Messages in this thread | | | Subject | Re: [PATCH v3 1/3] KVM: x86: Deflect unknown MSR accesses to user space | From | Alexander Graf <> | Date | Mon, 3 Aug 2020 13:34:28 +0200 |
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On 03.08.20 13:27, Vitaly Kuznetsov wrote: > Alexander Graf <graf@amazon.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@amazon.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 >> >> 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()?
It's even worse than that: We don't need it at all. I'll remove it for v4.
Alex
Amazon Development Center Germany GmbH Krausenstr. 38 10117 Berlin Geschaeftsfuehrung: Christian Schlaeger, Jonathan Weiss Eingetragen am Amtsgericht Charlottenburg unter HRB 149173 B Sitz: Berlin Ust-ID: DE 289 237 879
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