Re: [PATCH v2] i386: Make 'hv-reenlightenment' require explicit 'tsc-frequency' setting

[Vitaly Kuznetsov]

Eduardo Habkost <ehabkost@redhat.com> writes:

> My apologies, this was lost under the noise in my mail inbox.
> (I promise I'm trying to improve)
>
> On Wed, Mar 31, 2021 at 01:39:48PM +0200, Vitaly Kuznetsov wrote:
>> Commit 561dbb41b1d7 "i386: Make migration fail when Hyper-V reenlightenment
>> was enabled but 'user_tsc_khz' is unset" forbade migrations with when guest
>> has opted for reenlightenment notifications but 'tsc-frequency' wasn't set
>> explicitly on the command line. This works but the migration fails late and
>> this may come as an unpleasant surprise. To make things more explicit,
>> require 'tsc-frequency=' on the command line when 'hv-reenlightenment' was
>> enabled. Make the change affect 6.0+ machine types only to preserve
>> previously-valid configurations.
>> 
>> Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
>> Acked-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
>
> Even if the 6.0 release gets delayed, I wouldn't be comfortable
> including this in a -rc4.
>
> What if the user does not plan to live migrate the machine at
> all?  Why is this case different from the ~25
> migrate_add_blocker() calls in QEMU, where we block migration but
> still let the VM run?

The question is when do we want to let the user know about the
problem. By refusing to start with 'hv-reenlightenment' and without
'tsc-frequency' we make it sure he knows early. 

We can, indeed, replace this with migrate_add_blocker() call but the
fact that the VM is not migratable may come as a (late) surprise (we
can certainly print a warning but these may be hidden by upper layers). 

Also, v1 of this patch was implementing a slightly different approach
failing the migration late if we can't set tsc frequency on the
destination host. Explicit 'tsc-frequency=' was not required.

Personally, I'm comfortable with any approach, please advise.

-- 
Vitaly