linux-kernel - Re: [PATCH v3 03/14] KVM: x86: use physical LAPIC array for logical x2APIC

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Date:	Tue, 2 Aug 2016 21:46:59 +0800
From:	Wanpeng Li <kernellwp@...il.com>
To:	Radim Krčmář <rkrcmar@...hat.com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	kvm <kvm@...r.kernel.org>, Paolo Bonzini <pbonzini@...hat.com>,
	"Lan, Tianyu" <tianyu.lan@...el.com>,
	Igor Mammedov <imammedo@...hat.com>,
	Jan Kiszka <jan.kiszka@....de>, Peter Xu <peterx@...hat.com>,
	Yang Zhang <yang.zhang.wz@...il.com>
Subject: Re: [PATCH v3 03/14] KVM: x86: use physical LAPIC array for logical x2APIC

2016-07-13 4:09 GMT+08:00 Radim Krčmář <rkrcmar@...hat.com>:
> Logical x2APIC IDs map injectively to physical x2APIC IDs, so we can
> reuse the physical array for them.  This allows us to save space by
> separating logical xAPIC maps.
>
> Signed-off-by: Radim Krčmář <rkrcmar@...hat.com>
> ---
>  arch/x86/include/asm/kvm_host.h |  6 ++--
>  arch/x86/kvm/lapic.c            | 69 +++++++++++++++++++++--------------------
>  2 files changed, 39 insertions(+), 36 deletions(-)
>
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 53d39771842b..3194b19b9c7b 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -683,8 +683,10 @@ struct kvm_apic_map {
>         struct rcu_head rcu;
>         u8 mode;
>         struct kvm_lapic *phys_map[256];
> -       /* first index is cluster id second is cpu id in a cluster */
> -       struct kvm_lapic *logical_map[16][16];
> +       union {
> +               struct kvm_lapic *xapic_flat_map[8];
> +               struct kvm_lapic *xapic_cluster_map[16][4];
> +       };
>  };
>
>  /* Hyper-V emulation context */
> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> index 2987843657db..9880d03f533d 100644
> --- a/arch/x86/kvm/lapic.c
> +++ b/arch/x86/kvm/lapic.c
> @@ -115,26 +115,36 @@ static inline int apic_enabled(struct kvm_lapic *apic)
>         (LVT_MASK | APIC_MODE_MASK | APIC_INPUT_POLARITY | \
>          APIC_LVT_REMOTE_IRR | APIC_LVT_LEVEL_TRIGGER)
>
> -/* The logical map is definitely wrong if we have multiple
> - * modes at the same time.  (Physical map is always right.)
> - */
> -static inline bool kvm_apic_logical_map_valid(struct kvm_apic_map *map)
> -{
> -       return !(map->mode & (map->mode - 1));
> -}
> +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) {
> +       case KVM_APIC_MODE_X2APIC: {
> +               u32 offset = (dest_id >> 16) * 16;
> +               u32 max_apic_id = ARRAY_SIZE(map->phys_map) - 1;
>
> -static inline void
> -apic_logical_id(struct kvm_apic_map *map, u32 dest_id, u16 *cid, u16 *lid)
> -{
> -       unsigned lid_bits;
> +               if (offset <= max_apic_id) {
> +                       u8 cluster_size = min(max_apic_id - offset + 1, 16U);
>
> -       BUILD_BUG_ON(KVM_APIC_MODE_XAPIC_CLUSTER !=  4);
> -       BUILD_BUG_ON(KVM_APIC_MODE_XAPIC_FLAT    !=  8);
> -       BUILD_BUG_ON(KVM_APIC_MODE_X2APIC        != 16);
> -       lid_bits = map->mode;
> +                       *cluster = &map->phys_map[offset];
> +                       *mask = dest_id & (0xffff >> (16 - cluster_size));
> +               } else {
> +                       *mask = 0;
> +               }
>
> -       *cid = dest_id >> lid_bits;
> -       *lid = dest_id & ((1 << lid_bits) - 1);
> +               return true;
> +               }
> +       case KVM_APIC_MODE_XAPIC_FLAT:
> +               *cluster = map->xapic_flat_map;
> +               *mask = dest_id & 0xff;
> +               return true;
> +       case KVM_APIC_MODE_XAPIC_CLUSTER:
> +               *cluster = map->xapic_cluster_map[dest_id >> 4];
> +               *mask = dest_id & 0xf;
> +               return true;
> +       default:
> +               /* Not optimized. */
> +               return false;
> +       }
>  }
>
>  static void recalculate_apic_map(struct kvm *kvm)
> @@ -152,7 +162,8 @@ static void recalculate_apic_map(struct kvm *kvm)
>
>         kvm_for_each_vcpu(i, vcpu, kvm) {
>                 struct kvm_lapic *apic = vcpu->arch.apic;
> -               u16 cid, lid;
> +               struct kvm_lapic **cluster;
> +               u16 mask;
>                 u32 ldr, aid;
>
>                 if (!kvm_apic_present(vcpu))
> @@ -174,13 +185,11 @@ static void recalculate_apic_map(struct kvm *kvm)
>                                 new->mode |= KVM_APIC_MODE_XAPIC_CLUSTER;
>                 }
>
> -               if (!kvm_apic_logical_map_valid(new))
> +               if (!kvm_apic_map_get_logical_dest(new, ldr, &cluster, &mask))
>                         continue;

As the comments of the removed function kvm_apic_logical_map_valid()
mentioned: "The logical map is definitely wrong if we have multiple
modes at the same time". So once one lapic offend this, we will just
skip it. However, the offend mode is still not removed from the
new->mode, which results in  all the lapics after the offend one will
be skipped and they can't get the benefit of the apic map table. I can
make a patch once it make sense to you.

Regards,
Wanpeng Li