Re: [PATCH v4 2/4] xen/x86: introduce self modifying code test

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Wed, Dec 20, 2023 at 10:09:12AM +0100, Jan Beulich wrote:
> On 19.12.2023 21:31, Andrew Cooper wrote:
> > On 15/12/2023 11:18 am, Roger Pau Monne wrote:
> >> +        if ( system_state < SYS_STATE_active )
> >> +            printk(XENLOG_ERR "%s test failed\n", tests[i].name);
> > 
> > This is a test hypercall, for the purpose of running testing, in
> > combination with test livepatches.  Eliding the diagnostics isn't ok.
> > 
> > Logspam concerns aren't an issue.  If the user runs `while :; do
> > xen-test-smc; done` in dom0 then they get to have a full dmesg ring.
> > 
> > Don't let that get in the way of having a sensible time figuring out
> > what went wrong.
> 
> Since it was me who asked to suppress this when invoked through sysctl:
> I can live with an unconditional printk() here, but I think this goes
> against the "what can be done in user space should be done there"
> principle: If enough information is propagated back, user space should
> be able to provide all necessary output without even a need for the
> observer to run "xl dmesg".

But propagating such information to user-space is also not trivial,
and involves more logic in the hypervisor.

I think the point of "what can be done in user space should be done
there" is to avoid adding unnecessary code to Xen, but in this case
printing such detailed information in user-space would require adding
more code to Xen in order to propagate it.

Thanks, Roger.