On 04/01/21 22:24, Nitesh Narayan Lal wrote:
Hi, Last year I reported an issue of "suspicious RCU usage" [1] with the debug kernel which was fixed with the patch: 87fa7f3e98 "x86/kvm: Move context tracking where it belongs" Recently I have come across a possible regression because of this patch in the cpuacct.stats system time. With the latest upstream kernel (5.11-rc2) when we set up a VM and start observing the system time value from cpuacct.stat then it is significantly higher than value reported with the kernel that doesn't have the previously mentioned patch. FWIU the reason behind this increase is the moving of guest_exit_irqoff() to its proper location (near vmexit). This leads to the accounting of instructions that were previously accounted into the guest context as a part of the system time. IMO this should be an expected behavior after the previously mentioned change. Is that a right conclusion or I am missing something here?
Yes it's expected and I think it's more precise, since this is host overhead rather than guest operation .
Another question that I have is about the patch d7a08882a0 "KVM: x86: Unconditionally enable irqs in guest context" considering we are enabling irqs early now in the code path, do we still need this patch?
No, we don't. Since the code is a bit simpler without it, feel free to send a revert.
Thanks, Paolo
[1] https://lore.kernel.org/lkml/ece36eb1-253a-8ec6-c183-309c10bb35d5@xxxxxxxxxx/ -- Thanks Nitesh
