Hi, Thorsten here, the Linux kernel's regression tracker. Top-posting for once, to make this easily accessible to everyone. Was this regression ever resolved? Doesn't look like it, but maybe I just missed something. Ciao, Thorsten (wearing his 'the Linux kernel's regression tracker' hat) -- Everything you wanna know about Linux kernel regression tracking: https://linux-regtracking.leemhuis.info/about/#tldr If I did something stupid, please tell me, as explained on that page. #regzbot poke On 20.03.24 14:43, Nicholas Piggin wrote: > On Wed Mar 13, 2024 at 5:26 PM AEST, Vaibhav Jain wrote: >> This reverts commit 180c6b072bf360b686e53d893d8dcf7dbbaec6bb ("KVM: PPC: >> Book3S HV nestedv2: Do not cancel pending decrementer exception") which >> prevented cancelling a pending HDEC exception for nestedv2 KVM guests. It >> was done to avoid overhead of a H_GUEST_GET_STATE hcall to read the 'HDEC >> expiry TB' register which was higher compared to handling extra decrementer >> exceptions. >> >> This overhead of reading 'HDEC expiry TB' register has been mitigated >> recently by the L0 hypervisor(PowerVM) by putting the value of this >> register in L2 guest-state output buffer on trap to L1. From there the >> value of this register is cached, made available in kvmhv_run_single_vcpu() >> to compare it against host(L1) timebase and cancel the pending hypervisor >> decrementer exception if needed. > > Ah, I figured out the problem here. Guest entry never clears the > queued dec, because it's level triggered on the DEC MSB so it > doesn't go away when it's delivered. So upstream code is indeed > buggy and I think I take the blame for suggesting this nestedv2 > workaround. > > I actually don't think that is necessary though, we could treat it > like other interrupts. I think that would solve the problem without > having to test dec here. > > I am wondering though, what workload slows down that this patch > was needed in the first place. We'd only get here after a cede > returns, then we'd dequeue the dec and stop having to GET_STATE > it here. > > Thanks, > Nick > >> >> Fixes: 180c6b072bf3 ("KVM: PPC: Book3S HV nestedv2: Do not cancel pending decrementer exception") >> Signed-off-by: Vaibhav Jain <vaibhav@xxxxxxxxxxxxx> >> --- >> arch/powerpc/kvm/book3s_hv.c | 2 +- >> 1 file changed, 1 insertion(+), 1 deletion(-) >> >> diff --git a/arch/powerpc/kvm/book3s_hv.c b/arch/powerpc/kvm/book3s_hv.c >> index 0b921704da45..e47b954ce266 100644 >> --- a/arch/powerpc/kvm/book3s_hv.c >> +++ b/arch/powerpc/kvm/book3s_hv.c >> @@ -4856,7 +4856,7 @@ int kvmhv_run_single_vcpu(struct kvm_vcpu *vcpu, u64 time_limit, >> * entering a nested guest in which case the decrementer is now owned >> * by L2 and the L1 decrementer is provided in hdec_expires >> */ >> - if (!kvmhv_is_nestedv2() && kvmppc_core_pending_dec(vcpu) && >> + if (kvmppc_core_pending_dec(vcpu) && >> ((tb < kvmppc_dec_expires_host_tb(vcpu)) || >> (trap == BOOK3S_INTERRUPT_SYSCALL && >> kvmppc_get_gpr(vcpu, 3) == H_ENTER_NESTED))) >
