Messages in this thread | | | From | Vitaly Kuznetsov <> | Subject | Re: [PATCH] x86/intel_rdt: use rdmsr_safe() to workaround AWS host issue | Date | Thu, 10 Jan 2019 11:32:47 +0100 |
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Tony Luck <tony.luck@gmail.com> writes:
> On Wed, Jan 9, 2019 at 5:00 AM Borislav Petkov <bp@alien8.de> 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
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