Re: Intercepting RDTSC instruction by causing a VMEXIT

[Sean Christopherson <seanjc@google.com>]

On Mon, Apr 26, 2021, Arnabjyoti Kalita wrote:
> Hello Sean,
> 
> Thank you very much for your answer again. I could actually see that the
> RDTSC handler gets called successfully. I added a "printk" statement to the
> handler and I can see the messages being printed out to the kernel syslog.
> 
> However, I am not sure if a VMEXIT is happening in userspace. I use QEMU
> and I do not see any notification from QEMU that tells me that a VMEXIT
> happened due to RDTSC being executed. Is there a mechanism to confirm this?

Without further modification, KVM will _not_ exit to userspace in this case.

> My actual requirement to record tsc values read out as a result of RDTSC
> execution still stands.

Your requirement didn't clarify that userspace needed to record the values ;-)

Forcing an exit to userspace is very doable, but will tank guest performance and
possibly interfere with whatever you're trying to do.  I would try adding a
tracepoint and using that to capture the TSC values.  Adding guest RIP, etc...
as needed is trivial.

diff --git a/arch/x86/kvm/trace.h b/arch/x86/kvm/trace.h
index a61c015870e3..e962e813ba04 100644
--- a/arch/x86/kvm/trace.h
+++ b/arch/x86/kvm/trace.h
@@ -821,6 +821,23 @@ TRACE_EVENT(
                  __entry->gpa_match ? "GPA" : "GVA")
 );

+TRACE_EVENT(kvm_emulate_rdtsc,
+       TP_PROTO(unsigned int vcpu_id, __u64 tsc),
+       TP_ARGS(vcpu_id, tsc),
+
+       TP_STRUCT__entry(
+               __field( unsigned int,  vcpu_id         )
+               __field(        __u64,  tsc             )
+       ),
+
+       TP_fast_assign(
+               __entry->vcpu_id                = vcpu_id;
+               __entry->tsc                    = tsc;
+       ),
+
+       TP_printk("vcpu=%u tsc=%llu", __entry->vcpu_id, __entry->tsc)
+);
+
 TRACE_EVENT(kvm_write_tsc_offset,
        TP_PROTO(unsigned int vcpu_id, __u64 previous_tsc_offset,
                 __u64 next_tsc_offset),
diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index 61bf0b86770b..1fbeef520349 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -5254,6 +5254,8 @@ static int handle_rdtsc(struct kvm_vcpu *vcpu)
 {
        u64 tsc = kvm_read_l1_tsc(vcpu, rdtsc());

+       trace_kvm_emulate_rdtsc(vcpu->vcpu_id, tsc);
+
        kvm_rax_write(vcpu, tsc & -1u);
        kvm_rdx_write(vcpu, (tsc >> 32) & -1u);
        return kvm_skip_emulated_instruction(vcpu);