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

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Date:	Mon, 11 Jul 2016 16:14:46 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Radim Krčmář <rkrcmar@...hat.com>,
	Yang Zhang <yang.zhang.wz@...il.com>
Cc:	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 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?

>> 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?

> (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...

> 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...

Paolo

> Having apic id stored in big endian, or "0-7,8-31" format, would be
> safest.  I wanted to change the apic map to do incremental updates with
> with respect to the APIC that has changed, instead of being completely
> recomputed, so maybe the time is now. :)
>