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Date: Thu, 17 Sep 2020 09:29:43 -0700 From: Sean Christopherson <sean.j.christopherson@...el.com> To: Maxim Levitsky <mlevitsk@...hat.com> Cc: kvm@...r.kernel.org, Vitaly Kuznetsov <vkuznets@...hat.com>, Ingo Molnar <mingo@...hat.com>, Wanpeng Li <wanpengli@...cent.com>, "H. Peter Anvin" <hpa@...or.com>, Borislav Petkov <bp@...en8.de>, Jim Mattson <jmattson@...gle.com>, Paolo Bonzini <pbonzini@...hat.com>, Joerg Roedel <joro@...tes.org>, "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>, linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de> Subject: Re: [PATCH v4 2/2] KVM: nSVM: implement ondemand allocation of the nested state On Thu, Sep 17, 2020 at 01:10:48PM +0300, Maxim Levitsky wrote: > This way we don't waste memory on VMs which don't use > nesting virtualization even if it is available to them. > > If allocation of nested state fails (which should happen, > only when host is about to OOM anyway), use new KVM_REQ_OUT_OF_MEMORY > request to shut down the guest > > Signed-off-by: Maxim Levitsky <mlevitsk@...hat.com> > --- > arch/x86/kvm/svm/nested.c | 42 ++++++++++++++++++++++++++++++ > arch/x86/kvm/svm/svm.c | 54 ++++++++++++++++++++++----------------- > arch/x86/kvm/svm/svm.h | 7 +++++ > 3 files changed, 79 insertions(+), 24 deletions(-) > > diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c > index 09417f5197410..fe119da2ef836 100644 > --- a/arch/x86/kvm/svm/nested.c > +++ b/arch/x86/kvm/svm/nested.c > @@ -467,6 +467,9 @@ int nested_svm_vmrun(struct vcpu_svm *svm) > > vmcb12 = map.hva; > > + if (WARN_ON(!svm->nested.initialized)) > + return 1; > + > if (!nested_vmcb_checks(svm, vmcb12)) { > vmcb12->control.exit_code = SVM_EXIT_ERR; > vmcb12->control.exit_code_hi = 0; > @@ -684,6 +687,45 @@ int nested_svm_vmexit(struct vcpu_svm *svm) > return 0; > } > > +int svm_allocate_nested(struct vcpu_svm *svm) > +{ > + struct page *hsave_page; > + > + if (svm->nested.initialized) > + return 0; > + > + hsave_page = alloc_page(GFP_KERNEL_ACCOUNT | __GFP_ZERO); > + if (!hsave_page) > + goto error; goto is unnecessary, just do return -ENOMEM; > + > + svm->nested.hsave = page_address(hsave_page); > + > + svm->nested.msrpm = svm_vcpu_init_msrpm(); > + if (!svm->nested.msrpm) > + goto err_free_hsave; > + > + svm->nested.initialized = true; > + return 0; > +err_free_hsave: > + __free_page(hsave_page); > +error: > + return 1; As above, -ENOMEM would be preferable. > +} > + > +void svm_free_nested(struct vcpu_svm *svm) > +{ > + if (!svm->nested.initialized) > + return; > + > + svm_vcpu_free_msrpm(svm->nested.msrpm); > + svm->nested.msrpm = NULL; > + > + __free_page(virt_to_page(svm->nested.hsave)); > + svm->nested.hsave = NULL; > + > + svm->nested.initialized = false; > +} > + > /* > * Forcibly leave nested mode in order to be able to reset the VCPU later on. > */ > diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c > index 3da5b2f1b4a19..57ea4407dcf09 100644 > --- a/arch/x86/kvm/svm/svm.c > +++ b/arch/x86/kvm/svm/svm.c > @@ -266,6 +266,7 @@ static int get_max_npt_level(void) > void svm_set_efer(struct kvm_vcpu *vcpu, u64 efer) > { > struct vcpu_svm *svm = to_svm(vcpu); > + u64 old_efer = vcpu->arch.efer; > vcpu->arch.efer = efer; > > if (!npt_enabled) { > @@ -276,9 +277,26 @@ void svm_set_efer(struct kvm_vcpu *vcpu, u64 efer) > efer &= ~EFER_LME; > } > > - if (!(efer & EFER_SVME)) { > - svm_leave_nested(svm); > - svm_set_gif(svm, true); > + if ((old_efer & EFER_SVME) != (efer & EFER_SVME)) { > + if (!(efer & EFER_SVME)) { > + svm_leave_nested(svm); > + svm_set_gif(svm, true); > + > + /* > + * Free the nested state unless we are in SMM, in which > + * case the exit from SVM mode is only for duration of the SMI > + * handler > + */ > + if (!is_smm(&svm->vcpu)) > + svm_free_nested(svm); > + > + } else { > + if (svm_allocate_nested(svm)) { > + vcpu->arch.efer = old_efer; > + kvm_make_request(KVM_REQ_OUT_OF_MEMORY, vcpu); I really dislike KVM_REQ_OUT_OF_MEMORY. It's redundant with -ENOMEM and creates a huge discrepancy with respect to existing code, e.g. nVMX returns -ENOMEM in a similar situation. The deferred error handling creates other issues, e.g. vcpu->arch.efer is unwound but the guest's RIP is not. One thought for handling this without opening a can of worms would be to do: r = kvm_x86_ops.set_efer(vcpu, efer); if (r) { WARN_ON(r > 0); return r; } I.e. go with the original approach, but only for returning errors that will go all the way out to userspace. > + return; > + } > + } > } > > svm->vmcb->save.efer = efer | EFER_SVME;
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