On Mon, Sep 16, 2019 at 06:22:57PM +0200, Vitaly Kuznetsov wrote: > Hyper-V 2019 doesn't expose MD_CLEAR CPUID bit to guests when it cannot > guarantee that two virtual processors won't end up running on sibling SMT > threads without knowing about it. This is done as an optimization as in > this case there is nothing the guest can do to protect itself against MDS > and issuing additional flush requests is just pointless. On bare metal the > topology is known, however, when Hyper-V is running nested (e.g. on top of > KVM) it needs an additional piece of information: a confirmation that the > exposed topology (wrt vCPU placement on different SMT threads) is > trustworthy. > > NoNonArchitecturalCoreSharing (CPUID 0x40000004 EAX bit 18) is described in > TLFS as follows: "Indicates that a virtual processor will never share a > physical core with another virtual processor, except for virtual processors > that are reported as sibling SMT threads." From KVM we can give such > guarantee in two cases: > - SMT is unsupported or forcefully disabled (just 'disabled' doesn't work > as it can become re-enabled during the lifetime of the guest). > - vCPUs are properly pinned so the scheduler won't put them on sibling > SMT threads (when they're not reported as such). > > This patch reports NoNonArchitecturalCoreSharing bit in to userspace in the > first case. The second case is outside of KVM's domain of responsibility > (as vCPU pinning is actually done by someone who manages KVM's userspace - > e.g. libvirt pinning QEMU threads). This is purely about guest<->guest MDS, right? Ie. not worse than actual hardware.
