Re: [PATCH v2 7/7] kvm/x86: rework guest entry logic

[Nicolas Saenz Julienne]

On Wed, 2022-01-19 at 10:58 +0000, Mark Rutland wrote:
> For consistency and clarity, migrate x86 over to the generic helpers for
> guest timing and lockdep/RCU/tracing management, and remove the
> x86-specific helpers.
> 
> Prior to this patch, the guest timing was entered in
> kvm_guest_enter_irqoff() (called by svm_vcpu_enter_exit() and
> svm_vcpu_enter_exit()), and was exited by the call to
> vtime_account_guest_exit() within vcpu_enter_guest().
> 
> To minimize duplication and to more clearly balance entry and exit, both
> entry and exit of guest timing are placed in vcpu_enter_guest(), using
> the new guest_timing_{enter,exit}_irqoff() helpers. When context
> tracking is used a small amount of additional time will be accounted
> towards guests; tick-based accounting is unnaffected as IRQs are
> disabled at this point and not enabled until after the return from the
> guest.
> 
> This also corrects (benign) mis-balanced context tracking accounting
> introduced in commits:
> 
>   ae95f566b3d22ade ("KVM: X86: TSCDEADLINE MSR emulation fastpath")
>   26efe2fd92e50822 ("KVM: VMX: Handle preemption timer fastpath")
> 
> Where KVM can enter a guest multiple times, calling vtime_guest_enter()
> without a corresponding call to vtime_account_guest_exit(), and with
> vtime_account_system() called when vtime_account_guest() should be used.
> As account_system_time() checks PF_VCPU and calls account_guest_time(),
> this doesn't result in any functional problem, but is unnecessarily
> confusing.
> 
> Signed-off-by: Mark Rutland <mark.rutland@xxxxxxx>
> Cc: Borislav Petkov <bp@xxxxxxxxx>
> Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: Jim Mattson <jmattson@xxxxxxxxxx>
> Cc: Joerg Roedel <joro@xxxxxxxxxx>
> Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> Cc: Sean Christopherson <seanjc@xxxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
> Cc: Wanpeng Li <wanpengli@xxxxxxxxxxx>
> ---

Reviewed-by: Nicolas Saenz Julienne <nsaenzju@xxxxxxxxxx>

Thanks,

-- 
Nicolás Sáenz