Re: [PATCH v3 1/3] KVM: x86: Deflect unknown MSR accesses to user space

From: Vitaly Kuznetsov
Date: Mon Aug 03 2020 - 07:27:51 EST

Alexander Graf <graf@xxxxxxxxxx> 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@xxxxxxxxxx>
>
> ---
>
> 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
>
> v2 -> v3:
>
> - return r if r == X86EMUL_IO_NEEDED
> - s/KVM_EXIT_RDMSR/KVM_EXIT_X86_RDMSR/g
> - s/KVM_EXIT_WRMSR/KVM_EXIT_X86_WRMSR/g
> - Use complete_userspace_io logic instead of reply field
> - Simplify trapping code
> ---
> Documentation/virt/kvm/api.rst | 62 +++++++++++++++++++
> arch/x86/include/asm/kvm_host.h | 6 ++
> arch/x86/kvm/emulate.c | 18 +++++-
> arch/x86/kvm/x86.c | 106 ++++++++++++++++++++++++++++++--
> include/trace/events/kvm.h | 2 +-
> include/uapi/linux/kvm.h | 10 +++
> 6 files changed, 197 insertions(+), 7 deletions(-)
>
> diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
> index 320788f81a05..79c3e2fdfae4 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_X86_RDMSR / KVM_EXIT_X86_WRMSR */
> + struct {
> + __u8 error;
> + __u8 pad[3];
> + __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_X86_RDMSR exit for reads and KVM_EXIT_X86_WRMSR
> +exit for writes.
> +
> +For KVM_EXIT_X86_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 the respective data into the "data" field and must continue guest
> +execution to ensure the read data is transferred into guest register state.
> +
> +If the RDMSR request was unsuccessful, user space indicates that with a "1" in
> +the "error" field. This will inject a #GP into the guest when the VCPU is
> +executed again.
> +
> +For KVM_EXIT_X86_WRMSR, the "index" field tells user space which MSR the guest
> +wants to write. Once finished processing the event, user space must continue
> +vCPU execution. 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_X86_RDMSR
> +and KVM_EXIT_X86_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.
> +
> 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_X86_RDMSR and
> +KVM_EXIT_X86_WRMSR exit notifications.
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index be5363b21540..809eed0dbdea 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -829,6 +829,9 @@ struct kvm_vcpu_arch {
>
> /* AMD MSRC001_0015 Hardware Configuration */
> u64 msr_hwcr;
> +
> + /* User space is handling an MSR request */
> + bool pending_user_msr;
> };
>
> struct kvm_lpage_info {
> @@ -1002,6 +1005,9 @@ 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..744ab9c92b73 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 r;
> +
> + 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 r;
>
> - 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..24c72250f6df 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -1549,12 +1549,75 @@ int kvm_set_msr(struct kvm_vcpu *vcpu, u32 index, u64 data)
> }
> EXPORT_SYMBOL_GPL(kvm_set_msr);
>
> +static int complete_emulated_msr(struct kvm_vcpu *vcpu, bool is_read)
> +{
> + BUG_ON(!vcpu->arch.pending_user_msr);
> +
> + if (vcpu->run->msr.error) {
> + kvm_inject_gp(vcpu, 0);
> + } else if (is_read) {
> + kvm_rax_write(vcpu, (u32)vcpu->run->msr.data);
> + kvm_rdx_write(vcpu, vcpu->run->msr.data >> 32);
> + }
> +
> + return kvm_skip_emulated_instruction(vcpu);
> +}
> +
> +static int complete_emulated_rdmsr(struct kvm_vcpu *vcpu)
> +{
> + return complete_emulated_msr(vcpu, true);
> +}
> +
> +static int complete_emulated_wrmsr(struct kvm_vcpu *vcpu)
> +{
> + return complete_emulated_msr(vcpu, false);
> +}
> +
> +static int kvm_get_msr_user_space(struct kvm_vcpu *vcpu, u32 index)
> +{
> + if (!vcpu->kvm->arch.user_space_msr_enabled)
> + return 0;
> +
> + vcpu->run->exit_reason = KVM_EXIT_X86_RDMSR;
> + vcpu->run->msr.error = 0;
> + vcpu->run->msr.index = index;
> + vcpu->arch.pending_user_msr = true;
> + vcpu->arch.complete_userspace_io = complete_emulated_rdmsr;
> +
> + return 1;
> +}
> +
> +static int kvm_set_msr_user_space(struct kvm_vcpu *vcpu, u32 index, u64 data)
> +{
> + if (!vcpu->kvm->arch.user_space_msr_enabled)
> + return 0;
> +
> + vcpu->run->exit_reason = KVM_EXIT_X86_WRMSR;
> + vcpu->run->msr.error = 0;
> + vcpu->run->msr.index = index;
> + vcpu->run->msr.data = data;
> + vcpu->arch.pending_user_msr = true;
> + vcpu->arch.complete_userspace_io = complete_emulated_wrmsr;

I'm probably missing something but where do we reset
vcpu->arch.pending_user_msr? Shouldn't it be done in
complete_emulated_msr()?

> +
> + return 1;
> +}
> +
> 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);
>
> - if (kvm_get_msr(vcpu, ecx, &data)) {
> + /* MSR read failed? See if we should ask user space */
> + if (r && kvm_get_msr_user_space(vcpu, ecx)) {
> + /* Bounce to user space */
> + return 0;
> + }
> +
> + /* MSR read failed? Inject a #GP */
> + if (r) {
> trace_kvm_msr_read_ex(ecx);
> kvm_inject_gp(vcpu, 0);
> return 1;
> @@ -1572,8 +1635,18 @@ 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);
>
> - if (kvm_set_msr(vcpu, ecx, data)) {
> + /* MSR write failed? See if we should ask user space */
> + if (r && kvm_set_msr_user_space(vcpu, ecx, data)) {
> + /* Bounce to user space */
> + return 0;
> + }
> +
> + /* MSR write failed? Inject a #GP */
> + if (r) {
> trace_kvm_msr_write_ex(ecx, data);
> kvm_inject_gp(vcpu, 0);
> return 1;
> @@ -3476,6 +3549,7 @@ 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 +5064,10 @@ 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 +6397,33 @@ 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 && kvm_get_msr_user_space(vcpu, msr_index)) {
> + /* 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 && kvm_set_msr_user_space(vcpu, msr_index, data)) {
> + /* 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 9417a34aad08..26cfb0fa8e7e 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(ARM_NISV)
> + ERSN(HYPERV), ERSN(ARM_NISV), ERSN(X86_RDMSR), ERSN(X86_WRMSR)
>
> 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..13fc7de1eb50 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_X86_RDMSR 29
> +#define KVM_EXIT_X86_WRMSR 30
>
> /* For KVM_EXIT_INTERNAL_ERROR */
> /* Emulate instruction failed. */
> @@ -412,6 +414,13 @@ struct kvm_run {
> __u64 esr_iss;
> __u64 fault_ipa;
> } arm_nisv;
> + /* KVM_EXIT_X86_RDMSR / KVM_EXIT_X86_WRMSR */
> + struct {
> + __u8 error;
> + __u8 pad[3];
> + __u32 index;
> + __u64 data;
> + } msr;
> /* Fix the size of the union. */
> char padding[256];
> };
> @@ -1031,6 +1040,7 @@ 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