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

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.


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