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

[Alexander Graf]

On 03.08.20 13:27, Vitaly Kuznetsov wrote:
> 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()?

It's even worse than that: We don't need it at all. I'll remove it for v4.


Alex



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