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Re: [Qemu-devel] [Xen-devel] [BUG 1747]Guest could't find bootable devic


From: Stefano Stabellini
Subject: Re: [Qemu-devel] [Xen-devel] [BUG 1747]Guest could't find bootable device with memory more than 3600M
Date: Thu, 13 Jun 2013 16:40:52 +0100
User-agent: Alpine 2.02 (DEB 1266 2009-07-14)

On Thu, 13 Jun 2013, Ian Campbell wrote:
> On Thu, 2013-06-13 at 14:54 +0100, George Dunlap wrote:
> > On 13/06/13 14:44, Stefano Stabellini wrote:
> > > On Wed, 12 Jun 2013, George Dunlap wrote:
> > >> On 12/06/13 08:25, Jan Beulich wrote:
> > >>>>>> On 11.06.13 at 19:26, Stefano Stabellini
> > >>>>>> <address@hidden> wrote:
> > >>>> I went through the code that maps the PCI MMIO regions in hvmloader
> > >>>> (tools/firmware/hvmloader/pci.c:pci_setup) and it looks like it already
> > >>>> maps the PCI region to high memory if the PCI bar is 64-bit and the 
> > >>>> MMIO
> > >>>> region is larger than 512MB.
> > >>>>
> > >>>> Maybe we could just relax this condition and map the device memory to
> > >>>> high memory no matter the size of the MMIO region if the PCI bar is
> > >>>> 64-bit?
> > >>> I can only recommend not to: For one, guests not using PAE or
> > >>> PSE-36 can't map such space at all (and older OSes may not
> > >>> properly deal with 64-bit BARs at all). And then one would generally
> > >>> expect this allocation to be done top down (to minimize risk of
> > >>> running into RAM), and doing so is going to present further risks of
> > >>> incompatibilities with guest OSes (Linux for example learned only in
> > >>> 2.6.36 that PFNs in ioremap() can exceed 32 bits, but even in
> > >>> 3.10-rc5 ioremap_pte_range(), while using "u64 pfn", passes the
> > >>> PFN to pfn_pte(), the respective parameter of which is
> > >>> "unsigned long").
> > >>>
> > >>> I think this ought to be done in an iterative process - if all MMIO
> > >>> regions together don't fit below 4G, the biggest one should be
> > >>> moved up beyond 4G first, followed by the next to biggest one
> > >>> etc.
> > >> First of all, the proposal to move the PCI BAR up to the 64-bit range is 
> > >> a
> > >> temporary work-around.  It should only be done if a device doesn't fit 
> > >> in the
> > >> current MMIO range.
> > >>
> > >> We have three options here:
> > >> 1. Don't do anything
> > >> 2. Have hvmloader move PCI devices up to the 64-bit MMIO hole if they 
> > >> don't
> > >> fit
> > >> 3. Convince qemu to allow MMIO regions to mask memory (or what it thinks 
> > >> is
> > >> memory).
> > >> 4. Add a mechanism to tell qemu that memory is being relocated.
> > >>
> > >> Number 4 is definitely the right answer long-term, but we just don't 
> > >> have time
> > >> to do that before the 4.3 release.  We're not sure yet if #3 is 
> > >> possible; even
> > >> if it is, it may have unpredictable knock-on effects.
> > >>
> > >> Doing #2, it is true that many guests will be unable to access the device
> > >> because of 32-bit limitations.  However, in #1, *no* guests will be able 
> > >> to
> > >> access the device.  At least in #2, *many* guests will be able to do so. 
> > >>  In
> > >> any case, apparently #2 is what KVM does, so having the limitation on 
> > >> guests
> > >> is not without precedent.  It's also likely to be a somewhat tested
> > >> configuration (unlike #3, for example).
> > > I would avoid #3, because I don't think is a good idea to rely on that
> > > behaviour.
> > > I would also avoid #4, because having seen QEMU's code, it's wouldn't be
> > > easy and certainly not doable in time for 4.3.
> > >
> > > So we are left to play with the PCI MMIO region size and location in
> > > hvmloader.
> > >
> > > I agree with Jan that we shouldn't relocate unconditionally all the
> > > devices to the region above 4G. I meant to say that we should relocate
> > > only the ones that don't fit. And we shouldn't try to dynamically
> > > increase the PCI hole below 4G because clearly that doesn't work.
> > > However we could still increase the size of the PCI hole below 4G by
> > > default from start at 0xf0000000 to starting at 0xe0000000.
> > > Why do we know that is safe? Because in the current configuration
> > > hvmloader *already* increases the PCI hole size by decreasing the start
> > > address every time a device doesn't fit.
> > > So it's already common for hvmloader to set pci_mem_start to
> > > 0xe0000000, you just need to assign a device with a PCI hole size big
> > > enough.
> 
> Isn't this the exact case which is broken? And therefore not known safe
> at all?

hvmloader sets pci_mem_start to 0xe0000000 and works with
qemu-xen-traditional but it doesn't with qemu-xen (before the patch that
increases the default pci hole size in QEMU).
What I was trying to say is: "it's already common for hvmloader to set
pci_mem_start to 0xe0000000 with qemu-xen-traditional".


> > > My proposed solution is:
> > >
> > > - set 0xe0000000 as the default PCI hole start for everybody, including
> > > qemu-xen-traditional
> 
> What is the impact on existing qemu-trad guests?
> 
> It does mean that guest which were installed with a bit less than 4GB
> RAM may now find a little bit of RAM moves above 4GB to make room for
> the bigger whole. If they can dynamically enable PAE that might be ok.

Yes, the amount of below 4G ram is going to be a bit less.


> Does this have any impact on Windows activation?

I don't think so: I assigned graphic cards with less than 512MB of
videoram to Windows guests before without compromising the Windows
license. I'll get more info on this.


> > > - move above 4G everything that doesn't fit and support 64-bit bars
> > > - print an error if the device doesn't fit and doesn't support 64-bit
> > > bars
> > 
> > Also, as I understand it, at the moment:
> > 1. Some operating systems (32-bit XP) won't be able to use relocated devices
> > 2. Some devices (without 64-bit BARs) can't be relocated
> > 3. qemu-traditional is fine with a resized <4GiB MMIO hole.
> > 
> > So if we have #1 or #2, at the moment an option for a work-around is to 
> > use qemu-traditional.
> > 
> > However, if we add your "print an error if the device doesn't fit", then 
> > this option will go away -- this will be a regression in functionality 
> > from 4.2.
> 
> Only if print an error also involves aborting. It could print an error
> (lets call it a warning) and continue, which would leave the workaround
> viable.

Regardless of the warning we print and the strategy we decide to have to
fix the problem, we can always have a manually selectable workaround to
switch to the old behaviour if we want to (aside from the minimum PCI
hole size that might be harder to make selectable). It just
means more code in hvmloader.



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