Alex Williamson wrote:
When I try to boot guests using a recent Linux kernel (2.6.26+), memory
above 3.5G gets thrown away with an error like this:
WARNING: BIOS bug: CPU MTRRs don't cover all of memory, losing 4608MB of RAM.
And it's true, we're only providing MTRRs for memory below 4G. In fact
rombios32 knows very little, if anything, about memory above 4G, as seen
by memory reporting in the SMBIOS table.
It looks like the Linux kernel MTRR code does have a bail-out point for
kvm/qemu, but that was only effective before we started reporting MTRRs.
On real hardware, I have two systems that do this two different ways.
The first is an Intel based system, which reports MTRRs to cover the I/O
space, then defaults the rest of memory to WB. The second is an AMD
based system which uses MTRRs to cover memory below 4G, then seems to
have a special AMD MSR to describe the top of memory above 4G. Xen
appears to mimic the first approach.
Is there any reason that KVM sets the default MTRR type to UC, then only
sets up MTRRs for the memory below 4G?
The thinking is that if we hotplug a device, its memory must be set to
uncacheable by default.
the patch below is a possible
approach to continue down this path and enlighten rombios32 about the
real top of memory, and setup MTRRs appropriately. It doesn't address
SMBIOS or whatever causes grub to only report upper memory below 4G.
Alternatively we could switch to the Intel/Xen system approach, but it
seems rombios32 needs to understand the extra memory at some point
anyway. Thoughts? BTW, another benefit to the default WB approach is
that MTRRs are a limited resource and there will be memory sizes we
can't fully cover using the approach below.
Yes, especially with the pci hole causing any memory size to require
many MTRRs.
I'd like to switch to default WB + MTRRs covering the pci space, but I'd
like to get a clear understanding of how we handle hotplug. Meanwhile,
I've applied your patch.
--
error compiling committee.c: too many arguments to function
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