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

[Vitaly Kuznetsov]

Eduardo Habkost <ehabkost@redhat.com> writes:

> On Wed, Apr 14, 2021 at 03:51:37PM +0200, Vitaly Kuznetsov wrote:
>> 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.
>
> I agree with what you are trying to do, I just wonder why we
> wouldn't do exactly the same for all other migrate_add_blocker()
> calls too (whatever the solution we choose here).
>
> I'd suggest the following:
>
> - For people who use "-only-migratable", using
>   migrate_add_blocker() here already solves the problem.
>
> - For people who don't use "-only-migratable", we could change
>   migrate_add_blocker() to print a warning by default (only on
>   machine types where live migration is supported).
>
> - For people who don't need live migration and don't want to see
>   those warnings, we can introduce a "-no-migration" option.

All make sense to me, let me try to draft v3 based on your proposal.

Thanks!

-- 
Vitaly