Re: [PATCH 2/9] KVM: x86: dynamic kvm_apic_map

[Peter Xu <peterx@redhat.com>]

On Wed, May 25, 2016 at 06:15:00PM +0200, Radim Krčmář wrote:
> 2016-05-23 16:04+0800, Peter Xu:
> > Hi, Radim,
> > 
> > Got several questions inline.
> > 
> > On Fri, May 06, 2016 at 10:53:58PM +0200, Radim Krčmář wrote:
> >> x2APIC supports up to 2^32-1 LAPICs, but most guest in coming years will
> >> have slighly less VCPUs.  Dynamic size saves memory at the cost of
> >> turning one constant into a variable.
> > 
> > From the manual, I see x2apic only support 2^20-16 processors, not
> > 2^32-1.  Which one is correct?
> 
> "up to" is a crucial part.  Physical x2APIC can address 2^32-1, logical
> x2APIC can address (2^16-1)*16 LAPICs and the OS can choose which mode
> to use.
> 
> 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.
> 
> (Well, broadcasts would make even xAPIC "support" an unbounded amount of
>  LAPICs. :])
> 
> > 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.
> > 
> > [...]
> > 
> >> +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;
> >> +		// XXX: phys_map must be allocated as multiples of 16
> >> +		if (offset < map->size) {
> >> +			*cluster = &map->phys_map[offset];
> >> +			*mask = dest_id & 0xffff;
> > 
> > [1]
> 
> This function is called ...get_LOGICAL_dest, so only logical addresses
> are going to be passed to it.  Yes, it cannot handle APIC ID > 2^20-16.

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

-- peterx