Sean Christopherson <seanjc@xxxxxxxxxx> writes: > On Wed, Jan 06, 2021, Vitaly Kuznetsov wrote: >> >> Looking back, I don't quite understand why we wanted to account ticks >> between vmexit and exiting guest context as 'guest' in the first place; >> to my understanging 'guest time' is time spent within VMX non-root >> operation, the rest is KVM overhead (system). > > With tick-based accounting, if the tick IRQ is received after PF_VCPU is cleared > then that tick will be accounted to the host/system. The motivation for opening > an IRQ window after VM-Exit is to handle the case where the guest is constantly > exiting for a different reason _just_ before the tick arrives, e.g. if the guest > has its tick configured such that the guest and host ticks get synchronized > in a bad way. > > This is a non-issue when using CONFIG_VIRT_CPU_ACCOUNTING_GEN=y, at least with a > stable TSC, as the accounting happens during guest_exit_irqoff() itself. > Accounting might be less-than-stellar if TSC is unstable, but I don't think it > would be as binary of a failure as tick-based accounting. > Oh, yea, I vaguely remember we had to deal with a very similar problem but for userspace/kernel accounting. It was possible to observe e.g. a userspace task going 100% kernel while in reality it was just perfectly synchronized with the tick and doing a syscall just before it arrives (or something like that, I may be misremembering the details). So depending on the frequency, it is probably possible to e.g observe '100% host' with tick based accounting, the guest just has to synchronize exiting to KVM in a way that the tick will always arrive past guest_exit_irqoff(). It seems to me this is a fundamental problem in case the frequency of guest exits can match the frequency of the time accounting tick. -- Vitaly
