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

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Date:	Mon, 11 Jul 2016 18:04:51 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Radim Krčmář <rkrcmar@...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



On 11/07/2016 17:52, Radim Krčmář wrote:
> 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.

You can do

	if (x2apic mode)
		return lapic->vcpu->vcpu_id;
	else
		return get_reg(APIC_ID) >> 24;

The point is to avoid returning a shifted APIC_ID without shifting it.

The alternative of course is just caching it, which at this point is not
particularly harder...

Paolo

>>> (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.