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KVM: x86: Conditionally resched when restarting REP string emulation

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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: 5cd21917da ("KVM: x86 emulator: restart string instruction without going back to a guest.")
Cc: stable@vger.kernel.org
Cc: David Woodhouse <dwmw2@infradead.org>
Reported-by: Stefan Teodorescu <fane@google.com>
Suggested-by: Stefan Teodorescu <fane@google.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-ID: <20261028173638.1765329-1-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
This commit is contained in:
Sean Christopherson 2026-10-28 10:36:38 -07:00 committed by Paolo Bonzini
parent b2691b3a6c
commit ae85ac25eb
1 changed files with 4 additions and 2 deletions
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6
arch/x86/kvm/x86.c
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@ -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) {