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

On Fri, 26 Jan 2018, Andrew Cooper wrote:
> On 26/01/18 11:06, Jan Beulich wrote:
> >>>> On 25.01.18 at 21:04, <andrew.cooper3@xxxxxxxxxx> wrote:
> >> On 25/01/18 19:43, Stefano Stabellini wrote:
> >>> 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.
> >> Testing is good. I approve of testing.
> >>
> >> The problem is that what you are doing here is using a broken testing
> >> tool and instead of fixing the tool, you're bodging Xen to pro-actively
> >> search for your broken testing tool in all cases including release
> >> builds, and ignore one of Xen's safety checks.
> > What about a slightly different approach: Instead of skipping the
> > tests, issue a bright warning (along the lines of the sync_console
> > one) if the tests fail (remember that they're carried out in debug
> > builds only anyway). That'll allow people to use Xen in such an
> > environment, but makes them aware that their testing results may
> > be meaningless. And this wouldn't require probing whether we're
> > running on that qemu emulator.
> That seems reasonable.  IIRC, there is already infrastructure for adding
> extra warning text.

It is a good suggestion, I'll submit a patch to do that.
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