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Date: Tue, 07 Feb 2012 09:23:16 -0600
From: Anthony Liguori <anthony@codemonkey.ws>
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Subject: Re: [Qemu-devel] [RFC] Next gen kvm api
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To: Alexander Graf <agraf@suse.de>
Cc: KVM list <kvm@vger.kernel.org>, linux-kernel <linux-kernel@vger.kernel.org>, Avi Kivity <avi@redhat.com>, kvm-ppc <kvm-ppc@vger.kernel.org>, qemu-devel <qemu-devel@nongnu.org>

On 02/07/2012 07:40 AM, Alexander Graf wrote:
>
> Why? For the HPET timer register for example, we could have a simple MMIO hook that says
>
>    on_read:
>      return read_current_time() - shared_page.offset;
>    on_write:
>      handle_in_user_space();
>
> For IDE, it would be as simple as
>
>    register_pio_hook_ptr_r(PIO_IDE, SIZE_BYTE,&s->cmd[0]);
>    for (i = 1; i<  7; i++) {
>      register_pio_hook_ptr_r(PIO_IDE + i, SIZE_BYTE,&s->cmd[i]);
>      register_pio_hook_ptr_w(PIO_IDE + i, SIZE_BYTE,&s->cmd[i]);
>    }

You can't easily serialize updates to that address with the kernel since two 
threads are likely going to be accessing it at the same time.  That either means 
an expensive sync operation or a reliance on atomic instructions.

But not all architectures offer non-word sized atomic instructions so it gets 
fairly nasty in practice.

Regards,

Anthony Liguori