Message-ID: <87h7tpl7i0.fsf@vitty.brq.redhat.com>
Date:   Thu, 30 Jul 2020 10:41:43 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Alexander Graf <graf@...zon.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Cc:     Jonathan Corbet <corbet@....net>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        KarimAllah Raslan <karahmed@...zon.de>, kvm@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/3] KVM: x86: Deflect unknown MSR accesses to user space

Alexander Graf <graf@...zon.com> writes:

> MSRs are weird. Some of them are normal control registers, such as EFER.
> Some however are registers that really are model specific, not very
> interesting to virtualization workloads, and not performance critical.
> Others again are really just windows into package configuration.
>
> Out of these MSRs, only the first category is necessary to implement in
> kernel space. Rarely accessed MSRs, MSRs that should be fine tunes against
> certain CPU models and MSRs that contain information on the package level
> are much better suited for user space to process. However, over time we have
> accumulated a lot of MSRs that are not the first category, but still handled
> by in-kernel KVM code.
>
> This patch adds a generic interface to handle WRMSR and RDMSR from user
> space. With this, any future MSR that is part of the latter categories can
> be handled in user space.
>
> Furthermore, it allows us to replace the existing "ignore_msrs" logic with
> something that applies per-VM rather than on the full system. That way you
> can run productive VMs in parallel to experimental ones where you don't care
> about proper MSR handling.
>
> Signed-off-by: Alexander Graf <graf@...zon.com>
>
> ---
>
> v1 -> v2:
>
>   - s/ETRAP_TO_USER_SPACE/ENOENT/g
>   - deflect all #GP injection events to user space, not just unknown MSRs.
>     That was we can also deflect allowlist errors later
>   - fix emulator case
> ---
>  Documentation/virt/kvm/api.rst  |  62 ++++++++++++++++++
>  arch/x86/include/asm/kvm_host.h |   3 +
>  arch/x86/kvm/emulate.c          |  18 +++++-
>  arch/x86/kvm/x86.c              | 111 ++++++++++++++++++++++++++++++--
>  include/trace/events/kvm.h      |   2 +-
>  include/uapi/linux/kvm.h        |  11 ++++
>  6 files changed, 200 insertions(+), 7 deletions(-)
>
> diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
> index 320788f81a05..c1f991c1ffa6 100644
> --- a/Documentation/virt/kvm/api.rst
> +++ b/Documentation/virt/kvm/api.rst
> @@ -5155,6 +5155,35 @@ Note that KVM does not skip the faulting instruction as it does for
>  KVM_EXIT_MMIO, but userspace has to emulate any change to the processing state
>  if it decides to decode and emulate the instruction.
>  
> +::
> +
> +		/* KVM_EXIT_RDMSR / KVM_EXIT_WRMSR */
> +		struct {
> +			__u8 reply;
> +			__u8 error;
> +			__u8 pad[2];
> +			__u32 index;
> +			__u64 data;
> +		} msr;
> +
> +Used on x86 systems. When the VM capability KVM_CAP_X86_USER_SPACE_MSR is
> +enabled, MSR accesses to registers that would invoke a #GP by KVM kernel code
> +will instead trigger a KVM_EXIT_RDMSR exit for reads and KVM_EXIT_WRMSR exit for
> +writes.
> +
> +For KVM_EXIT_RDMSR, the "index" field tells user space which MSR the guest
> +wants to read. To respond to this request with a successful read, user space
> +writes a 1 into the "reply" field and the respective data into the "data" field.
> +
> +If the RDMSR request was unsuccessful, user space indicates that with a "1"
> +in the "reply" field and a "1" in the "error" field. This will inject a #GP
> +into the guest when the VCPU is executed again.
> +
> +For KVM_EXIT_WRMSR, the "index" field tells user space which MSR the guest
> +wants to write. Once finished processing the event, user space sets the "reply"
> +field to "1". If the MSR write was unsuccessful, user space also sets the
> +"error" field to "1".
> +
>  ::
>  
>  		/* Fix the size of the union. */
> @@ -5844,6 +5873,28 @@ controlled by the kvm module parameter halt_poll_ns. This capability allows
>  the maximum halt time to specified on a per-VM basis, effectively overriding
>  the module parameter for the target VM.
>  
> +7.21 KVM_CAP_X86_USER_SPACE_MSR
> +----------------------
> +
> +:Architectures: x86
> +:Target: VM
> +:Parameters: args[0] is 1 if user space MSR handling is enabled, 0 otherwise
> +:Returns: 0 on success; -1 on error
> +
> +This capability enables trapping of #GP invoking RDMSR and WRMSR instructions
> +into user space.
> +
> +When a guest requests to read or write an MSR, KVM may not implement all MSRs
> +that are relevant to a respective system. It also does not differentiate by
> +CPU type.
> +
> +To allow more fine grained control over MSR handling, user space may enable
> +this capability. With it enabled, MSR accesses that would usually trigger
> +a #GP event inside the guest by KVM will instead trigger KVM_EXIT_RDMSR
> +and KVM_EXIT_WRMSR exit notifications which user space can then handle to
> +implement model specific MSR handling and/or user notifications to inform
> +a user that an MSR was not handled.

(I'm not against the idea, just a spare thought):

#GP on WRMSR may be triggered because we violate some restrictions which
can also be conditional (e.g. some other feature enablement), userspace
should be aware of that if it cares enough to get the exit.

> +
>  8. Other capabilities.
>  ======================
>  
> @@ -6151,3 +6202,14 @@ KVM can therefore start protected VMs.
>  This capability governs the KVM_S390_PV_COMMAND ioctl and the
>  KVM_MP_STATE_LOAD MP_STATE. KVM_SET_MP_STATE can fail for protected
>  guests when the state change is invalid.
> +
> +8.24 KVM_CAP_X86_USER_SPACE_MSR
> +----------------------------
> +
> +:Architectures: x86
> +
> +This capability indicates that KVM supports deflection of MSR reads and
> +writes to user space. It can be enabled on a VM level. If enabled, MSR
> +accesses that would usually trigger a #GP by KVM into the guest will
> +instead get bounced to user space through the KVM_EXIT_RDMSR and
> +KVM_EXIT_WRMSR exit notifications.
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index be5363b21540..2f2307e71342 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -1002,6 +1002,9 @@ struct kvm_arch {
>  	bool guest_can_read_msr_platform_info;
>  	bool exception_payload_enabled;
>  
> +	/* Deflect RDMSR and WRMSR to user space when they trigger a #GP */
> +	bool user_space_msr_enabled;
> +
>  	struct kvm_pmu_event_filter *pmu_event_filter;
>  	struct task_struct *nx_lpage_recovery_thread;
>  };
> diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
> index d0e2825ae617..d85c4883e37c 100644
> --- a/arch/x86/kvm/emulate.c
> +++ b/arch/x86/kvm/emulate.c
> @@ -3689,11 +3689,18 @@ static int em_dr_write(struct x86_emulate_ctxt *ctxt)
>  
>  static int em_wrmsr(struct x86_emulate_ctxt *ctxt)
>  {
> +	u64 msr_index = reg_read(ctxt, VCPU_REGS_RCX);
>  	u64 msr_data;
> +	int r;
>  
>  	msr_data = (u32)reg_read(ctxt, VCPU_REGS_RAX)
>  		| ((u64)reg_read(ctxt, VCPU_REGS_RDX) << 32);
> -	if (ctxt->ops->set_msr(ctxt, reg_read(ctxt, VCPU_REGS_RCX), msr_data))
> +	r = ctxt->ops->set_msr(ctxt, msr_index, msr_data);
> +
> +	if (r == X86EMUL_IO_NEEDED)
> +		return X86EMUL_IO_NEEDED;

'return r' would've been shorter :-)

> +
> +	if (r)
>  		return emulate_gp(ctxt, 0);
>  
>  	return X86EMUL_CONTINUE;
> @@ -3701,9 +3708,16 @@ static int em_wrmsr(struct x86_emulate_ctxt *ctxt)
>  
>  static int em_rdmsr(struct x86_emulate_ctxt *ctxt)
>  {
> +	u64 msr_index = reg_read(ctxt, VCPU_REGS_RCX);
>  	u64 msr_data;
> +	int r;
> +
> +	r = ctxt->ops->get_msr(ctxt, msr_index, &msr_data);
> +
> +	if (r == X86EMUL_IO_NEEDED)
> +		return X86EMUL_IO_NEEDED;
>  
> -	if (ctxt->ops->get_msr(ctxt, reg_read(ctxt, VCPU_REGS_RCX), &msr_data))
> +	if (r)
>  		return emulate_gp(ctxt, 0);
>  
>  	*reg_write(ctxt, VCPU_REGS_RAX) = (u32)msr_data;
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 88c593f83b28..11e94a780656 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -1549,12 +1549,71 @@ int kvm_set_msr(struct kvm_vcpu *vcpu, u32 index, u64 data)
>  }
>  EXPORT_SYMBOL_GPL(kvm_set_msr);
>  
> +static int kvm_get_msr_user_space(struct kvm_vcpu *vcpu, u32 index, u64 *data)
> +{
> +	if (!vcpu->kvm->arch.user_space_msr_enabled)
> +		return 1;
> +
> +	if (vcpu->run->exit_reason == KVM_EXIT_RDMSR && vcpu->run->msr.reply) {
> +		vcpu->run->msr.reply = 0;
> +
> +		if (vcpu->run->msr.error)
> +			return 1;
> +
> +		*data = vcpu->run->msr.data;
> +		return 0;
> +	}
> +
> +	vcpu->run->exit_reason = KVM_EXIT_RDMSR;
> +	vcpu->run->msr.reply = 0;
> +	vcpu->run->msr.error = 0;
> +	vcpu->run->msr.index = index;
> +
> +	return -ENOENT;
> +}
> +
> +static int kvm_set_msr_user_space(struct kvm_vcpu *vcpu, u32 index, u64 data)
> +{
> +	if (!vcpu->kvm->arch.user_space_msr_enabled)
> +		return 1;
> +
> +	if (vcpu->run->exit_reason == KVM_EXIT_WRMSR && vcpu->run->msr.reply) {
> +		vcpu->run->msr.reply = 0;
> +
> +		if (vcpu->run->msr.error)
> +			return 1;
> +
> +		return 0;
> +	}
> +
> +	vcpu->run->exit_reason = KVM_EXIT_WRMSR;
> +	vcpu->run->msr.reply = 0;
> +	vcpu->run->msr.error = 0;
> +	vcpu->run->msr.index = index;
> +	vcpu->run->msr.data = data;
> +
> +	return -ENOENT;
> +}
> +
>  int kvm_emulate_rdmsr(struct kvm_vcpu *vcpu)
>  {
>  	u32 ecx = kvm_rcx_read(vcpu);
>  	u64 data;
> +	int r;
> +
> +	r = kvm_get_msr(vcpu, ecx, &data);
> +
> +	/* MSR read failed? See if we should ask user space */
> +	if (r) {
> +		r = kvm_get_msr_user_space(vcpu, ecx, &data);
> +		if (r == -ENOENT) {
> +			/* Bounce to user space */
> +			return 0;
> +		}
> +	}
>  
> -	if (kvm_get_msr(vcpu, ecx, &data)) {
> +	/* MSR read failed? Inject a #GP */
> +	if (r) {
>  		trace_kvm_msr_read_ex(ecx);
>  		kvm_inject_gp(vcpu, 0);
>  		return 1;
> @@ -1572,8 +1631,21 @@ int kvm_emulate_wrmsr(struct kvm_vcpu *vcpu)
>  {
>  	u32 ecx = kvm_rcx_read(vcpu);
>  	u64 data = kvm_read_edx_eax(vcpu);
> +	int r;
> +
> +	r = kvm_set_msr(vcpu, ecx, data);
> +
> +	/* MSR write failed? See if we should ask user space */
> +	if (r) {
> +		r = kvm_set_msr_user_space(vcpu, ecx, data);
> +		if (r == -ENOENT) {
> +			/* Bounce to user space */
> +			return 0;
> +		}
> +	}
>  
> -	if (kvm_set_msr(vcpu, ecx, data)) {
> +	/* MSR write failed? Inject a #GP */
> +	if (r) {
>  		trace_kvm_msr_write_ex(ecx, data);
>  		kvm_inject_gp(vcpu, 0);
>  		return 1;
> @@ -3476,6 +3548,7 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
>  	case KVM_CAP_MSR_PLATFORM_INFO:
>  	case KVM_CAP_EXCEPTION_PAYLOAD:
>  	case KVM_CAP_SET_GUEST_DEBUG:
> +	case KVM_CAP_X86_USER_SPACE_MSR:
>  		r = 1;
>  		break;
>  	case KVM_CAP_SYNC_REGS:
> @@ -4990,6 +5063,10 @@ int kvm_vm_ioctl_enable_cap(struct kvm *kvm,
>  		kvm->arch.exception_payload_enabled = cap->args[0];
>  		r = 0;
>  		break;
> +	case KVM_CAP_X86_USER_SPACE_MSR:
> +		kvm->arch.user_space_msr_enabled = cap->args[0];
> +		r = 0;
> +		break;
>  	default:
>  		r = -EINVAL;
>  		break;
> @@ -6319,13 +6396,39 @@ static void emulator_set_segment(struct x86_emulate_ctxt *ctxt, u16 selector,
>  static int emulator_get_msr(struct x86_emulate_ctxt *ctxt,
>  			    u32 msr_index, u64 *pdata)
>  {
> -	return kvm_get_msr(emul_to_vcpu(ctxt), msr_index, pdata);
> +	struct kvm_vcpu *vcpu = emul_to_vcpu(ctxt);
> +	int r;
> +
> +	r = kvm_get_msr(vcpu, msr_index, pdata);
> +
> +	if (r) {
> +		r = kvm_get_msr_user_space(vcpu, msr_index, pdata);
> +		if (r == -ENOENT) {
> +			/* Bounce to user space */
> +			return X86EMUL_IO_NEEDED;
> +		}
> +	}
> +
> +	return r;
>  }
>  
>  static int emulator_set_msr(struct x86_emulate_ctxt *ctxt,
>  			    u32 msr_index, u64 data)
>  {
> -	return kvm_set_msr(emul_to_vcpu(ctxt), msr_index, data);
> +	struct kvm_vcpu *vcpu = emul_to_vcpu(ctxt);
> +	int r;
> +
> +	r = kvm_set_msr(emul_to_vcpu(ctxt), msr_index, data);
> +
> +	if (r) {
> +		r = kvm_set_msr_user_space(vcpu, msr_index, data);
> +		if (r == -ENOENT) {
> +			/* Bounce to user space */
> +			return X86EMUL_IO_NEEDED;
> +		}
> +	}
> +
> +	return r;
>  }
>  
>  static u64 emulator_get_smbase(struct x86_emulate_ctxt *ctxt)
> diff --git a/include/trace/events/kvm.h b/include/trace/events/kvm.h
> index 2c735a3e6613..09509dee4968 100644
> --- a/include/trace/events/kvm.h
> +++ b/include/trace/events/kvm.h
> @@ -17,7 +17,7 @@
>  	ERSN(NMI), ERSN(INTERNAL_ERROR), ERSN(OSI), ERSN(PAPR_HCALL),	\
>  	ERSN(S390_UCONTROL), ERSN(WATCHDOG), ERSN(S390_TSCH), ERSN(EPR),\
>  	ERSN(SYSTEM_EVENT), ERSN(S390_STSI), ERSN(IOAPIC_EOI),          \
> -	ERSN(HYPERV)
> +	ERSN(HYPERV), ERSN(ARM_NISV), ERSN(RDMSR), ERSN(WRMSR)

Nit: adding ARM_NISV here is definitely a good thing but I'd suggest we
do this in a separate patch (just think about poor backporters :-)

>  
>  TRACE_EVENT(kvm_userspace_exit,
>  	    TP_PROTO(__u32 reason, int errno),
> diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
> index 4fdf30316582..df237bf2bdc2 100644
> --- a/include/uapi/linux/kvm.h
> +++ b/include/uapi/linux/kvm.h
> @@ -248,6 +248,8 @@ struct kvm_hyperv_exit {
>  #define KVM_EXIT_IOAPIC_EOI       26
>  #define KVM_EXIT_HYPERV           27
>  #define KVM_EXIT_ARM_NISV         28
> +#define KVM_EXIT_RDMSR            29
> +#define KVM_EXIT_WRMSR            30

It is actually weird we still don't have KVM_EXIT_X86_* exits (but we
have KVM_EXIT_ARM_*/KVM_EXIT_S390_*), maybe we should take the
opportunity (as these are difinitely X86 specific)?

>  
>  /* For KVM_EXIT_INTERNAL_ERROR */
>  /* Emulate instruction failed. */
> @@ -412,6 +414,14 @@ struct kvm_run {
>  			__u64 esr_iss;
>  			__u64 fault_ipa;
>  		} arm_nisv;
> +		/* KVM_EXIT_RDMSR / KVM_EXIT_WRMSR */
> +		struct {
> +			__u8 reply;
> +			__u8 error;
> +			__u8 pad[2];
> +			__u32 index;
> +			__u64 data;
> +		} msr;
>  		/* Fix the size of the union. */
>  		char padding[256];
>  	};
> @@ -1031,6 +1041,7 @@ struct kvm_ppc_resize_hpt {
>  #define KVM_CAP_PPC_SECURE_GUEST 181
>  #define KVM_CAP_HALT_POLL 182
>  #define KVM_CAP_ASYNC_PF_INT 183
> +#define KVM_CAP_X86_USER_SPACE_MSR 184
>  
>  #ifdef KVM_CAP_IRQ_ROUTING

-- 
Vitaly

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