Re: [PATCH v5 2/6] evtchn: convert domain event lock to an r/w one

[Julien Grall <julien@xen.org>]

Hi Jan,

As discussed in [1], I think it would good to revive this patch.

AFAICT, this patch was dropped because the performance was thought to be 
minimal. However, I think it would be a better way to resolve the 
problem that one is trying to address [1].

So I will do another review of this patch.

On 27/01/2021 08:16, Jan Beulich wrote:
> Especially for the use in evtchn_move_pirqs() (called when moving a vCPU
> across pCPU-s) and the ones in EOI handling in PCI pass-through code,
> serializing perhaps an entire domain isn't helpful when no state (which
> isn't e.g. further protected by the per-channel lock) changes.
> 
> Unfortunately this implies dropping of lock profiling for this lock,
> until r/w locks may get enabled for such functionality.
> 
> While ->notify_vcpu_id is now meant to be consistently updated with the
> per-channel lock held, an extension applies to ECS_PIRQ: The field is
> also guaranteed to not change with the per-domain event lock held for
> writing. Therefore the link_pirq_port() call from evtchn_bind_pirq()
> could in principle be moved out of the per-channel locked regions, but
> this further code churn didn't seem worth it.

This doesn't seem to apply on upstream anymore. Would you be able to 
respin it?

I have looked at the place where you use read_lock() rather than 
write_lock(). They all look fine to me, so I would be fine to give my 
reviewed-by on the next version (assuming there are nothing wrong with 
the rebase :)).

Cheers,

[1] 
https://lore.kernel.org/xen-devel/acd0dfae-b045-8505-3f6c-30ce72653660@suse.com/

-- 
Julien Grall