From mboxrd@z Thu Jan  1 00:00:00 1970
From: Gleb Natapov <gleb@redhat.com>
Subject: Re: [PATCH RFC] virtio-pci: new config layout: using memory BAR
Date: Thu, 6 Jun 2013 18:10:12 +0300
Message-ID: <20130606151012.GD4725@redhat.com>
References: <87bo7ktvaw.fsf@codemonkey.ws> <20130605162029.GB26561@redhat.com>
	<51AFA92B.2030203@zytor.com> <87fvwwmdmf.fsf@codemonkey.ws>
	<51AFB3DD.1050902@zytor.com> <874ndcb47i.fsf@codemonkey.ws>
	<51AFC4A6.6070607@zytor.com> <877gi8nk9e.fsf@codemonkey.ws>
	<20130606063432.GQ4725@redhat.com> <51B0A578.2030104@redhat.com>
Mime-Version: 1.0
Content-Type: text/plain; charset="us-ascii"
Content-Transfer-Encoding: 7bit
Return-path: <virtualization-bounces@lists.linux-foundation.org>
Content-Disposition: inline
In-Reply-To: <51B0A578.2030104@redhat.com>
List-Unsubscribe: <https://lists.linuxfoundation.org/mailman/options/virtualization>,
	<mailto:virtualization-request@lists.linux-foundation.org?subject=unsubscribe>
List-Archive: <http://lists.linuxfoundation.org/pipermail/virtualization/>
List-Post: <mailto:virtualization@lists.linux-foundation.org>
List-Help: <mailto:virtualization-request@lists.linux-foundation.org?subject=help>
List-Subscribe: <https://lists.linuxfoundation.org/mailman/listinfo/virtualization>,
	<mailto:virtualization-request@lists.linux-foundation.org?subject=subscribe>
Sender: virtualization-bounces@lists.linux-foundation.org
Errors-To: virtualization-bounces@lists.linux-foundation.org
To: Gerd Hoffmann <kraxel@redhat.com>
Cc: Peter Maydell <peter.maydell@linaro.org>, Anthony Liguori <aliguori@us.ibm.com>, kvm@vger.kernel.org, "Michael S. Tsirkin" <mst@redhat.com>, virtualization@lists.linux-foundation.org, Stefan Hajnoczi <stefanha@redhat.com>, "H. Peter Anvin" <hpa@zytor.com>, Paolo Bonzini <pbonzini@redhat.com>, KONRAD Frederic <fred.konrad@greensocs.com>
List-Id: virtualization@lists.linuxfoundation.org

On Thu, Jun 06, 2013 at 05:06:32PM +0200, Gerd Hoffmann wrote:
> On 06/06/13 08:34, Gleb Natapov wrote:
> > On Wed, Jun 05, 2013 at 07:41:17PM -0500, Anthony Liguori wrote:
> >>
> >> Oh, you mean in real mode.
> >>
> >> SeaBIOS runs the virtio code in 32-bit mode with a flat memory layout.
> >> There are loads of ASSERT32FLAT()s in the code to make sure of this.
> >>
> > Well, not exactly. Initialization is done in 32bit, but disk
> > reads/writes are done in 16bit mode since it should work from int13
> > interrupt handler.
> 
> Exactly.  It's "only" the initialization code which has ASSERt32FLAT()
> all over the place.  Which actually is the majority of the code in most
> cases as all the hardware detection and initialization code is there.
> But kicking I/O requests must work from 16bit mode too.
> 
> > The only way I know to access MMIO bars from 16 bit
> > is to use SMM which we do not have in KVM.
> 
> For seabios itself this isn't a big issue, see pci_{readl,writel} in
> src/pci.c.  When called in 16bit mode it goes into 32bit mode
> temporarily, just for accessing the mmio register.  ahci driver uses it,
> xhci driver (wip atm) will use that too, and virtio-{blk,scsi} drivers
> in seabios can do the same.
> 
Isn't this approach broken? How can SeaBIOS be sure it restores real
mode registers to exactly same state they were before entering 32bit
mode?

> But as hpa mentioned it will be more tricky for option roms (aka
> virtio-net).
> 
> cheers,
>   Gerd
> 

--
			Gleb.