It has been reported that the internal workings of kvm_gmem_prepare_folio() incurs noticeable overhead for large guests even for platforms where kvm_arch_gmem_prepare() is a no-op. Provide a new kvm_arch_gmem_prepare_needed() hook so that architectures that set CONFIG_HAVE_KVM_GMEM_PREPARE can still opt-out of issuing the kvm_arch_gmem_prepare() callback if the particular KVM instance doesn't require any sort of special preparation of its gmem pages prior to use. Link: https://lore.kernel.org/lkml/20240228202906.GB10568@xxxxxxxxxxxxxxxxxxxxx/ Suggested-by: Isaku Yamahata <isaku.yamahata@xxxxxxxxx> Signed-off-by: Michael Roth <michael.roth@xxxxxxx> --- include/linux/kvm_host.h | 1 + virt/kvm/guest_memfd.c | 10 ++++++++++ 2 files changed, 11 insertions(+) diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h index 2f5074eff958..5b8308b5e4af 100644 --- a/include/linux/kvm_host.h +++ b/include/linux/kvm_host.h @@ -2466,6 +2466,7 @@ static inline int kvm_gmem_undo_get_pfn(struct kvm *kvm, #ifdef CONFIG_HAVE_KVM_GMEM_PREPARE int kvm_arch_gmem_prepare(struct kvm *kvm, gfn_t gfn, kvm_pfn_t pfn, int max_order); +bool kvm_arch_gmem_prepare_needed(struct kvm *kvm); #endif #ifdef CONFIG_HAVE_KVM_GMEM_INVALIDATE diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c index 74e19170af8a..4ce0056d1149 100644 --- a/virt/kvm/guest_memfd.c +++ b/virt/kvm/guest_memfd.c @@ -13,6 +13,13 @@ struct kvm_gmem { struct list_head entry; }; +#ifdef CONFIG_HAVE_KVM_GMEM_PREPARE +bool __weak kvm_arch_gmem_prepare_needed(struct kvm *kvm) +{ + return false; +} +#endif + static int kvm_gmem_prepare_folio(struct inode *inode, pgoff_t index, struct folio *folio) { #ifdef CONFIG_HAVE_KVM_GMEM_PREPARE @@ -27,6 +34,9 @@ static int kvm_gmem_prepare_folio(struct inode *inode, pgoff_t index, struct fol gfn_t gfn; int rc; + if (!kvm_arch_gmem_prepare_needed(kvm)) + continue; + slot = xa_load(&gmem->bindings, index); if (!slot) continue; -- 2.25.1
