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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: |
Fri, 24 Jun 2016 07:55:33 +0200 |
On Do, 2016-06-23 at 19:38 +0300, Michael S. Tsirkin wrote:
> On Thu, Jun 23, 2016 at 10:40:03AM +0200, Gerd Hoffmann wrote:
> > Hi,
> >
> > > > Well the crash of guest phys bits > host phys bits, should be easy to
> > > > reproduce by booting a 65GB guest on a 64GB RAM + 2GB swap host with
> > > > 36 host phys bits using the upstream qemu that forces the guest phys
> > > > bits to 40.
> > >
> > > So you supply more RAM than host can address, and guest crashes?
> >
> > Yep. The only reason we don't see this happening in practice is that
> > it's probably next to impossible to find a machine which has (a) only 36
> > physical address lines and (b) allows to plug that much RAM.
> >
> > > Why are we worried about it?
> >
> > It's more a issue with pci ressources. In theory seabios/edk2 could go
> > figure how big the physical address space is, then map 64bit pci bars as
> > high as possible, thereby making stuff like etc/reserved-memory-end in
> > fw_cfg unnecessary.
> >
> > But with qemu saying 40 phys bits are available even if they are not
> > this approach isn't going to fly ...
> >
> > cheers,
> > Gerd
>
> Nah, x86 guests really need to go by _CRS.
Yep, we can implement the "soft-phys-bits" that way.
> bios doesn't want to parse that
> so it can go by some fw cfg file instead.
firmware can't use it anyway because the firmware first maps the bars,
the loads acpi tables (while qemu generates _CRS entries according to
the bios mappings).
> Going by phys bits won't work on old qemu so I don't believe it's
> practical.
Indeed, so I guess we'll have to stick to the current approach of
mapping 64bit bars above ram (or etc/reserved-memory-end if present).
cheers,
Gerd
- 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), Paolo Bonzini, 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] Default for phys-addr-bits? (was Re: [PATCH 4/5] x86: Allow physical address bits to be set), Paolo Bonzini, 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] 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] 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 <=
- 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, 2016/06/30
- 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