Re: [Xen-devel] [PATCH] xen,x86: introduce tcg_errata

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Thu, 25 Jan 2018, Andrew Cooper wrote:
> On 25/01/18 18:37, Stefano Stabellini wrote:
> > The TCG emulator in QEMU is not good enough to pass the the tests in
> > stub_selftest. Detect if Xen is running on TCG early, then drop the
> > tests if it is the case.
> >
> > Signed-off-by: Stefano Stabellini <sstabellini@xxxxxxxxxx>
> 
> I'm still opposed to this change.  The selftests demonstrate that TCG
> doesn't work for an architectural area we depend, and simply pretending
> its not buggy isn't ok.  If this were a piece of real hardware, it would
> be blacklisted in a similar fashion to XSA-9.

> I still haven't seen a convincing enough usecase to cause Xen to
> proactively look for Qemu in all cases including release builds on real
> hardware.

Testing is a very good use case. It doesn't have to replace testing on
real hardware, it can complement it. You can try it yourself, it might
become your new favorite test environment :-) Just launch
qemu-system-x86_64 with -cpu qemu64,+svm. FYI this is my full command
line:

qemu-system-x86_64 -m 4G -smp 2 -cpu qemu64,+svm \
    -nographic -serial stdio -monitor none \
    --bios /usr/share/ovmf/OVMF.fd \
    -netdev user,id=hostnet0 -device 
virtio-net-pci,netdev=hostnet0,bus=pci.0,addr=0x3 \
    -device virtio-balloon-pci,id=balloon0,bus=pci.0,addr=0x6 \
    -drive file=$DISK,if=none,id=drive-virtio-disk0,format=raw,cache=none \
    -device 
virtio-blk-pci,bus=pci.0,addr=0x4,drive=drive-virtio-disk0,id=virtio-disk0,bootindex=1
 \
    -device virtio-rng-pci


I haven't investigated yet the QEMU side of the problem. I can dig
deeper there, then, once we understand the full extent of the issue,
we could decide what to do with this patch. However, I performed a few
tests so far and I haven't seen any other issues. I can start HVM and
PVH guests without issues.

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