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Re: [Qemu-devel] Default for phys-addr-bits? (was Re: [PATCH 4/5] x86: A
From: |
Dr. David Alan Gilbert |
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 18:12:34 +0100 |
User-agent: |
Mutt/1.6.1 (2016-04-27) |
* Gerd Hoffmann (address@hidden) wrote:
> > 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.
Hmm for me it's saying it mapped 64M on the setup with 64T maxmem and 48bit
phys-bits;
even though the bar is showing it as OK; how is the guest ioremap detecting a
problem?
> > 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.
Hmm OK let me think about this; using current BIOS+old virtio+host phys-bits
on a 36bit host, with maxmem=4T would apparently work because
nothing would actually get mapped that high.
The same with OVMF would work as well, because generally you wouldn't have
any 64bit bars; but then you turn on virtio 1.0 and.. well then what happens?
The guest sees it's 36bit phys-address bits so linux probably drops the
bars associated with the virtio? Hmm.
With current downstream qemu's 40bit physical address bit you'd
get those bar's mapped; so it might break badly - except if
we can figure out what causes my 2GB qxl bar not to happen
as at the start of this message.
Dave
> cheers,
> Gerd
>
--
Dr. David Alan Gilbert / address@hidden / Manchester, UK
- Re: [Qemu-devel] Default for phys-addr-bits? (was Re: [PATCH 4/5] x86: Allow physical address bits to be set), (continued)
- Re: [Qemu-devel] Default for phys-addr-bits? (was Re: [PATCH 4/5] x86: Allow physical address bits to be set), Andrea Arcangeli, 2016/06/22
- Re: [Qemu-devel] Default for phys-addr-bits? (was Re: [PATCH 4/5] x86: Allow physical address bits to be set), Michael S. Tsirkin, 2016/06/22
- Re: [Qemu-devel] Default for phys-addr-bits? (was Re: [PATCH 4/5] x86: Allow physical address bits to be set), Gerd Hoffmann, 2016/06/23
- Re: [Qemu-devel] Default for phys-addr-bits? (was Re: [PATCH 4/5] x86: Allow physical address bits to be set), Michael S. Tsirkin, 2016/06/23
- Re: [Qemu-devel] Default for phys-addr-bits? (was Re: [PATCH 4/5] x86: Allow physical address bits to be set), Gerd Hoffmann, 2016/06/24
- Re: [Qemu-devel] Default for phys-addr-bits? (was Re: [PATCH 4/5] x86: Allow physical address bits to be set), Michael S. Tsirkin, 2016/06/24
- Re: [Qemu-devel] Default for phys-addr-bits? (was Re: [PATCH 4/5] x86: Allow physical address bits to be set), Dr. David Alan Gilbert, 2016/06/29
- Re: [Qemu-devel] Default for phys-addr-bits? (was Re: [PATCH 4/5] x86: Allow physical address bits to be set), Gerd Hoffmann, 2016/06/30
- Re: [Qemu-devel] Default for phys-addr-bits? (was Re: [PATCH 4/5] x86: Allow physical address bits to be set), Dr. David Alan Gilbert, 2016/06/30
- Re: [Qemu-devel] Default for phys-addr-bits? (was Re: [PATCH 4/5] x86: Allow physical address bits to be set), Gerd Hoffmann, 2016/06/30
- Re: [Qemu-devel] Default for phys-addr-bits? (was Re: [PATCH 4/5] x86: Allow physical address bits to be set),
Dr. David Alan Gilbert <=
- Re: [Qemu-devel] Default for phys-addr-bits? (was Re: [PATCH 4/5] x86: Allow physical address bits to be set), Michael S. Tsirkin, 2016/06/22
- Re: [Qemu-devel] Default for phys-addr-bits? (was Re: [PATCH 4/5] x86: Allow physical address bits to be set), Andrea Arcangeli, 2016/06/22
- Re: [Qemu-devel] [PATCH 4/5] x86: Allow physical address bits to be set, Michael S. Tsirkin, 2016/06/18
- Re: [Qemu-devel] [PATCH 4/5] x86: Allow physical address bits to be set, Paolo Bonzini, 2016/06/20
- Re: [Qemu-devel] [PATCH 4/5] x86: Allow physical address bits to be set, Marcel Apfelbaum, 2016/06/17