linux-kernel - Re: [Qemu-devel] [RFC] Next gen kvm api

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Date:	Tue, 7 Feb 2012 16:28:37 +0100
From:	Alexander Graf <agraf@...e.de>
To:	Anthony Liguori <anthony@...emonkey.ws>
Cc:	Avi Kivity <avi@...hat.com>, qemu-devel <qemu-devel@...gnu.org>,
	kvm-ppc <kvm-ppc@...r.kernel.org>,
	KVM list <kvm@...r.kernel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [Qemu-devel] [RFC] Next gen kvm api


On 07.02.2012, at 16:23, Anthony Liguori wrote:

> 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.

Yes. Essentially we want a mutex for them.

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

Well, we can always require fields to be word sized.


Alex

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