Hi David, Thanks for the series. I noticed the testing results, but didn't see a performance comparison for the SRCU change itself.
Do you happen to have any numbers comparing the GPC invalidation latency with regular SRCU vs. `synchronize_srcu_atomic()`? If there are also numbers with the reader-free fastpath, that would be useful for understanding its impact as well. No need to run anything specifically if you don't already have the numbers. Thanks, Kunwu On Mon, Sep 21, 2026 at 5:19 AM David Woodhouse <[email protected]> wrote: > > Now that Paul et al have so kindly provided atomic SRCU for us, it would > be churlish not to use it. > > Based on the merge of kvm/next (d4b7fb647204) with Paul's dev.2026.09.17a¹ > as posted last week², this series: > > • Converts the GPC to use atomic SRCU, ditching the rwlock which was a > scaling and latency problem, and which PREEMPT_RT hated because it > turned the rwlock into a sleeping lock. > > • Cleans up the intentional lack of dirty-tracking of Xen shinfo and > vcpu_info pages (Sean). > > • Converts steal-time / preempted status to use gfn_to_pfn_cache. (Carsten) > > • Reinstates guest-mode pinning: replacing the old GUEST_USES_PFN mode, > letting nested state pages be cached and pinned while in guest context. > > • nVMX: Uses GPC for the L1 MSR bitmap, and guest mode for APIC access > and vAPIC pages. (Fred) > > • nSVM: Uses GPC for vmcb12 / MSR-permissions / IO-permissions pages. This > cuts the vmcb12 transition cost measurably (selftest included). > > • Returns -EAGAIN from cache lookups which race with memslot updates, > and re-posts KVM_REQ_GET_NESTED_STATE_PAGES on memslot generation > bumps so pinned caches are revalidated lazily. > > Tested: rcutorture atomic-SRCU (srcud, reader_flavor=0x10) on three > hosts; KVM selftests including new invalidation and transition tests; > >1 week soak of VM lifecycle + invalidation reproducers on 128-way > RT+KASAN+lockdep, 192-way RT and 192-way PREEMPT_DYNAMIC hosts. And > booted an actual Xen guest in QEMU a few times... > > ¹ https://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu.git > dev.2026.09.17a > ² https://lore.kernel.org/rcu/[email protected]/ > > There have been previous series which attempt to deal with various parts > of GPC locking, and the steal time one has definitely been posted before > a few times, but let's just call this v1: > https://git.infradead.org/?p=users/dwmw2/linux.git;a=shortlog;h=refs/heads/gpc-srcu > > Carsten Stollmaier (1): > KVM: x86: Use gfn_to_pfn_cache for steal time / preempted status > > David Woodhouse (10): > KVM: pfncache: Use atomic SRCU for readers instead of a rwlock > KVM: x86: Request the guest TLB flush from record_steal_time() > KVM: pfncache: Add guest-mode pinning (GUEST_USES_PFN successor) > KVM: pfncache: Return -EAGAIN for a lookup which hits an invalid memslot > KVM: x86: Post KVM_REQ_GET_NESTED_STATE_PAGES on memslot updates > KVM: x86: Move nested GPC lock helpers to x86.h as kvm_gpc_lock_page() > KVM: nSVM: Use a gfn_to_pfn_cache for the vmcb12 page > KVM: nSVM: Cache L1's MSR permissions map pages > KVM: nSVM: Cache L1's IO permissions map pages > KVM: selftests: Add nested transition benchmark > > Fred Griffoul (3): > KVM: nVMX: Implement cache for L1 MSR bitmap > KVM: nVMX: Use pinned pfncache for L1 APIC virtualization pages > KVM: selftests: Add nested VMX APIC cache invalidation test > > Sean Christopherson (3): > KVM: x86/xen: Extract delivery of event to vCPU into a separate helper > KVM: x86/xen: Explicitly tag "shared info" page as never being dirty > tracked > KVM: x86/xen: Don't dirty track "vCPU info" page > > > arch/x86/include/asm/kvm_host.h | 2 +- > arch/x86/kvm/msrs.c | 7 +- > arch/x86/kvm/svm/nested.c | 112 +++- > arch/x86/kvm/svm/svm.h | 16 + > arch/x86/kvm/vmx/nested.c | 356 ++++++++++-- > arch/x86/kvm/vmx/vmx.c | 11 +- > arch/x86/kvm/vmx/vmx.h | 16 +- > arch/x86/kvm/x86.c | 181 ++++--- > arch/x86/kvm/x86.h | 33 ++ > arch/x86/kvm/xen.c | 279 +++++----- > include/linux/kvm_host.h | 86 ++- > include/linux/kvm_types.h | 52 +- > include/linux/srcu.h | 7 + > tools/testing/selftests/kvm/Makefile.kvm | 2 + > .../selftests/kvm/x86/nested_transition_bench.c | 204 +++++++ > .../selftests/kvm/x86/vmx_apic_update_test.c | 299 +++++++++++ > virt/kvm/kvm_main.c | 9 + > virt/kvm/pfncache.c | 596 > +++++++++++++++++---- > 18 files changed, 1876 insertions(+), 392 deletions(-)
