| Message ID | 20240802195120.325560-1-seanjc@google.com (mailing list archive) |
|---|---|
| Headers | show |
| Series | KVM: x86: Fastpath cleanup, fix, and enhancement | expand |
On Fri, 02 Aug 2024 12:51:15 -0700, Sean Christopherson wrote: > This series was prompted by observations of HLT-exiting when debugging > a throughput issue related to posted interrupts. When KVM is running in > a nested scenario, a rather surprising number of HLT exits occur with an > unmasked interrupt already pending. I didn't debug too deeply into the > guest side of things, but I suspect what is happening is that it's fairly > easy for L2 to be interrupted (by L1 or L0) between checking if it (the > CPU) should enter an idle state and actually executing STI;HLT. > > [...] Applied to kvm-x86 misc, I gave myself enough confidence the fastpath fix is correct with a selftest update[*] (which I'll get applied next week). [*] https://lore.kernel.org/all/20240830044448.130449-1-seanjc@google.com [1/5] KVM: x86: Re-enter guest if WRMSR(X2APIC_ICR) fastpath is successful https://github.com/kvm-x86/linux/commit/0dd45f2cd8cc [2/5] KVM: x86: Dedup fastpath MSR post-handling logic https://github.com/kvm-x86/linux/commit/ea60229af7fb [3/5] KVM: x86: Exit to userspace if fastpath triggers one on instruction skip https://github.com/kvm-x86/linux/commit/f7f39c50edb9 [4/5] KVM: x86: Reorganize code in x86.c to co-locate vCPU blocking/running helpers https://github.com/kvm-x86/linux/commit/70cdd2385106 [5/5] KVM: x86: Add fastpath handling of HLT VM-Exits https://github.com/kvm-x86/linux/commit/1876dd69dfe8 -- https://github.com/kvm-x86/linux/tree/next
On Sat, Aug 31, 2024 at 2:21 AM Sean Christopherson <seanjc@google.com> wrote: > Applied to kvm-x86 misc, I gave myself enough confidence the fastpath fix is > correct with a selftest update[*] (which I'll get applied next week). Sorry for not reviewing this before vacation; done this belatedly and patches 1/2 may require another thought (or a revert). Hopefully I'm wrong. Paolo > [*] https://lore.kernel.org/all/20240830044448.130449-1-seanjc@google.com > > [1/5] KVM: x86: Re-enter guest if WRMSR(X2APIC_ICR) fastpath is successful > https://github.com/kvm-x86/linux/commit/0dd45f2cd8cc > [2/5] KVM: x86: Dedup fastpath MSR post-handling logic > https://github.com/kvm-x86/linux/commit/ea60229af7fb > [3/5] KVM: x86: Exit to userspace if fastpath triggers one on instruction skip > https://github.com/kvm-x86/linux/commit/f7f39c50edb9 > [4/5] KVM: x86: Reorganize code in x86.c to co-locate vCPU blocking/running helpers > https://github.com/kvm-x86/linux/commit/70cdd2385106 > [5/5] KVM: x86: Add fastpath handling of HLT VM-Exits > https://github.com/kvm-x86/linux/commit/1876dd69dfe8 > > -- > https://github.com/kvm-x86/linux/tree/next >
This series was prompted by observations of HLT-exiting when debugging a throughput issue related to posted interrupts. When KVM is running in a nested scenario, a rather surprising number of HLT exits occur with an unmasked interrupt already pending. I didn't debug too deeply into the guest side of things, but I suspect what is happening is that it's fairly easy for L2 to be interrupted (by L1 or L0) between checking if it (the CPU) should enter an idle state and actually executing STI;HLT. AFAICT, a non-nested setup doesn't benefit much, if at all. But, I don't see any downside to checking for a wake event in the fastpath, e.g. it's basically a "zero" time halt-polling mechanism. The other patches fix flaws found by inspection when adding HLT-exiting to the faspath. Note, the userspace-exit logic is basically untested, i.e. I probably need to write a selftest... Sean Christopherson (5): KVM: x86: Re-enter guest if WRMSR(X2APIC_ICR) fastpath is successful KVM: x86: Dedup fastpath MSR post-handling logic KVM: x86: Exit to userspace if fastpath triggers one on instruction skip KVM: x86: Reorganize code in x86.c to co-locate vCPU blocking/running helpers KVM: x86: Add fastpath handling of HLT VM-Exits arch/x86/include/asm/kvm_host.h | 1 + arch/x86/kvm/svm/svm.c | 13 +- arch/x86/kvm/vmx/vmx.c | 2 + arch/x86/kvm/x86.c | 319 +++++++++++++++++--------------- arch/x86/kvm/x86.h | 1 + 5 files changed, 188 insertions(+), 148 deletions(-) base-commit: 332d2c1d713e232e163386c35a3ba0c1b90df83f