On Tue, Jan 12, 2021, Nitesh Narayan Lal wrote: > > On 1/7/21 4:33 AM, Vitaly Kuznetsov wrote: > > 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. > > > > Just to make sure that I am understanding things correctly. > There are two issues: > 1. The first issue is with the tick IRQs that arrive after PF_VCPU is > cleared as they are then accounted into the system context atleast on > the setup where CONFIG_VIRT_CPU_ACCOUNTING_GEN is not enabled. With the > patch "KVM: x86: Unconditionally enable irqs in guest context", we are > atleast taking care of the scenario where the guest context is exiting > constantly just before the arrival of the tick. Yep. > 2. The second issue that Sean mentioned was introduced because of moving > guest_exit_irqoff() closer to VM-exit. Due to this change, any ticks that > happen after IRQs are disabled are incorrectly accounted into the system > context. This is because we exit the guest context early without > ensuring if the required guest states to handle IRQs are restored. Yep. > So, the increase in the system time (reported by cpuacct.stats) that I was > observing is not entirely correct after all. It's correct, but iff CONFIG_VIRT_CPU_ACCOUNTING_GEN=y, as that doesn't rely on ticks and so closer to VM-Enter is better. The problem is that it completely breaks CONFIG_VIRT_CPU_ACCOUNTING_GEN=n (#2 above) because KVM will never service an IRQ, ticks included, with PF_VCPU set. > Am I missing anything here?
