On Sat, Feb 01, 2020 at 01:47:10AM +0800, Xiaoyao Li wrote: > On 1/31/2020 11:37 PM, Andy Lutomirski wrote: > > > >>On Jan 30, 2020, at 11:22 PM, Xiaoyao Li <xiaoyao.li@xxxxxxxxx> wrote: > >> > >> On 1/31/2020 1:16 AM, Andy Lutomirski wrote: ... > >>>Can we get a credible description of how this would work? I suggest: Intel > >>>adds and documents a new CPUID bit or core capability bit that means > >>>“split lock detection is forced on”. If this bit is set, the MSR bit > >>>controlling split lock detection is still writable, but split lock > >>>detection is on regardless of the value. Operating systems are expected > >>>to set the bit to 1 to indicate to a hypervisor, if present, that they > >>>understand that split lock detection is on. This would be an SDM-only > >>>change, but it would also be a commitment to certain behavior for future > >>>CPUs that don’t implement split locks. > >> > >>It sounds a PV solution for virtualization that it doesn't need to be > >>defined in Intel-SDM but in KVM document. > >> > >>As you suggested, we can define new bit in KVM_CPUID_FEATURES (0x40000001) > >>as KVM_FEATURE_SLD_FORCED and reuse MSR_TEST_CTL or use a new virtualized > >>MSR for guest to tell hypervisor it understand split lock detection is > >>forced on. > > > >Of course KVM can do this. But this missed the point. Intel added a new CPU > >feature, complete with an enumeration mechanism, that cannot be correctly > >used if a hypervisor is present. > > Why it cannot be correctly used if a hypervisor is present? Because it needs > to disable split lock detection when running a vcpu for guest as this patch > wants to do? Because SMT. Unless vCPUs are pinned 1:1 with pCPUs, and the guest is given an accurate topology, disabling/enabling split-lock #AC may (or may not) also disable/enable split-lock #AC on a random vCPU in the guest. > >As it stands, without specific hypervisor and guest support of a non-Intel > >interface, it is *impossible* to give architecturally correct behavior to a > >guest. If KVM implements your suggestion, *Windows* guests will still > >malfunction on Linux. > > Actually, KVM don't need to implement my suggestion. It can just virtualize > and expose this feature (MSR_IA32_CORE_CAPABILITIES and MSR_TEST_CTRL) to > guest, (but it may have some requirement that HT is disabled and host is > sld_off) then guest can use it architecturally. This is essentially what I proposed a while back. KVM would allow enabling split-lock #AC in the guest if and only if SMT is disabled or the enable bit is per-thread, *or* the host is in "warn" mode (can live with split-lock #AC being randomly disabled/enabled) and userspace has communicated to KVM that it is pinning vCPUs.
