KVM: x86: Defer vtime accounting 'til after IRQ handling

KVM: x86: Defer vtime accounting 'til after IRQ handling

Defer the call to account guest time until after servicing any IRQ(s)
that happened in the guest or immediately after VM-Exit.  Tick-based
accounting of vCPU time relies on PF_VCPU being set when the tick IRQ
handler runs, and IRQs are blocked throughout the main sequence of
vcpu_enter_guest(), including the call into vendor code to actually
enter and exit the guest.

This fixes a bug where reported guest time remains '0', even when
running an infinite loop in the guest:

  https://bugzilla.kernel.org/show_bug.cgi?id=209831

Fixes: 87fa7f3e98a131 ("x86/kvm: Move context tracking where it belongs")
Suggested-by: Thomas Gleixner <[email protected]>
Co-developed-by: Sean Christopherson <[email protected]>
Signed-off-by: Wanpeng Li <[email protected]>
Signed-off-by: Sean Christopherson <[email protected]>
Signed-off-by: Thomas Gleixner <[email protected]>
Cc: [email protected]
Link: https://lore.kernel.org/r/[email protected]