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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 08:10:46 +0200


> Something somewhere in qemu/ kernel/ firmware is already reading the number
> of physical bits to determine PCI mapping; if I do:
> ./x86_64-softmmu/qemu-system-x86_64 -m 4096,slots=16,maxmem=128T

No, it's not the physbits.  You add some memory hotplug slots here.
Qemu will ask seabios to reserve address space for those, which seabios
promptly does and maps 64bit pci bars above the reserved address space.

>    -vga none -device qxl-vga,bus=pcie.0,ram_size_mb=2048,vram64_size_mb=2048 
> -vnc /home/vms/7.2a.qcow2 -chardev stdio,mux=on,id=mon -mon 
> chardev=mon,mode=readline -cpu host,phys-bits=48
> it will happily map the qxl VRAM right up high, but if I lower
> the phys-bits down to 46 it won't.

I suspect the linux kernel remaps the bar because the seabios mapping is
unreachable.  Check dmesg.


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