: how can we be sure that there's no hypervisor exposing these feature already? Even if open-source hypervisors like KVM/Xen don't do it it doesn't prove anything: there are numerous proprietary hypervisors and who knows what they do. The original issue which triggered the discussion was discovered on AWS Xen where the host is buggy and I suggested a simple short-term workaround, I'm no expert in rdt/qos so I'm leaving this up to the maintainers to decide which fix deserves to go in (if any). -- Vitaly
