linux-kernel - Re: [PATCH] KVM: x86: inline kvm_ioapic_handles

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Message-ID: <20150324001743.GA16287@amt.cnet>
Date:	Mon, 23 Mar 2015 21:17:44 -0300
From:	Marcelo Tosatti <mtosatti@...hat.com>
To:	Radim Krčmář <rkrcmar@...hat.com>
Cc:	linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [PATCH] KVM: x86: inline kvm_ioapic_handles_vector()

On Thu, Mar 19, 2015 at 09:52:41PM +0100, Radim Krčmář wrote:
> An overhead from function call is not appropriate for its size and
> frequency of execution.
> 
> Suggested-by: Paolo Bonzini <pbonzini@...hat.com>
> Signed-off-by: Radim Krčmář <rkrcmar@...hat.com>
> ---
>   I'm not very fond of that smp_rmb(): there is no real synchronization
>   against update_handled_vectors(), 

Yes, because the guest OS should provide synchronization (it should
shutdown interrupts before attempting to modify IOAPIC table).

The smp_wmb is necessary.

>  so the only point I see is to drop
>   cached value of handled_vectors, which seems like bad use of LFENCE.

test_bit has volatile on *addr, so don't see why the smp_rmb is
necessary at all.

Applied, thanks.

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