On Thu, 2024-12-19 at 18:33 +0100, Paolo Bonzini wrote:
> On 9/10/24 22:03, Maxim Levitsky wrote:
> > Add 3 new tracepoints for nested VM exits which are intended
> > to capture extra information to gain insights about the nested guest
> > behavior.
> >
> > The new tracepoints are:
> >
> > - kvm_nested_msr
> > - kvm_nested_hypercall
> >
> > These tracepoints capture extra register state to be able to know
> > which MSR or which hypercall was done.
> >
> > - kvm_nested_page_fault
> >
> > This tracepoint allows to capture extra info about which host pagefault
> > error code caused the nested page fault.
> >
> > Signed-off-by: Maxim Levitsky <mlevitsk@xxxxxxxxxx>
> > ---
> > arch/x86/kvm/svm/nested.c | 22 +++++++++++
> > arch/x86/kvm/trace.h | 82 +++++++++++++++++++++++++++++++++++++--
> > arch/x86/kvm/vmx/nested.c | 27 +++++++++++++
> > arch/x86/kvm/x86.c | 3 ++
> > 4 files changed, 131 insertions(+), 3 deletions(-)
> >
> > diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c
> > index 6f704c1037e51..2020307481553 100644
> > --- a/arch/x86/kvm/svm/nested.c
> > +++ b/arch/x86/kvm/svm/nested.c
> > @@ -38,6 +38,8 @@ static void nested_svm_inject_npf_exit(struct kvm_vcpu *vcpu,
> > {
> > struct vcpu_svm *svm = to_svm(vcpu);
> > struct vmcb *vmcb = svm->vmcb;
> > + u64 host_error_code = vmcb->control.exit_info_1;
> > +
> >
> > if (vmcb->control.exit_code != SVM_EXIT_NPF) {
> > /*
> > @@ -48,11 +50,15 @@ static void nested_svm_inject_npf_exit(struct kvm_vcpu *vcpu,
> > vmcb->control.exit_code_hi = 0;
> > vmcb->control.exit_info_1 = (1ULL << 32);
> > vmcb->control.exit_info_2 = fault->address;
> > + host_error_code = 0;
> > }
> >
> > vmcb->control.exit_info_1 &= ~0xffffffffULL;
> > vmcb->control.exit_info_1 |= fault->error_code;
> >
> > + trace_kvm_nested_page_fault(fault->address, host_error_code,
> > + fault->error_code);
> > +
>
> I disagree with Sean about trace_kvm_nested_page_fault. It's a useful
> addition and it is easier to understand what's happening with a
> dedicated tracepoint (especially on VMX).
>
> Tracepoint are not an exact science and they aren't entirely kernel API.
> At least they can just go away at any time (changing them is a lot
> more tricky, but their presence is not guaranteed). The one below has
> the slight ugliness of having to do some computation in
> nested_svm_vmexit(), this one should go in.
>
> > nested_svm_vmexit(svm);
> > }
> >
> > @@ -1126,6 +1132,22 @@ int nested_svm_vmexit(struct vcpu_svm *svm)
> > vmcb12->control.exit_int_info_err,
> > KVM_ISA_SVM);
> >
> > + /* Collect some info about nested VM exits */
> > + switch (vmcb12->control.exit_code) {
> > + case SVM_EXIT_MSR:
> > + trace_kvm_nested_msr(vmcb12->control.exit_info_1 == 1,
> > + kvm_rcx_read(vcpu),
> > + (vmcb12->save.rax & 0xFFFFFFFFull) |
> > + (((u64)kvm_rdx_read(vcpu) << 32)));
> > + break;
> > + case SVM_EXIT_VMMCALL:
> > + trace_kvm_nested_hypercall(vmcb12->save.rax,
> > + kvm_rbx_read(vcpu),
> > + kvm_rcx_read(vcpu),
> > + kvm_rdx_read(vcpu));
> > + break;
>
> Here I probably would have preferred an unconditional tracepoint giving
> RAX/RBX/RCX/RDX after a nested vmexit. This is not exactly what Sean
> wanted but perhaps it strikes a middle ground? I know you wrote this
> for a debugging tool, do you really need to have everything in a single
> tracepoint, or can you correlate the existing exit tracepoint with this
> hypothetical trace_kvm_nested_exit_regs, to pick RDMSR vs. WRMSR?
Hi!
If the new trace_kvm_nested_exit_regs tracepoint has a VM exit number argument, then
I can enable this new tracepoint twice with a different filter (vm_exit_num number == msr and vm_exit_num == vmcall),
and each instance will count the events that I need.
So this can work.
Thanks!
Best regards,
Maxim Levitsky
>
> Paolo
>
