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SubjectRe: [patch V5 part 2 15/18] x86/kvm/svm: Handle hardirqs proper on guest enter/exit
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On 07/05/20 16:44, Thomas Gleixner wrote:
> Entering guest mode is more or less the same as returning to user
> space. From an instrumentation point of view both leave kernel mode and the
> transition to guest or user mode reenables interrupts on the host. In user
> mode an interrupt is served directly and in guest mode it causes a VM exit
> which then handles or reinjects the interrupt.
>
> The transition from guest mode or user mode to kernel mode disables
> interrupts, which needs to be recorded in instrumentation to set the
> correct state again.
>
> This is important for e.g. latency analysis because otherwise the execution
> time in guest or user mode would be wrongly accounted as interrupt disabled
> and could trigger false positives.
>
> Add hardirq tracing to guest enter/exit functions in the same way as it
> is done in the user mode enter/exit code, respecting the RCU requirements.
>
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> Cc: Paolo Bonzini <pbonzini@redhat.com>
> Cc: Sean Christopherson <sean.j.christopherson@intel.com>
> ---
> V5: Adjust comments and changelog

Apart from the subject being svm and not vmx, it looks great. Thanks!

Paolo

> ---
> arch/x86/kvm/vmx/vmx.c | 27 +++++++++++++++++++++++++--
> 1 file changed, 25 insertions(+), 2 deletions(-)
>
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -6604,9 +6604,21 @@ static void vmx_vcpu_run(struct kvm_vcpu
> x86_spec_ctrl_set_guest(vmx->spec_ctrl, 0);
>
> /*
> - * Tell context tracking that this CPU is about to enter guest mode.
> + * VMENTER enables interrupts (host state), but the kernel state is
> + * interrupts disabled when this is invoked. Also tell RCU about
> + * it. This is the same logic as for exit_to_user_mode().
> + *
> + * This ensures that e.g. latency analysis on the host observes
> + * guest mode as interrupt enabled.
> + *
> + * guest_enter_irqoff() informs context tracking about the
> + * transition to guest mode and if enabled adjusts RCU state
> + * accordingly.
> */
> + trace_hardirqs_on_prepare();
> + lockdep_hardirqs_on_prepare(CALLER_ADDR0);
> guest_enter_irqoff();
> + lockdep_hardirqs_on(CALLER_ADDR0);
>
> /* L1D Flush includes CPU buffer clear to mitigate MDS */
> if (static_branch_unlikely(&vmx_l1d_should_flush))
> @@ -6623,9 +6635,20 @@ static void vmx_vcpu_run(struct kvm_vcpu
> vcpu->arch.cr2 = read_cr2();
>
> /*
> - * Tell context tracking that this CPU is back.
> + * VMEXIT disables interrupts (host state), but tracing and lockdep
> + * have them in state 'on' as recorded before entering guest mode.
> + * Same as enter_from_user_mode().
> + *
> + * guest_exit_irqoff() restores host context and reinstates RCU if
> + * enabled and required.
> + *
> + * This needs to be done before the below as native_read_msr()
> + * contains a tracepoint and x86_spec_ctrl_restore_host() calls
> + * into world and some more.
> */
> + lockdep_hardirqs_off(CALLER_ADDR0);
> guest_exit_irqoff();
> + trace_hardirqs_off_prepare();
>
> /*
> * We do not use IBRS in the kernel. If this vCPU has used the
>

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