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Re: [Qemu-devel] Default for phys-addr-bits? (was Re: [PATCH 4/5] x86: A

From: Gerd Hoffmann
Subject: Re: [Qemu-devel] Default for phys-addr-bits? (was Re: [PATCH 4/5] x86: Allow physical address bits to be set)
Date: Thu, 30 Jun 2016 18:14:49 +0200

> So that's mapped at an address beyond host phys-bits.
> And it hasn't failed/crashed etc - but I guess maybe nothing is using that 2G 
> space?

address@hidden ~# dmesg | grep Surface
[    4.830095] [drm] qxl: 2048M of Surface memory size

qxl bar 4 (64bit) and qxl bar 1 (32bit) are the same thing.  The 64bit
bar can be alot larger obviously.  The 32bit bar is just an alias for
the first portion of the 64bit bar.  So I guess qxl just falls back to
use bar 1 instead of bar 4 because ioremap() on bar 4 fails.

> Obviously 128T is a bit silly for maxmem at the moment, however I was 
> worrying what
> happens with 36/39/40bit hosts, and it's not unusual to pick a maxmem that's 
> a few TB
> even if the VMs you're initially creating are only a handful of GB. 
> (oVirt/RHEV seems to use
> a 4TB default for maxmem).

Oh, ok.  Should be fixed I guess.

> Still, this only hits as a problem if you hit the combination of:
>    a) You use large PCI bars

ovmf will map all 64bit bars high, even without running out of 32bit
address space.  And with virtio 1.0 pretty much every virtual machine
will have 64bit bars.


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