Re: [PATCH v5 33/33] KVM: x86: Add helpers to recalc physical vs. logical optimized APIC maps

Maxim Levitsky
2023-01-08T15:33:48Z

On Fri, 2023-01-06 at 01:13 +0000, Sean Christopherson wrote:
> Move the guts of kvm_recalculate_apic_map()'s main loop to two separate
> helpers to handle recalculating the physical and logical pieces of the
> optimized map. Having 100+ lines of code in the for-loop makes it hard
> to understand what is being calculated where.
>
> No functional change intended.
>
> Suggested-by: Maxim Levitsky <mlevitsk@redhat.com>
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> ---
> arch/x86/kvm/lapic.c | 250 +++++++++++++++++++++++--------------------
> 1 file changed, 133 insertions(+), 117 deletions(-)
>
> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> index 856e81a2dc64..669ea125b7e2 100644
> --- a/arch/x86/kvm/lapic.c
> +++ b/arch/x86/kvm/lapic.c
> @@ -218,6 +218,134 @@ static void kvm_apic_map_free(struct rcu_head *rcu)
> kvfree(map);
> }
>
> +static int kvm_recalculate_phys_map(struct kvm_apic_map *new,
> + struct kvm_vcpu *vcpu,
> + bool *xapic_id_mismatch)
> +{
> + struct kvm_lapic *apic = vcpu->arch.apic;
> + u32 x2apic_id = kvm_x2apic_id(apic);
> + u32 xapic_id = kvm_xapic_id(apic);
> + u32 physical_id;
> +
> + /*
> + * Deliberately truncate the vCPU ID when detecting a mismatched APIC
> + * ID to avoid false positives if the vCPU ID, i.e. x2APIC ID, is a
> + * 32-bit value. Any unwanted aliasing due to truncation results will
> + * be detected below.
> + */
> + if (!apic_x2apic_mode(apic) && xapic_id != (u8)vcpu->vcpu_id)
> + *xapic_id_mismatch = true;
> +
> + /*
> + * Apply KVM's hotplug hack if userspace has enable 32-bit APIC IDs.
> + * Allow sending events to vCPUs by their x2APIC ID even if the target
> + * vCPU is in legacy xAPIC mode, and silently ignore aliased xAPIC IDs
> + * (the x2APIC ID is truncated to 8 bits, causing IDs > 0xff to wrap
> + * and collide).
> + *
> + * Honor the architectural (and KVM's non-optimized) behavior if
> + * userspace has not enabled 32-bit x2APIC IDs. Each APIC is supposed
> + * to process messages independently. If multiple vCPUs have the same
> + * effective APIC ID, e.g. due to the x2APIC wrap or because the guest
> + * manually modified its xAPIC IDs, events targeting that ID are
> + * supposed to be recognized by all vCPUs with said ID.
> + */
> + if (vcpu->kvm->arch.x2apic_format) {
> + /* See also kvm_apic_match_physical_addr(). */
> + if ((apic_x2apic_mode(apic) || x2apic_id > 0xff) &&
> + x2apic_id <= new->max_apic_id)
> + new->phys_map[x2apic_id] = apic;
> +
> + if (!apic_x2apic_mode(apic) && !new->phys_map[xapic_id])
> + new->phys_map[xapic_id] = apic;
> + } else {
> + /*
> + * Disable the optimized map if the physical APIC ID is already
> + * mapped, i.e. is aliased to multiple vCPUs. The optimized
> + * map requires a strict 1:1 mapping between IDs and vCPUs.
> + */
> + if (apic_x2apic_mode(apic))
> + physical_id = x2apic_id;
> + else
> + physical_id = xapic_id;
> +
> + if (new->phys_map[physical_id])
> + return -EINVAL;
Very small nitpick: -EINVAL feels like redundant here,
I'll say just return -1 or returning boolean would be a tiny bit better.
But this really doesn't matter much.

> +
> + new->phys_map[physical_id] = apic;
> + }
> +
> + return 0;
> +}
> +
> +static void kvm_recalculate_logical_map(struct kvm_apic_map *new,
> + struct kvm_vcpu *vcpu)
> +{
> + struct kvm_lapic *apic = vcpu->arch.apic;
> + enum kvm_apic_logical_mode logical_mode;
> + struct kvm_lapic **cluster;
> + u16 mask;
> + u32 ldr;
> +
> + if (new->logical_mode == KVM_APIC_MODE_MAP_DISABLED)
> + return;
> +
> + if (!kvm_apic_sw_enabled(apic))
> + return;
> +
> + ldr = kvm_lapic_get_reg(apic, APIC_LDR);
> + if (!ldr)
> + return;
> +
> + if (apic_x2apic_mode(apic)) {
> + logical_mode = KVM_APIC_MODE_X2APIC;
> + } else {
> + ldr = GET_APIC_LOGICAL_ID(ldr);
> + if (kvm_lapic_get_reg(apic, APIC_DFR) == APIC_DFR_FLAT)
> + logical_mode = KVM_APIC_MODE_XAPIC_FLAT;
> + else
> + logical_mode = KVM_APIC_MODE_XAPIC_CLUSTER;
> + }
> +
> + /*
> + * To optimize logical mode delivery, all software-enabled APICs must
> + * be configured for the same mode.
> + */
> + if (new->logical_mode == KVM_APIC_MODE_SW_DISABLED) {
> + new->logical_mode = logical_mode;
> + } else if (new->logical_mode != logical_mode) {
> + new->logical_mode = KVM_APIC_MODE_MAP_DISABLED;
> + return;
> + }
> +
> + /*
> + * In x2APIC mode, the LDR is read-only and derived directly from the
> + * x2APIC ID, thus is guaranteed to be addressable. KVM reuses
> + * kvm_apic_map.phys_map to optimize logical mode x2APIC interrupts by
> + * reversing the LDR calculation to get cluster of APICs, i.e. no
> + * additional work is required.
> + */
> + if (apic_x2apic_mode(apic)) {
> + WARN_ON_ONCE(ldr != kvm_apic_calc_x2apic_ldr(kvm_x2apic_id(apic)));
> + return;
> + }
> +
> + if (WARN_ON_ONCE(!kvm_apic_map_get_logical_dest(new, ldr,
> + &cluster, &mask))) {
> + new->logical_mode = KVM_APIC_MODE_MAP_DISABLED;
> + return;
> + }
> +
> + if (!mask)
> + return;
> +
> + ldr = ffs(mask) - 1;
> + if (!is_power_of_2(mask) || cluster[ldr])
> + new->logical_mode = KVM_APIC_MODE_MAP_DISABLED;
> + else
> + cluster[ldr] = apic;
> +}
> +
> /*
> * CLEAN -> DIRTY and UPDATE_IN_PROGRESS -> DIRTY changes happen without a lock.
> *
> @@ -272,128 +400,16 @@ void kvm_recalculate_apic_map(struct kvm *kvm)
> new->logical_mode = KVM_APIC_MODE_SW_DISABLED;
>
> kvm_for_each_vcpu(i, vcpu, kvm) {
> - struct kvm_lapic *apic = vcpu->arch.apic;
> - struct kvm_lapic **cluster;
> - enum kvm_apic_logical_mode logical_mode;
> - u32 x2apic_id, physical_id;
> - u16 mask;
> - u32 ldr;
> - u8 xapic_id;
> -
> if (!kvm_apic_present(vcpu))
> continue;
>
> - xapic_id = kvm_xapic_id(apic);
> - x2apic_id = kvm_x2apic_id(apic);
> -
> - /*
> - * Deliberately truncate the vCPU ID when detecting a mismatched
> - * APIC ID to avoid false positives if the vCPU ID, i.e. x2APIC
> - * ID, is a 32-bit value. Any unwanted aliasing due to
> - * truncation results will be detected below.
> - */
> - if (!apic_x2apic_mode(apic) && xapic_id != (u8)vcpu->vcpu_id)
> - xapic_id_mismatch = true;
> -
> - /*
> - * Apply KVM's hotplug hack if userspace has enable 32-bit APIC
> - * IDs. Allow sending events to vCPUs by their x2APIC ID even
> - * if the target vCPU is in legacy xAPIC mode, and silently
> - * ignore aliased xAPIC IDs (the x2APIC ID is truncated to 8
> - * bits, causing IDs > 0xff to wrap and collide).
> - *
> - * Honor the architectural (and KVM's non-optimized) behavior
> - * if userspace has not enabled 32-bit x2APIC IDs. Each APIC
> - * is supposed to process messages independently. If multiple
> - * vCPUs have the same effective APIC ID, e.g. due to the
> - * x2APIC wrap or because the guest manually modified its xAPIC
> - * IDs, events targeting that ID are supposed to be recognized
> - * by all vCPUs with said ID.
> - */
> - if (kvm->arch.x2apic_format) {
> - /* See also kvm_apic_match_physical_addr(). */
> - if ((apic_x2apic_mode(apic) || x2apic_id > 0xff) &&
> - x2apic_id <= new->max_apic_id)
> - new->phys_map[x2apic_id] = apic;
> -
> - if (!apic_x2apic_mode(apic) && !new->phys_map[xapic_id])
> - new->phys_map[xapic_id] = apic;
> - } else {
> - /*
> - * Disable the optimized map if the physical APIC ID is
> - * already mapped, i.e. is aliased to multiple vCPUs.
> - * The optimized map requires a strict 1:1 mapping
> - * between IDs and vCPUs.
> - */
> - if (apic_x2apic_mode(apic))
> - physical_id = x2apic_id;
> - else
> - physical_id = xapic_id;
> -
> - if (new->phys_map[physical_id]) {
> - kvfree(new);
> - new = NULL;
> - goto out;
> - }
> - new->phys_map[physical_id] = apic;
> - }
> -
> - if (new->logical_mode == KVM_APIC_MODE_MAP_DISABLED ||
> - !kvm_apic_sw_enabled(apic))
> - continue;
> -
> - ldr = kvm_lapic_get_reg(apic, APIC_LDR);
> - if (!ldr)
> - continue;
> -
> - if (apic_x2apic_mode(apic)) {
> - logical_mode = KVM_APIC_MODE_X2APIC;
> - } else {
> - ldr = GET_APIC_LOGICAL_ID(ldr);
> - if (kvm_lapic_get_reg(apic, APIC_DFR) == APIC_DFR_FLAT)
> - logical_mode = KVM_APIC_MODE_XAPIC_FLAT;
> - else
> - logical_mode = KVM_APIC_MODE_XAPIC_CLUSTER;
> + if (kvm_recalculate_phys_map(new, vcpu, &xapic_id_mismatch)) {
> + kvfree(new);
> + new = NULL;
> + goto out;
> }
>
> - /*
> - * To optimize logical mode delivery, all software-enabled APICs must
> - * be configured for the same mode.
> - */
> - if (new->logical_mode == KVM_APIC_MODE_SW_DISABLED) {
> - new->logical_mode = logical_mode;
> - } else if (new->logical_mode != logical_mode) {
> - new->logical_mode = KVM_APIC_MODE_MAP_DISABLED;
> - continue;
> - }
> -
> - /*
> - * In x2APIC mode, the LDR is read-only and derived directly
> - * from the x2APIC ID, thus is guaranteed to be addressable.
> - * KVM reuses kvm_apic_map.phys_map to optimize logical mode
> - * x2APIC interrupts by reversing the LDR calculation to get
> - * cluster of APICs, i.e. no additional work is required.
> - */
> - if (apic_x2apic_mode(apic)) {
> - WARN_ON_ONCE(ldr != kvm_apic_calc_x2apic_ldr(x2apic_id));
> - continue;
> - }
> -
> - if (WARN_ON_ONCE(!kvm_apic_map_get_logical_dest(new, ldr,
> - &cluster, &mask))) {
> - new->logical_mode = KVM_APIC_MODE_MAP_DISABLED;
> - continue;
> - }
> -
> - if (!mask)
> - continue;
> -
> - ldr = ffs(mask) - 1;
> - if (!is_power_of_2(mask) || cluster[ldr]) {
> - new->logical_mode = KVM_APIC_MODE_MAP_DISABLED;
> - continue;
> - }
> - cluster[ldr] = apic;
> + kvm_recalculate_logical_map(new, vcpu);
> }
> out:
> /*

Looks good, much cleaner now.

Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>

Best regards,
Maxim Levitsky