Re: [Qemu-devel] [RFC PATCH v3 3/3] fw/pci: Allocate IGD stolen memory

[Kevin O'Connor]

On Sat, Feb 13, 2016 at 01:57:09PM -0700, Alex Williamson wrote:
> On Sat, 13 Feb 2016 15:05:09 -0500
> "Kevin O'Connor" <address@hidden> wrote:
> > On Sat, Feb 13, 2016 at 11:51:51AM -0700, Alex Williamson wrote:
> > > On Sat, 13 Feb 2016 13:18:39 -0500
> > > "Kevin O'Connor" <address@hidden> wrote:  
> > > > On Sat, Feb 13, 2016 at 08:12:09AM -0700, Alex Williamson wrote:  
> > > > > On Fri, 12 Feb 2016 21:49:04 -0500
> > > > > "Kevin O'Connor" <address@hidden> wrote:    
> > > > > > On Fri, Feb 12, 2016 at 05:23:18PM -0700, Alex Williamson wrote:    
> > > > > > > Intel IGD makes use of memory allocated and marked reserved by the
> > > > > > > BIOS as a stolen memory range.  For the most part, guest drivers 
> > > > > > > don't
> > > > > > > make use of this, but our achilles heel is the vBIOS.  The vBIOS
> > > > > > > programs the device to use the host stolen memory range and it's 
> > > > > > > used
> > > > > > > in the pre-boot environment.  Generally the guest won't have 
> > > > > > > access to
> > > > > > > the host stolen memory area, so these accesses either land in VM
> > > > > > > memory or unassigned space and generate IOMMU faults.  By 
> > > > > > > allocating
> > > > > > > this range in SeaBIOS and programming it into the device, QEMU 
> > > > > > > (via
> > > > > > > vfio) can make sure this guest allocated stolen memory range is 
> > > > > > > used
> > > > > > > instead.      
> > > > > > 
> > > > > > What does "vBIOS" mean in this context?  Is it the video device 
> > > > > > option
> > > > > > rom or something else?    
> > > > > 
> > > > > vBIOS = video BIOS, you're correct, it's just the video device option
> > > > > ROM.    
> > > > 
> > > > Is the problem from when the host runs the video option rom, or is the
> > > > problem from the guest (via SeaBIOS) running the video option rom?  If
> > > > the guest is running the option rom, is it the first time the option
> > > > rom has been run for the machine (ie, was the option rom not executed
> > > > on the host when the host machine first booted)?
> > > > 
> > > > FWIW, many of the chromebooks use coreboot with Intel graphics and a
> > > > number of users have installed SeaBIOS (running natively) on their
> > > > machines.  Running the intel video option rom more than once has been
> > > > known to cause issues.  
> > > 
> > > The issue is in the VM and it occurs every time the option ROM is
> > > executed.  Standard VGA text mode displays fine (ex. SeaBIOS version
> > > string and boot menu), but any sort of extended graphics mode (ex. live
> > > CD boot menu) tries to make use of the host memory area which
> > > corresponds to the stolen memory area of the physical device.  We're
> > > not really sure how the ROM execution arrives at these addresses (not
> > > from the device according to access traces), but we can see when the
> > > ROM is writing these addresses to the device and modify they addresses
> > > to point at a VM memory range which we've allocated.  That's what this
> > > code attempts to do, allocate a buffer and tell QEMU about it via the
> > > BDSM (Base Data of Stolen Memory) register.  
> > 
> > Forgive me if I'm not fully understanding this.  If I read what you're
> > saying then the sequence is something like:
> > 
> > 1 - the host system bios (or vgabios) programs the GTT/stolen memory
> >     base register at host system bootup time and reserves it in the
> >     host e820 map.
> > 
> > 2 - upon running qemu, the guest reruns the vga bios option rom which
> >     seems to work (ie, text mode works)
> > 
> > 3 - in the guest, upon running a bootloader that uses graphics mode,
> >     the bootloader calls the vgabios to switch to graphics mode, and
> >     the vgabios sends commands to the graphics hardware that somehow
> >     reference the host stolen memory
> 
> What exactly happens here isn't clear to me, but this is a plausible
> explanation.  What we see in tracing access to the hardware is that a
> bunch of addresses are written to the device that fall within the host
> e820 reserved area and then the device starts generating IOMMU faults
> trying to access those addresses.
> 
> > 4 - your patch causes QEMU to catch these commands with references to
> >     the host stolen memory and replace them with references to the
> >     guest stolen memory (which seabios creates)
> > 
> > Am I understanding the above correctly?
> 
> Yes.
>  
> > Is the only reason to run the intel option rom in the guest for
> > bootloader graphic mode support?  Last time I looked, the intel vga
> > hardware could fully emulate a legacy vga device - if the device is in
> > vga compatibility mode then it may be possible to have seavgabios
> > drive mode changes.
> 
> I have a SandyBridge based laptop (Lenovo W520) where the LCD panel
> won't turn on without the vBIOS.

This confuses me - why didn't the host system BIOS turn on the LCD
panel during host bootup?

>Another desktop IvyBridge system
> doesn't really care about the vBIOS so long as we don't ask it to
> output anything before the guest native drivers are loaded.  If we
> could, I think we'd just enable vBIOS for laptop panel support, but
> that's really not an option, it's going to run as a boot option ROM as
> well, so we need to fix the issues that it generates there.

>From my experience with coreboot, running the vga option rom multiple
times during a given boot is very fragile.  (By multiple times, I mean
either the host running it and then a guest, or running it multiple
times from multiple guests.)  YMMV.

> > > > [...]  
> > > > > The write to 0x5C is used by QEMU code that traps writes to the
> > > > > device I/O port BAR and replaces the host stolen memory address
> > > > > with the new guest address generated here.  0x5C is initialized to
> > > > > 0x0 by kernel vfio code, so we can detect whether it has been
> > > > > written.  If not written, QEMU has no place to redirect to for
> > > > > stolen memory and it will either overlap VM memory or an unassigned
> > > > > area.  The former may corrupt VM memory, the latter throws host
> > > > > errors.  We could in QEMU halt with a hardware error if 0x5C hasn't
> > > > > been programmed.    
> > > > 
> > > > So, if I understand correctly, 0x5C is not a "real" register on the
> > > > hardware, but is instead just a mechanism to give QEMU the address of
> > > > some guest visible ram?  
> > > 
> > > It is a real register, BDSM that is virtualized by vfio turning it
> > > effectively into a scratch register.  On physical hardware the
> > > register is read-only.
> > >    
> > > > BTW, is 0xFC a "real" register in the hardware?  How does the guest
> > > > find the location of the "OpRegion" if SeaBIOS allocates it?  
> > > 
> > > 0xFC is the ASL Storage register, the guest finds the location of the
> > > OpRegion using this register.  This is another register that is
> > > read-only on real hardware but virtualized through vfio so we can
> > > relocate the OpRegion into the VM address space.
> > > 
> > > I've found that allocating a dummy MMIO BAR does work as an alternative
> > > for mapping space for this stolen memory into the VM address space.
> > > For a Linux guest it works to allocate BAR5 on the IGD device.
> > > Windows10 is not so happy with this, but does work if I allocate the
> > > BAR on something like the ISA bridge device.  My guess is that the IGD
> > > driver in Windows freaks out at finding this strange new BAR on its
> > > device.  So I'll need to come up with an algorithm for either creating
> > > a dummy PCI device to host this BAR or trying to add it to other
> > > existing devices.  It's certainly a more self-contained solution this
> > > way, so I expect we'll only need patch 1/3 from this series.  Thanks,  
> > 
> > Okay.  (I'm not saying patch 3 is bad, but okay.)
> > 
> > If you go through the trouble of mapping the BDSM through a pci bar,
> > then why not do the same with ASLS then too?
> 
> I suppose we could do that.  There are a few nuances to the fake BAR
> solution:
> 
> 1) The BAR needs to get mapped and not remapped while in use - usually
>    not a problem.
> 
> 2) The guest needs to not disable the device we attach the BARs to,
>    which it might do if it doesn't recognize the device.
> 
> 3) We need to be careful about adding BARs to devices the guest does
>    have drivers for or we might overlap real functionality.
> 
> 4) If we create a dummy device with bogus IDs, it will show up with an
>    exclamation mark in device manager, which makes people unhappy.
> 
> So from a perspective of being self contained, the fake BAR solution is
> very good, but it's not without issue.  I'll try to think of what sort
> of dummy device we could create that would always have a guest driver,
> but nothing that a couple extra BARs would interfere with.  Maybe a
> generic PCI bridge.  Thanks,

Okay.  Again, I'm not stating a preferred direction.

BTW, I wonder if the recent discussion between Michael and Igor is
relevant here:
  https://lists.gnu.org/archive/html/qemu-devel/2016-01/msg05602.html

-Kevin