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Date: Mon, 14 Dec 2020 13:00:36 -0600
From: Tom Lendacky <thomas.lendacky@....com>
To: Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
linux-kernel@...r.kernel.org, x86@...nel.org
Cc: Jim Mattson <jmattson@...gle.com>, Joerg Roedel <joro@...tes.org>,
Sean Christopherson <seanjc@...gle.com>,
Vitaly Kuznetsov <vkuznets@...hat.com>,
Wanpeng Li <wanpengli@...cent.com>,
Borislav Petkov <bp@...en8.de>, Ingo Molnar <mingo@...hat.com>,
Thomas Gleixner <tglx@...utronix.de>,
Brijesh Singh <brijesh.singh@....com>
Subject: Re: [PATCH v5 07/34] KVM: SVM: Add required changes to support
intercepts under SEV-ES
On 12/14/20 9:33 AM, Paolo Bonzini wrote:
> On 10/12/20 18:09, Tom Lendacky wrote:
>> @@ -2797,7 +2838,27 @@ static int svm_set_msr(struct kvm_vcpu *vcpu,
>> struct msr_data *msr)
>> static int wrmsr_interception(struct vcpu_svm *svm)
>> {
>> - return kvm_emulate_wrmsr(&svm->vcpu);
>> + u32 ecx;
>> + u64 data;
>> +
>> + if (!sev_es_guest(svm->vcpu.kvm))
>> + return kvm_emulate_wrmsr(&svm->vcpu);
>> +
>> + ecx = kvm_rcx_read(&svm->vcpu);
>> + data = kvm_read_edx_eax(&svm->vcpu);
>> + if (kvm_set_msr(&svm->vcpu, ecx, data)) {
>> + trace_kvm_msr_write_ex(ecx, data);
>> + ghcb_set_sw_exit_info_1(svm->ghcb, 1);
>> + ghcb_set_sw_exit_info_2(svm->ghcb,
>> + X86_TRAP_GP |
>> + SVM_EVTINJ_TYPE_EXEPT |
>> + SVM_EVTINJ_VALID);
>> + return 1;
>> + }
>> +
>> + trace_kvm_msr_write(ecx, data);
>> +
>> + return kvm_skip_emulated_instruction(&svm->vcpu);
>> }
>> static int msr_interception(struct vcpu_svm *svm)
>
> This code duplication is ugly, and does not work with userspace MSR
> filters too.
Agree and I missed that the userspace MSR support went in.
>
> But we can instead trap the completion of the MSR read/write to use
> ghcb_set_sw_exit_info_1 instead of kvm_inject_gp, with a callback like
>
> static int svm_complete_emulated_msr(struct kvm_vcpu *vcpu, int err)
> {
> if (!sev_es_guest(svm->vcpu.kvm) || !err)
> return kvm_complete_insn_gp(&svm->vcpu, err);
>
> ghcb_set_sw_exit_info_1(svm->ghcb, 1);
> ghcb_set_sw_exit_info_2(svm->ghcb,
> X86_TRAP_GP |
> SVM_EVTINJ_TYPE_EXEPT |
> SVM_EVTINJ_VALID);
> return 1;
> }
If we use the kvm_complete_insn_gp() we lose the tracing and it needs to
be able to deal with read completion setting the registers. It also needs
to deal with both kvm_emulate_rdmsr/wrmsr() when not bouncing to
userspace. Let me take a shot at covering all the cases and see what I can
come up with.
I noticed that the userspace completion path doesn't have tracing
invocations, trace_kvm_msr_read/write_ex() or trace_kvm_msr_read/write(),
is that by design?
>
>
> ...
> .complete_emulated_msr = svm_complete_emulated_msr,
>
>> @@ -2827,7 +2888,14 @@ static int interrupt_window_interception(struct
>> vcpu_svm *svm)
>> static int pause_interception(struct vcpu_svm *svm)
>> {
>> struct kvm_vcpu *vcpu = &svm->vcpu;
>> - bool in_kernel = (svm_get_cpl(vcpu) == 0);
>> + bool in_kernel;
>> +
>> + /*
>> + * CPL is not made available for an SEV-ES guest, so just set
>> in_kernel
>> + * to true.
>> + */
>> + in_kernel = (sev_es_guest(svm->vcpu.kvm)) ? true
>> + : (svm_get_cpl(vcpu) == 0);
>> if (!kvm_pause_in_guest(vcpu->kvm))
>> grow_ple_window(vcpu);
>
> See below.
>
>> @@ -3273,6 +3351,13 @@ bool svm_interrupt_blocked(struct kvm_vcpu *vcpu)
>> struct vcpu_svm *svm = to_svm(vcpu);
>> struct vmcb *vmcb = svm->vmcb;
>> + /*
>> + * SEV-ES guests to not expose RFLAGS. Use the VMCB interrupt mask
>> + * bit to determine the state of the IF flag.
>> + */
>> + if (sev_es_guest(svm->vcpu.kvm))
>> + return !(vmcb->control.int_state & SVM_GUEST_INTERRUPT_MASK);
>
> This seems wrong, you have to take into account SVM_INTERRUPT_SHADOW_MASK
> as well. Also, even though GIF is not really used by SEV-ES guests, I
> think it's nicer to put this check afterwards.
>
> That is:
>
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index 4372e45c8f06..2dd9c9698480 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -3247,7 +3247,14 @@ bool svm_interrupt_blocked(struct kvm_vcpu *vcpu)
> if (!gif_set(svm))
> return true;
>
> - if (is_guest_mode(vcpu)) {
> + if (sev_es_guest(svm->vcpu.kvm)) {
> + /*
> + * SEV-ES guests to not expose RFLAGS. Use the VMCB interrupt mask
> + * bit to determine the state of the IF flag.
> + */
> + if (!(vmcb->control.int_state & SVM_GUEST_INTERRUPT_MASK))
> + return true;
> + } else if (is_guest_mode(vcpu)) {
> /* As long as interrupts are being delivered... */
> if ((svm->nested.ctl.int_ctl & V_INTR_MASKING_MASK)
> ? !(svm->nested.hsave->save.rflags & X86_EFLAGS_IF)
>
>
Yup, I'll make that change.
>
>> if (!gif_set(svm))
>> return true;
>> @@ -3458,6 +3543,12 @@ static void svm_complete_interrupts(struct
>> vcpu_svm *svm)
>> svm->vcpu.arch.nmi_injected = true;
>> break;
>> case SVM_EXITINTINFO_TYPE_EXEPT:
>> + /*
>> + * Never re-inject a #VC exception.
>> + */
>> + if (vector == X86_TRAP_VC)
>> + break;
>> +
>> /*
>> * In case of software exceptions, do not reinject the vector,
>> * but re-execute the instruction instead. Rewind RIP first
>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>> index a3fdc16cfd6f..b6809a2851d2 100644
>> --- a/arch/x86/kvm/x86.c
>> +++ b/arch/x86/kvm/x86.c
>> @@ -4018,7 +4018,7 @@ void kvm_arch_vcpu_put(struct kvm_vcpu *vcpu)
>> {
>> int idx;
>> - if (vcpu->preempted)
>> + if (vcpu->preempted && !vcpu->arch.guest_state_protected)
>> vcpu->arch.preempted_in_kernel = !kvm_x86_ops.get_cpl(vcpu);
>
> This has to be true, otherwise no directed yield will be done at all:
>
> if (READ_ONCE(vcpu->preempted) && yield_to_kernel_mode &&
> !kvm_arch_vcpu_in_kernel(vcpu))
> continue;
>
> Or more easily, just use in_kernel == false in pause_interception, like
>
> + /*
> + * CPL is not made available for an SEV-ES guest, therefore
> + * vcpu->arch.preempted_in_kernel can never be true. Just
> + * set in_kernel to false as well.
> + */
> + in_kernel = !sev_es_guest(svm->vcpu.kvm) && svm_get_cpl(vcpu) == 0;
Sounds good, I'll make that change.
>
>> /*
>> @@ -8161,7 +8161,9 @@ static void post_kvm_run_save(struct kvm_vcpu *vcpu)
>> {
>> struct kvm_run *kvm_run = vcpu->run;
>> - kvm_run->if_flag = (kvm_get_rflags(vcpu) & X86_EFLAGS_IF) != 0;
>> + kvm_run->if_flag = (vcpu->arch.guest_state_protected)
>> + ? kvm_arch_interrupt_allowed(vcpu)
>> + : (kvm_get_rflags(vcpu) & X86_EFLAGS_IF) != 0;
>
> Here indeed you only want the interrupt allowed bit, not the interrupt
> window. But we can just be bold and always set it to true.
>
> - for userspace irqchip, kvm_run->ready_for_interrupt_injection is set
> just below and it will always be false if kvm_arch_interrupt_allowed is false
>
> - for in-kernel APIC, if_flag is documented to be invalid (though it
> actually is valid). For split irqchip, they can just use
> kvm_run->ready_for_interrupt_injection; for entirely in-kernel interrupt
> handling, userspace does not need if_flag at all.
Ok, I'll make that change.
Thanks,
Tom
>
> Paolo
>
>> kvm_run->flags = is_smm(vcpu) ? KVM_RUN_X86_SMM : 0;
>> kvm_run->cr8 = kvm_get_cr8(vcpu);
>> kvm_run->apic_base = kvm_get_apic_base(vcpu);
>>
>
>
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