linux-kernel - Re: [Question] the check of ioeventfd collision in kvm_*assign_ioeventfd

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Date:   Fri, 31 Jul 2020 14:39:29 +0800
From:   Zhenyu Ye <yezhenyu2@...wei.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
CC:     "S. Tsirkin, Michael" <mst@...hat.com>, <gleb@...hat.com>,
        <linux-kernel@...r.kernel.org>,
        <linux-arm-kernel@...ts.infradead.org>, <kvm@...r.kernel.org>,
        <kvmarm@...ts.cs.columbia.edu>,
        Xiexiangyou <xiexiangyou@...wei.com>
Subject: Re: [Question] the check of ioeventfd collision in
 kvm_*assign_ioeventfd_idx

On 2020/7/31 2:03, Paolo Bonzini wrote:
> Yes, I think it's not needed. Probably the deassign check can be turned into an assertion?
> 
> Paolo
> 

I think we can do this in the same function, and turnt he check of
p->eventfd into assertion in kvm_deassign_ioeventfd_idx(). Just like:

---8<---
static inline struct _ioeventfd *
get_ioeventfd(struct kvm *kvm, enum kvm_bus bus_idx,
              struct kvm_ioeventfd *args)
{
        static struct _ioeventfd *_p;
        bool wildcard = !(args->flags & KVM_IOEVENTFD_FLAG_DATAMATCH);

        list_for_each_entry(_p, &kvm->ioeventfds, list)
                if (_p->bus_idx == bus_idx &&
                    _p->addr == args->addr &&
                    (!_p->length || !args->len ||
                     (_p->length == args->len &&
                      (_p->wildcard || wildcard ||
                       _p->datamatch == args->datamatch))))
                        return _p;

        return NULL;
}

kvm_deassign_ioeventfd_idx() {
	...
	p = get_ioeventfd(kvm, bus_idx, args);
	if (p) {
		assert(p->eventfd == eventfd);
		...
	}

---8<----

This may be easier to understand (keep the same logic in assign/deassign).

I will send a formal patch soon.

Thanks,
Zhenyu


> Il gio 30 lug 2020, 16:36 Zhenyu Ye <yezhenyu2@...wei.com <mailto:yezhenyu2@...wei.com>> ha scritto:
> 
>     Hi all,
> 
>     There are checks of ioeventfd collision in both kvm_assign_ioeventfd_idx()
>     and kvm_deassign_ioeventfd_idx(), however, with different logic.
> 
>     In kvm_assign_ioeventfd_idx(), this is done by ioeventfd_check_collision():
>     ---8<---
>             if (_p->bus_idx == p->bus_idx &&
>                 _p->addr == p->addr &&
>                 (!_p->length || !p->length ||
>                  (_p->length == p->length &&
>                   (_p->wildcard || p->wildcard ||
>                    _p->datamatch == p->datamatch))))
>                     // then we consider the two are the same
>     ---8<---
> 
>     The logic in kvm_deassign_ioeventfd_idx() is as follows:
>     ---8<---
>             if (p->bus_idx != bus_idx ||
>                 p->eventfd != eventfd  ||
>                 p->addr != args->addr  ||
>                 p->length != args->len ||
>                 p->wildcard != wildcard)
>                     continue;
> 
>             if (!p->wildcard && p->datamatch != args->datamatch)
>                     continue;
> 
>             // then we consider the two are the same
>     ---8<---
> 
>     As we can see, there is extra check of p->eventfd in
> 
>     ().  Why we don't check p->eventfd
>     in kvm_assign_ioeventfd_idx()? Or should we delete this in
>     kvm_deassign_ioeventfd_idx()?
> 
> 
>     Thanks,
>     Zhenyu
>