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KVM: x86: Fold WRMSR fastpath helpers into the main handler

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Fold the per-MSR WRMSR fastpath helpers into the main handler now that the
IPI path in particular is relatively tiny.  In addition to eliminating a
decent amount of boilerplate, this removes the ugly -errno/1/0 => bool
conversion (which is "necessitated" by kvm_x2apic_icr_write_fast()).

Opportunistically drop the comment about IPIs, as the purpose of the
fastpath is hopefully self-evident, and _if_ it needs more documentation,
the documentation (and rules!) should be placed in a more central location.

No functional change intended.

Link: https://lore.kernel.org/r/20250805190526.1453366-9-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>
This commit is contained in:
Sean Christopherson 2025-08-05 12:05:16 -07:00
parent aa2e4f0293
commit d618fb4e43
1 changed files with 5 additions and 29 deletions
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34
arch/x86/kvm/x86.c
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@ -2134,48 +2134,24 @@ static inline bool kvm_vcpu_exit_request(struct kvm_vcpu *vcpu)
kvm_request_pending(vcpu) || xfer_to_guest_mode_work_pending();
}
/*
* The fast path for frequent and performance sensitive wrmsr emulation,
* i.e. the sending of IPI, sending IPI early in the VM-Exit flow reduces
* the latency of virtual IPI by avoiding the expensive bits of transitioning
* from guest to host, e.g. reacquiring KVM's SRCU lock. In contrast to the
* other cases which must be called after interrupts are enabled on the host.
*/
static int handle_fastpath_set_x2apic_icr_irqoff(struct kvm_vcpu *vcpu, u64 data)
{
if (!lapic_in_kernel(vcpu) || !apic_x2apic_mode(vcpu->arch.apic))
return 1;
return kvm_x2apic_icr_write_fast(vcpu->arch.apic, data);
}
static int handle_fastpath_set_tscdeadline(struct kvm_vcpu *vcpu, u64 data)
{
kvm_set_lapic_tscdeadline_msr(vcpu, data);
return 0;
}
fastpath_t handle_fastpath_set_msr_irqoff(struct kvm_vcpu *vcpu)
{
u64 data = kvm_read_edx_eax(vcpu);
u32 msr = kvm_rcx_read(vcpu);
bool handled;
int r;
switch (msr) {
case APIC_BASE_MSR + (APIC_ICR >> 4):
handled = !handle_fastpath_set_x2apic_icr_irqoff(vcpu, data);
if (!lapic_in_kernel(vcpu) || !apic_x2apic_mode(vcpu->arch.apic) ||
kvm_x2apic_icr_write_fast(vcpu->arch.apic, data))
return EXIT_FASTPATH_NONE;
break;
case MSR_IA32_TSC_DEADLINE:
handled = !handle_fastpath_set_tscdeadline(vcpu, data);
kvm_set_lapic_tscdeadline_msr(vcpu, data);
break;
default:
handled = false;
break;
}
if (!handled)
return EXIT_FASTPATH_NONE;
}
kvm_vcpu_srcu_read_lock(vcpu);
r = kvm_skip_emulated_instruction(vcpu);