linux-kernel - Re: [PATCH RFC 11/39] KVM: x86/xen: evtchn signaling via eventfd

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Message-ID: <874d1fa922cb56238676b90bbeeba930d0706500.camel@infradead.org>
Date:   Mon, 30 Nov 2020 09:41:16 +0000
From:   David Woodhouse <dwmw2@...radead.org>
To:     Joao Martins <joao.m.martins@...cle.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Cc:     Ankur Arora <ankur.a.arora@...cle.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org
Subject: Re: [PATCH RFC 11/39] KVM: x86/xen: evtchn signaling via eventfd

On Wed, 2019-02-20 at 20:15 +0000, Joao Martins wrote:
> userspace registers a @port to an @eventfd, that is bound to a
>  @vcpu. This information is then used when the guest does an
> EVTCHNOP_send with a port registered with the kernel.

Why do I want this part?

> EVTCHNOP_send short-circuiting happens by marking the event as pending
> in the shared info and vcpu info pages and doing the upcall. For IPIs
> and interdomain event channels, we do the upcall on the assigned vcpu.

This part I understand, 'peeking' at the EVTCHNOP_send hypercall so
that we can short-circuit IPI delivery without it having to bounce
through userspace.

But why would I then want then short-circuit the short-circuit,
providing an eventfd for it to signal... so that I can then just
receive the event in userspace in a *different* form to the original
hypercall exit I would have got?


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