linux-kernel - Re: [PATCH v2 04/13] KVM: x86: dynamic kvm_apic

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Date:	Mon, 11 Jul 2016 17:52:29 +0200
From:	Radim Krčmář <rkrcmar@...hat.com>
To:	Paolo Bonzini <pbonzini@...hat.com>
Cc:	Yang Zhang <yang.zhang.wz@...il.com>, linux-kernel@...r.kernel.org,
	kvm@...r.kernel.org, "Lan, Tianyu" <tianyu.lan@...el.com>,
	Igor Mammedov <imammedo@...hat.com>,
	Jan Kiszka <jan.kiszka@....de>, Peter Xu <peterx@...hat.com>
Subject: Re: [PATCH v2 04/13] KVM: x86: dynamic kvm_apic_map

2016-07-11 16:14+0200, Paolo Bonzini:
> On 11/07/2016 15:48, Radim Krčmář wrote:
>>>> I guess the easiest solution is to replace kvm_apic_id with a field in
>>>> struct kvm_lapic, which is already shifted right by 24 in xAPIC mode.
>> 
>> (I guess the fewest LOC is to look at vcpu->vcpu_id, which is equal to
>>  x2apic id.  xapic id cannot be greater than 255 and all of those are
>>  covered by the initial value of max_id.)
> 
> Yes, this would work too.  Or even better perhaps, look at vcpu->vcpu_id
> in kvm_apic_id?

APIC ID is writeable in xAPIC mode, which would make the implementation
weird without an extra variable.  Always read-only APIC ID would be
best, IMO.

>>> Or we can just simply put the assignment of apic_base to the end.
>> 
>> Yes, this would work, I'd also remove recalculates from
>> kvm_apic_set_*apic_id() and add a compiler barrier with comment for good
>> measure, even though set_virtual_x2apic_mode() serves as one.
> 
> Why a compiler barrier?

True, it should be a proper pair of smp_wmb() and smp_rmb() in
recalculate ... and current kvm_apic_id() reads in a wrong order, so
changing the apic_base alone update wouldn't get rid of this race.

>> (What makes a bit wary is that it doesn't avoid the same problem if we
>>  changed KVM to reset apic id to xapic id first when disabling apic.)
> 
> Yes, this is why I prefer it fixed once and for all in kvm_apic_id...

Seems most reasonable.  We'll need to be careful to have a correct value
in the apic page, but there shouldn't be any races there.

>> Races in recalculation and APIC ID changes also lead to invalid physical
>> maps, which haven't been taken care of properly ...
> 
> Hmm, true, but can be fixed separately.  Probably the mutex should be
> renamed so that it can be taken outside recalculate_apic_map...

Good point, it'll make reasoning easier and shouldn't introduce any
extra scalability issues.