linux-kernel - Re: [PATCH] KVM: x86: fix deadlock for KVM_XEN_EVTCHN

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Message-ID: <3619dc3b-d1b9-ab76-a008-2c45799d5803@redhat.com>
Date:   Wed, 28 Dec 2022 14:21:51 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     "Woodhouse, David" <dwmw@...zon.co.uk>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "seanjc@...gle.com" <seanjc@...gle.com>,
        Michal Luczaj <mhal@...x.co>
Subject: Re: [PATCH] KVM: x86: fix deadlock for KVM_XEN_EVTCHN_RESET

On 12/28/22 12:55, Woodhouse, David wrote:
> 
> IIRC the send_port and type aren't used from hcall_send so we could 
> stick those in a union with a 'next' pointer and use it to build a list 
> instead of having to allocate the array (pathological worst case 32KiB).

Yeah I thought about using a list.  The union is doable as you say, but 
I wasn't sure if things were going to change in kvm_xen_evtchn_send later.

And the worst case is order 3, which is within PAGE_ALLOC_COSTLY_ORDER, 
so I decided to go with the array which only consumes memory on reset 
rather than always; it's much more likely that the array will be smaller 
than a page.

Paolo

> Or if the union is a bit icky, we could just add the 'next' pointer 
> unconditionally.
>