On Thu, Oct 28, 2021, Maxim Levitsky wrote: > On Fri, 2021-10-08 at 19:12 -0700, Sean Christopherson wrote: > > Move the WARN sanity checks out of the PI descriptor update loop so as > > not to spam the kernel log if the condition is violated and the update > > takes multiple attempts due to another writer. This also eliminates a > > few extra uops from the retry path. > > > > Technically not checking every attempt could mean KVM will now fail to > > WARN in a scenario that would have failed before, but any such failure > > would be inherently racy as some other agent (CPU or device) would have > > to concurrent modify the PI descriptor. ... > Don't know for sure if this is desired. I'll would just use WARN_ON_ONCE instead > if the warning spams the log. > > If there is a race I would rather want to catch it even if rare. Paolo had similar concerns[*]. I copied the most relevant part of the discussion below, let me know if you object to the outcome. Thanks for the reviews! [*] https://lore.kernel.org/all/YXllGfrjPX1pVUx6@xxxxxxxxxx/T/#u On Wed, Oct 27, 2021 at 8:38 AM Paolo Bonzini <pbonzini@xxxxxxxxxx> wrote: > On 27/10/21 17:28, Sean Christopherson wrote: > > On Wed, Oct 27, 2021, Paolo Bonzini wrote: > > > On 27/10/21 16:41, Sean Christopherson wrote: > > > > The other thing I don't like about having the WARN in the loop is that it suggests > > > > that something other than the vCPU can modify the NDST and SN fields, which is > > > > wrong and confusing (for me). > > > > > > Yeah, I can agree with that. Can you add it in a comment above the cmpxchg > > > loop, it can be as simple as > > > > > > /* The processor can set ON concurrently. */ > > > > > > when you respin patch 21 and the rest of the series? > > > > I can definitely add a comment, but I think that comment is incorrect. > > It's completely backwards indeed. I first had "the hardware" and then > shut down my brain for a second to replace it. > > > So something like this? > > > > /* ON can be set concurrently by a different vCPU or by hardware. */ > > Yes, of course.
