From mboxrd@z Thu Jan 1 00:00:00 1970 From: Avi Kivity Subject: Re: [Qemu-devel] [RFC] Next gen kvm api Date: Wed, 15 Feb 2012 15:33:18 +0200 Message-ID: <4F3BB41E.60405@redhat.com> References: <4F2AB552.2070909@redhat.com> <4F2B41D6.8020603@codemonkey.ws> <51470503-DEE0-478D-8D01-020834AF6E8C@suse.de> <4F3117E5.6000105@redhat.com> <4F31241C.70404@redhat.com> <4F3141E4.8080902@codemonkey.ws> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Cc: Alexander Graf , qemu-devel , kvm-ppc , KVM list , linux-kernel To: Anthony Liguori Return-path: In-Reply-To: <4F3141E4.8080902@codemonkey.ws> Sender: kvm-ppc-owner@vger.kernel.org List-Id: kvm.vger.kernel.org On 02/07/2012 05:23 PM, 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. > > But not all architectures offer non-word sized atomic instructions so > it gets fairly nasty in practice. > I doubt that any guest accesses IDE registers from two threads in parallel. The guest will have some lock, so we could have a lock as well and be assured that there will never be contention. -- error compiling committee.c: too many arguments to function