From mboxrd@z Thu Jan  1 00:00:00 1970
From: Peter Xu <peterx@redhat.com>
Subject: Re: [PATCH 2/9] KVM: x86: dynamic kvm_apic_map
Date: Mon, 30 May 2016 13:24:09 +0800
Message-ID: <20160530052409.GA6656@pxdev.xzpeter.org>
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Cc: kvm@vger.kernel.org, Paolo Bonzini <pbonzini@redhat.com>,
	"Lan, Tianyu" <tianyu.lan@intel.com>,
	Igor Mammedov <imammedo@redhat.com>,
	Jan Kiszka <jan.kiszka@web.de>
To: Radim =?utf-8?B?S3LEjW3DocWZ?= <rkrcmar@redhat.com>
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On Wed, May 25, 2016 at 06:15:00PM +0200, Radim Kr=C4=8Dm=C3=A1=C5=99 w=
rote:
> 2016-05-23 16:04+0800, Peter Xu:
> > Hi, Radim,
> >=20
> > Got several questions inline.
> >=20
> > On Fri, May 06, 2016 at 10:53:58PM +0200, Radim Kr=C4=8Dm=C3=A1=C5=99=
 wrote:
> >> x2APIC supports up to 2^32-1 LAPICs, but most guest in coming year=
s will
> >> have slighly less VCPUs.  Dynamic size saves memory at the cost of
> >> turning one constant into a variable.
> >=20
> > From the manual, I see x2apic only support 2^20-16 processors, not
> > 2^32-1.  Which one is correct?
>=20
> "up to" is a crucial part.  Physical x2APIC can address 2^32-1, logic=
al
> x2APIC can address (2^16-1)*16 LAPICs and the OS can choose which mod=
e
> to use.
>=20
> I think that machines with APIC IDs >2^20 will still be able to use
> logical x2APIC, but higher APIC IDs are only be addressable with
> physical x2APIC.
>=20
> (Well, broadcasts would make even xAPIC "support" an unbounded amount=
 of
>  LAPICs. :])
>=20
> > From below codes [1], I still see 2^20-16, since we are using high
> > 16 bits of dest ID as cluster ID (0-0xfffe, while I guess 0xffff
> > should be reserved), and lower 16 bits as processor/lapic mask.
> >=20
> > [...]
> >=20
> >> +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 =3D (dest_id >> 16) * 16;
> >> +		// XXX: phys_map must be allocated as multiples of 16
> >> +		if (offset < map->size) {
> >> +			*cluster =3D &map->phys_map[offset];
> >> +			*mask =3D dest_id & 0xffff;
> >=20
> > [1]
>=20
> This function is called ...get_LOGICAL_dest, so only logical addresse=
s
> are going to be passed to it.  Yes, it cannot handle APIC ID > 2^20-1=
6.

Thanks for the explanation. I thought x2apic only support logical
mode, seems I was wrong. :)

-- peterx