Re: [RFC] libxl: relax readonly check introduced by XSA-142 fix

[Ian Campbell <ian.campbell@citrix.com>]

On Thu, 2015-11-12 at 08:45 -0700, Jim Fehlig wrote:
> 
> > The commit message doesn't say anything about AHCI. Are AHCI disks
> > actually emulated correctly by QEMU with readonly=on?
> 
> I just double checked, and good thing since AHCI + readonly is another
> rejected
> combination
> 
> /usr/lib/xen/bin/qemu-system-i386 -device ahci,id=ahci0 \
>  -drive file=/tmp/disk.raw,if=none,id=ahcidisk-0,format=raw,readonly=on \
>  -device ide-hd,bus=ahci0.0,unit=0,drive=ahcidisk-0
> qemu-system-i386: -device ide-hd,bus=ahci0.0,unit=0,drive=ahcidisk-0:
> Can't use
> a read-only drive
> 
> So IDE/SATA/AHCI are all incompatible with readonly=on. I'll fix this and
> ammend
> the commit message in V2.

Just to clarify when you say "rejected" and "incompatible" do you mean that
qemu will fail to start if you try, or that it will ignore you and give a
writeable disk?

If, as I think, it will fail, why don't we just always ask and rely on qemu
to reject, instead of trying to whitelist the ones we know work in the
libxl code?

That way as people add r/o to various device types in QEMU it'll Just Work.

Ian.


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