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Message-ID: <875xghoaac.fsf@redhat.com>
Date: Fri, 27 Jun 2025 10:31:39 +0200
From: Vitaly Kuznetsov <vkuznets@...hat.com>
To: Jeremi Piotrowski <jpiotrowski@...ux.microsoft.com>, Sean Christopherson
<seanjc@...gle.com>, Paolo Bonzini <pbonzini@...hat.com>,
kvm@...r.kernel.org
Cc: Dave Hansen <dave.hansen@...ux.intel.com>, linux-kernel@...r.kernel.org,
alanjiang@...rosoft.com, chinang.ma@...rosoft.com,
andrea.pellegrini@...rosoft.com, Kevin Tian <kevin.tian@...el.com>, "K. Y.
Srinivasan" <kys@...rosoft.com>, Haiyang Zhang <haiyangz@...rosoft.com>,
Wei Liu <wei.liu@...nel.org>, Dexuan Cui <decui@...rosoft.com>,
linux-hyperv@...r.kernel.org, Jeremi Piotrowski
<jpiotrowski@...ux.microsoft.com>
Subject: Re: [RFC PATCH 1/1] KVM: VMX: Use Hyper-V EPT flush for local TLB
flushes
Jeremi Piotrowski <jpiotrowski@...ux.microsoft.com> writes:
> Use Hyper-V's HvCallFlushGuestPhysicalAddressSpace for local TLB flushes.
> This makes any KVM_REQ_TLB_FLUSH_CURRENT (such as on root alloc) visible to
> all CPUs which means we no longer need to do a KVM_REQ_TLB_FLUSH on CPU
> migration.
>
> The goal is to avoid invept-global in KVM_REQ_TLB_FLUSH. Hyper-V uses a
> shadow page table for the nested hypervisor (KVM) and has to invalidate all
> EPT roots when invept-global is issued. This has a performance impact on
> all nested VMs. KVM issues KVM_REQ_TLB_FLUSH on CPU migration, and under
> load the performance hit causes vCPUs to use up more of their slice of CPU
> time, leading to more CPU migrations. This has a snowball effect and causes
> CPU usage spikes.
>
> By issuing the hypercall we are now guaranteed that any root modification
> that requires a local TLB flush becomes visible to all CPUs. The same
> hypercall is already used in kvm_arch_flush_remote_tlbs and
> kvm_arch_flush_remote_tlbs_range. The KVM expectation is that roots are
> flushed locally on alloc and we achieve consistency on migration by
> flushing all roots - the new behavior of achieving consistency on alloc on
> Hyper-V is a superset of the expected guarantees. This makes the
> KVM_REQ_TLB_FLUSH on CPU migration no longer necessary on Hyper-V.
Sounds reasonable overall, my only concern (not sure if valid or not) is
that using the hypercall for local flushes is going to be more expensive
than invept-context we do today and thus while the performance is
improved for the scenario when vCPUs are migrating a lot, we will take a
hit in other cases.
>
> Coincidentally - we now match the behavior of SVM on Hyper-V.
>
> Signed-off-by: Jeremi Piotrowski <jpiotrowski@...ux.microsoft.com>
> ---
> arch/x86/include/asm/kvm_host.h | 1 +
> arch/x86/kvm/vmx/vmx.c | 20 +++++++++++++++++---
> arch/x86/kvm/vmx/vmx_onhyperv.h | 6 ++++++
> arch/x86/kvm/x86.c | 3 +++
> 4 files changed, 27 insertions(+), 3 deletions(-)
>
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index b4a391929cdb..d3acab19f425 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -1077,6 +1077,7 @@ struct kvm_vcpu_arch {
>
> #if IS_ENABLED(CONFIG_HYPERV)
> hpa_t hv_root_tdp;
> + bool hv_vmx_use_flush_guest_mapping;
> #endif
> };
>
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 4953846cb30d..f537e0df56fc 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -1485,8 +1485,12 @@ void vmx_vcpu_load_vmcs(struct kvm_vcpu *vcpu, int cpu)
> /*
> * Flush all EPTP/VPID contexts, the new pCPU may have stale
> * TLB entries from its previous association with the vCPU.
> + * Unless we are running on Hyper-V where we promotes local TLB
s,promotes,promote, or, as Sean doesn't like pronouns,
"... where local TLB flushes are promoted ..."
> + * flushes to be visible across all CPUs so no need to do again
> + * on migration.
> */
> - kvm_make_request(KVM_REQ_TLB_FLUSH, vcpu);
> + if (!vmx_hv_use_flush_guest_mapping(vcpu))
> + kvm_make_request(KVM_REQ_TLB_FLUSH, vcpu);
>
> /*
> * Linux uses per-cpu TSS and GDT, so set these when switching
> @@ -3243,11 +3247,21 @@ void vmx_flush_tlb_current(struct kvm_vcpu *vcpu)
> if (!VALID_PAGE(root_hpa))
> return;
>
> - if (enable_ept)
> + if (enable_ept) {
> + /*
> + * hyperv_flush_guest_mapping() has the semantics of
> + * invept-single across all pCPUs. This makes root
> + * modifications consistent across pCPUs, so an invept-global
> + * on migration is no longer required.
> + */
> + if (vmx_hv_use_flush_guest_mapping(vcpu))
> + return (void)WARN_ON_ONCE(hyperv_flush_guest_mapping(root_hpa));
> +
HvCallFlushGuestPhysicalAddressSpace sounds like a heavy operation as it
affects all processors. Is there any visible perfomance impact of this
change when there are no migrations (e.g. with vCPU pinning)? Or do we
believe that Hyper-V actually handles invept-context the exact same way?
> ept_sync_context(construct_eptp(vcpu, root_hpa,
> mmu->root_role.level));
> - else
> + } else {
> vpid_sync_context(vmx_get_current_vpid(vcpu));
> + }
> }
>
> void vmx_flush_tlb_gva(struct kvm_vcpu *vcpu, gva_t addr)
> diff --git a/arch/x86/kvm/vmx/vmx_onhyperv.h b/arch/x86/kvm/vmx/vmx_onhyperv.h
> index cdf8cbb69209..a5c64c90e49e 100644
> --- a/arch/x86/kvm/vmx/vmx_onhyperv.h
> +++ b/arch/x86/kvm/vmx/vmx_onhyperv.h
> @@ -119,6 +119,11 @@ static inline void evmcs_load(u64 phys_addr)
> }
>
> void evmcs_sanitize_exec_ctrls(struct vmcs_config *vmcs_conf);
> +
> +static inline bool vmx_hv_use_flush_guest_mapping(struct kvm_vcpu *vcpu)
> +{
> + return vcpu->arch.hv_vmx_use_flush_guest_mapping;
> +}
> #else /* !IS_ENABLED(CONFIG_HYPERV) */
> static __always_inline bool kvm_is_using_evmcs(void) { return false; }
> static __always_inline void evmcs_write64(unsigned long field, u64 value) {}
> @@ -128,6 +133,7 @@ static __always_inline u64 evmcs_read64(unsigned long field) { return 0; }
> static __always_inline u32 evmcs_read32(unsigned long field) { return 0; }
> static __always_inline u16 evmcs_read16(unsigned long field) { return 0; }
> static inline void evmcs_load(u64 phys_addr) {}
> +static inline bool vmx_hv_use_flush_guest_mapping(struct kvm_vcpu *vcpu) { return false; }
> #endif /* IS_ENABLED(CONFIG_HYPERV) */
>
> #endif /* __ARCH_X86_KVM_VMX_ONHYPERV_H__ */
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index b58a74c1722d..cbde795096a6 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -25,6 +25,7 @@
> #include "tss.h"
> #include "kvm_cache_regs.h"
> #include "kvm_emulate.h"
> +#include "kvm_onhyperv.h"
> #include "mmu/page_track.h"
> #include "x86.h"
> #include "cpuid.h"
> @@ -12390,6 +12391,8 @@ int kvm_arch_vcpu_create(struct kvm_vcpu *vcpu)
>
> #if IS_ENABLED(CONFIG_HYPERV)
> vcpu->arch.hv_root_tdp = INVALID_PAGE;
> + vcpu->arch.hv_vmx_use_flush_guest_mapping =
> + (kvm_x86_ops.flush_remote_tlbs == hv_flush_remote_tlbs);
> #endif
>
> r = kvm_x86_call(vcpu_create)(vcpu);
--
Vitaly
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