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 09:34:33 +0300
Message-ID: <20130606063432.GQ4725@redhat.com>
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To: Anthony Liguori <aliguori@us.ibm.com>
Cc: Peter Maydell <peter.maydell@linaro.org>, 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 Wed, Jun 05, 2013 at 07:41:17PM -0500, Anthony Liguori wrote:
> "H. Peter Anvin" <hpa@zytor.com> writes:
> 
> > On 06/05/2013 03:08 PM, Anthony Liguori wrote:
> >>>
> >>> Definitely an option.  However, we want to be able to boot from native
> >>> devices, too, so having an I/O BAR (which would not be used by the OS
> >>> driver) should still at the very least be an option.
> >> 
> >> What makes it so difficult to work with an MMIO bar for PCI-e?
> >> 
> >> With legacy PCI, tracking allocation of MMIO vs. PIO is pretty straight
> >> forward.  Is there something special about PCI-e here?
> >> 
> >
> > It's not tracking allocation.  It is that accessing memory above 1 MiB
> > is incredibly painful in the BIOS environment, which basically means
> > MMIO is inaccessible.
> 
> 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. The only way I know to access MMIO bars from 16 bit
is to use SMM which we do not have in KVM.

--
			Gleb.