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Date: Wed, 31 Aug 2022 21:42:09 +0300
From: Maxim Levitsky <mlevitsk@...hat.com>
To: Sean Christopherson <seanjc@...gle.com>,
Paolo Bonzini <pbonzini@...hat.com>
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
Suravee Suthikulpanit <suravee.suthikulpanit@....com>,
Li RongQing <lirongqing@...du.com>
Subject: Re: [PATCH 16/19] KVM: x86: Explicitly track all possibilities for
APIC map's logical modes
On Wed, 2022-08-31 at 00:35 +0000, Sean Christopherson wrote:
> Track all possibilities for the optimized APIC map's logical modes
> instead of overloading the pseudo-bitmap and treating any "unknown" value
> as "invalid".
>
> As documented by the now-stale comment above the mode values, the values
> did have meaning when the optimized map was originally added. That
> dependent logical was removed by commit e45115b62f9a ("KVM: x86: use
> physical LAPIC array for logical x2APIC"), but the obfuscated behavior
> and its comment were left behind.
>
> Opportunistically rename "mode" to "logical_mode", partly to make it
> clear that the "disabled" case applies only to the logical map, but also
> to prove that there is no lurking code that expects "mode" to be a bitmap.
>
> Functionally, this is a glorified nop.
>
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
> arch/x86/include/asm/kvm_host.h | 21 ++++++++++--------
> arch/x86/kvm/lapic.c | 38 ++++++++++++++++++++++++---------
> 2 files changed, 40 insertions(+), 19 deletions(-)
>
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 1f51411f3112..0184e64ab555 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -955,19 +955,22 @@ struct kvm_arch_memory_slot {
> };
>
> /*
> - * We use as the mode the number of bits allocated in the LDR for the
> - * logical processor ID. It happens that these are all powers of two.
> - * This makes it is very easy to detect cases where the APICs are
> - * configured for multiple modes; in that case, we cannot use the map and
> - * hence cannot use kvm_irq_delivery_to_apic_fast either.
> + * Track the mode of the optimized logical map, as the rules for decoding the
> + * destination vary per mode. Enabling the optimized logical map requires all
> + * software-enabled local APIs to be in the same mode, each addressable APIC to
> + * be mapped to only one MDA, and each MDA to map to at most one APIC.
> */
> -#define KVM_APIC_MODE_XAPIC_CLUSTER 4
> -#define KVM_APIC_MODE_XAPIC_FLAT 8
> -#define KVM_APIC_MODE_X2APIC 16
> +enum kvm_apic_logical_mode {
> + KVM_APIC_MODE_SW_DISABLED,
> + KVM_APIC_MODE_XAPIC_CLUSTER,
> + KVM_APIC_MODE_XAPIC_FLAT,
> + KVM_APIC_MODE_X2APIC,
> + KVM_APIC_MODE_MAP_DISABLED,
> +};
>
> struct kvm_apic_map {
> struct rcu_head rcu;
> - u8 mode;
> + enum kvm_apic_logical_mode logical_mode;
> u32 max_apic_id;
> union {
> struct kvm_lapic *xapic_flat_map[8];
> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> index 8209caffe3ab..3b6ef36b3963 100644
> --- a/arch/x86/kvm/lapic.c
> +++ b/arch/x86/kvm/lapic.c
> @@ -168,7 +168,12 @@ static bool kvm_use_posted_timer_interrupt(struct kvm_vcpu *vcpu)
>
> static inline bool kvm_apic_map_get_logical_dest(struct kvm_apic_map *map,
> u32 dest_id, struct kvm_lapic ***cluster, u16 *mask) {
> - switch (map->mode) {
> + switch (map->logical_mode) {
> + case KVM_APIC_MODE_SW_DISABLED:
> + /* Arbitrarily use the flat map so that @cluster isn't NULL. */
> + *cluster = map->xapic_flat_map;
> + *mask = 0;
> + return true;
> case KVM_APIC_MODE_X2APIC: {
> u32 offset = (dest_id >> 16) * 16;
> u32 max_apic_id = map->max_apic_id;
> @@ -193,8 +198,10 @@ static inline bool kvm_apic_map_get_logical_dest(struct kvm_apic_map *map,
> *cluster = map->xapic_cluster_map[(dest_id >> 4) & 0xf];
> *mask = dest_id & 0xf;
> return true;
> + case KVM_APIC_MODE_MAP_DISABLED:
> + return false;
> default:
> - /* Not optimized. */
> + WARN_ON_ONCE(1);
BTW unless I am mistaken, this warning is guest triggerable, and thus as you say when
'panic_on_warn=1', this will panic the host kernel.
Best regards,
Maxim Levitsky
> return false;
> }
> }
> @@ -256,10 +263,12 @@ void kvm_recalculate_apic_map(struct kvm *kvm)
> goto out;
>
> new->max_apic_id = max_id;
> + 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;
> @@ -314,7 +323,8 @@ void kvm_recalculate_apic_map(struct kvm *kvm)
> new->phys_map[physical_id] = apic;
> }
>
> - if (!kvm_apic_sw_enabled(apic))
> + if (new->logical_mode == KVM_APIC_MODE_MAP_DISABLED ||
> + !kvm_apic_sw_enabled(apic))
> continue;
>
> ldr = kvm_lapic_get_reg(apic, APIC_LDR);
> @@ -322,25 +332,33 @@ void kvm_recalculate_apic_map(struct kvm *kvm)
> continue;
>
> if (apic_x2apic_mode(apic)) {
> - new->mode |= KVM_APIC_MODE_X2APIC;
> + logical_mode = KVM_APIC_MODE_X2APIC;
> } else {
> ldr = GET_APIC_LOGICAL_ID(ldr);
> if (kvm_lapic_get_reg(apic, APIC_DFR) == APIC_DFR_FLAT)
> - new->mode |= KVM_APIC_MODE_XAPIC_FLAT;
> + logical_mode = KVM_APIC_MODE_XAPIC_FLAT;
> else
> - new->mode |= KVM_APIC_MODE_XAPIC_CLUSTER;
> + logical_mode = KVM_APIC_MODE_XAPIC_CLUSTER;
> }
> + if (new->logical_mode != KVM_APIC_MODE_SW_DISABLED &&
> + new->logical_mode != logical_mode) {
> + new->logical_mode = KVM_APIC_MODE_MAP_DISABLED;
> + continue;
> + }
> + new->logical_mode = logical_mode;
>
> - if (!kvm_apic_map_get_logical_dest(new, ldr, &cluster, &mask))
> + 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->mode = KVM_APIC_MODE_XAPIC_FLAT |
> - KVM_APIC_MODE_XAPIC_CLUSTER;
> + new->logical_mode = KVM_APIC_MODE_MAP_DISABLED;
> continue;
> }
> cluster[ldr] = apic;
> @@ -993,7 +1011,7 @@ static bool kvm_apic_is_broadcast_dest(struct kvm *kvm, struct kvm_lapic **src,
> {
> if (kvm->arch.x2apic_broadcast_quirk_disabled) {
> if ((irq->dest_id == APIC_BROADCAST &&
> - map->mode != KVM_APIC_MODE_X2APIC))
> + map->logical_mode != KVM_APIC_MODE_X2APIC))
> return true;
> if (irq->dest_id == X2APIC_BROADCAST)
> return true;
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