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

From: George Dunlap
Subject: Re: [Qemu-devel] [Xen-devel] [BUG 1747]Guest could't find bootable device with memory more than 3600M
Date: Wed, 12 Jun 2013 11:05:23 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/20130510 Thunderbird/17.0.6

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
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

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).


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