Re: [PATCH v13 00/21] KVM: xen: update shared_info and vcpu_info handling

David Woodhouse
2024-02-20T16:22:21Z

On 20 February 2024 17:15:06 CET, Sean Christopherson <seanjc@google.com> wrote:
>On Tue, Feb 20, 2024, Paul Durrant wrote:
>> On 20/02/2024 15:55, Sean Christopherson wrote:
>> > On Thu, 15 Feb 2024 15:28:55 +0000, Paul Durrant wrote:
>> > > From: Paul Durrant <pdurrant@amazon.com>
>> > >
>> > > This series contains a new patch from Sean added since v12 [1]:
>> > >
>> > > * KVM: s390: Refactor kvm_is_error_gpa() into kvm_is_gpa_in_memslot()
>> > >
>> > > This frees up the function name kvm_is_error_gpa() such that it can then be
>> > > re-defined in:
>> > >
>> > > [...]
>> >
>> > *sigh*
>> >
>> > I forgot to hit "send" on this yesterday. But lucky for me, that worked out in
>> > my favor as I needed to rebase on top of kvm/kvm-uapi to avoid pointless conflicts
>> > in the uapi headeres.
>> >
>> > So....
>> >
>> > Applied to kvm-x86 xen, minus 18 and 19 (trylock stuff) and 21 (locking cleanup
>> > that we're doing elsewhere).
>> >
>>
>> Looks like you meant 17 & 18?
>
>Doh, yes.
>
>> > Paul and David, please take (another) look at the end result to make sure you don't
>> > object to any of my tweaks and that I didn't botch anything.
>> >
>>
>> What was the issue with 17? It was reasonable clean-up and I'd like to keep
>> it even without 18 being applied (and I totally understand your reasons for
>> that).
>
>I omitted it purely to avoid creating an unnecessary dependency for the trylock
>patch. That way the trylock patch (or whatever it morphs into) can be applied on
>any branch (along with the cleanup), i.e. doesn't need to be taken through kvm-x86/xen.

What about if (in_atomic() && read_trylock()) return -EAGAIN; else read_lock();

That way we don't have any even theoretical fairness issues because the trylock can fail just *once* which kicks us to the slow path and that'll take the lock normally now.

The condition might not actually be in_atomic() but I'm not working this week and you get the idea.