On Mon, Sep 16, 2019 at 9:23 AM Vitaly Kuznetsov <vkuznets@xxxxxxxxxx> 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). That's a nice bit of information. Have you considered a mechanism for communicating this information to kvm guests in a way that doesn't require Hyper-V enlightenments? > 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). > > Signed-off-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx> > --- > arch/x86/include/asm/hyperv-tlfs.h | 7 +++++++ > arch/x86/kvm/hyperv.c | 4 +++- > 2 files changed, 10 insertions(+), 1 deletion(-) > > diff --git a/arch/x86/include/asm/hyperv-tlfs.h b/arch/x86/include/asm/hyperv-tlfs.h > index af78cd72b8f3..989a1efe7f5e 100644 > --- a/arch/x86/include/asm/hyperv-tlfs.h > +++ b/arch/x86/include/asm/hyperv-tlfs.h > @@ -170,6 +170,13 @@ > /* Recommend using enlightened VMCS */ > #define HV_X64_ENLIGHTENED_VMCS_RECOMMENDED BIT(14) > > +/* > + * Virtual processor will never share a physical core with another virtual > + * processor, except for virtual processors that are reported as sibling SMT > + * threads. > + */ > +#define HV_X64_NO_NONARCH_CORESHARING BIT(18) > + > /* Nested features. These are HYPERV_CPUID_NESTED_FEATURES.EAX bits. */ > #define HV_X64_NESTED_GUEST_MAPPING_FLUSH BIT(18) > #define HV_X64_NESTED_MSR_BITMAP BIT(19) > diff --git a/arch/x86/kvm/hyperv.c b/arch/x86/kvm/hyperv.c > index fff790a3f4ee..9c187d16a9cd 100644 > --- a/arch/x86/kvm/hyperv.c > +++ b/arch/x86/kvm/hyperv.c > @@ -23,6 +23,7 @@ > #include "ioapic.h" > #include "hyperv.h" > > +#include <linux/cpu.h> > #include <linux/kvm_host.h> > #include <linux/highmem.h> > #include <linux/sched/cputime.h> > @@ -1864,7 +1865,8 @@ int kvm_vcpu_ioctl_get_hv_cpuid(struct kvm_vcpu *vcpu, struct kvm_cpuid2 *cpuid, > ent->eax |= HV_X64_EX_PROCESSOR_MASKS_RECOMMENDED; > if (evmcs_ver) > ent->eax |= HV_X64_ENLIGHTENED_VMCS_RECOMMENDED; > - > + if (!cpu_smt_possible()) > + ent->eax |= HV_X64_NO_NONARCH_CORESHARING; > /* > * Default number of spinlock retry attempts, matches > * HyperV 2016. > -- > 2.20.1 >
