From ae85ac25eb671d174811cc3101549448671fd505 Mon Sep 17 00:00:00 2001 From: Sean Christopherson Date: Wed, 28 Oct 2026 10:36:38 -0700 Subject: [PATCH] KVM: x86: Conditionally resched when restarting REP string emulation When restarting emulation, e.g. of a REP-prefixed string instruction, conditionally reschedule as emulating guest writes can be deceptively slow and expensive, e.g. when shadow paging is in use and each write can trigger write tracking. In a perfect world, restarting emulation (or not) would be less arbitrary, e.g. would honor not only host events (NEED_RESCHED, signals, etc.) but also guest events (e.g. by checking kvm_vcpu_has_events()). Unfortunately, as pointed out by Sashiko, pio_in_emulated()'s read-ahead logic is baked into the restart logic, as KVM relies on restarting emulation to drain the cache and avoid duplicate reads from the I/O port. I.e. re-entering the guest on kvm_vcpu_has_events() or __xfer_to_guest_mode_work_pending() would introduce architecturally-incorrect, guest-visible changes. Fixes: 5cd21917da24 ("KVM: x86 emulator: restart string instruction without going back to a guest.") Cc: stable@vger.kernel.org Cc: David Woodhouse Reported-by: Stefan Teodorescu Suggested-by: Stefan Teodorescu Signed-off-by: Sean Christopherson Message-ID: <20261028173638.1765329-1-seanjc@google.com> Signed-off-by: Paolo Bonzini --- arch/x86/kvm/x86.c | 6 ++++-- 1 file changed, 4 insertions(+), 2 deletions(-) diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c index 32563a91fba9..c4a06214cde6 100644 --- a/arch/x86/kvm/x86.c +++ b/arch/x86/kvm/x86.c @@ -6558,10 +6558,12 @@ int x86_emulate_instruction(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa, } else if (vcpu->arch.complete_userspace_io) { writeback = false; r = 0; - } else if (r == EMULATION_RESTART) + } else if (r == EMULATION_RESTART) { + cond_resched(); goto restart; - else + } else { r = 1; + } writeback: if (writeback) {