Re: [PATCH 2/3] KVM: x86: hyper-v: set NoNonArchitecturalCoreSharing CPUID bit when SMT is impossible

[Jim Mattson <jmattson@google.com>]

On Mon, Sep 16, 2019 at 9:23 AM Vitaly Kuznetsov <vkuznets@redhat.com> 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@redhat.com>
> ---
>  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
>