linux-kernel - Re: [PATCH V4 4/4] xen: privcmd: Add support for ioeventfd

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Message-ID: <9da983d7-7acf-4c8e-8ab6-4651d43786d7@suse.com>
Date:   Mon, 16 Oct 2023 09:25:44 +0200
From:   Juergen Gross <jgross@...e.com>
To:     Viresh Kumar <viresh.kumar@...aro.org>,
        Stefano Stabellini <sstabellini@...nel.org>,
        Oleksandr Tyshchenko <oleksandr_tyshchenko@...m.com>
Cc:     Vincent Guittot <vincent.guittot@...aro.org>,
        Alex Bennée <alex.bennee@...aro.org>,
        stratos-dev@...lists.linaro.org,
        Erik Schilling <erik.schilling@...aro.org>,
        Manos Pitsidianakis <manos.pitsidianakis@...aro.org>,
        Mathieu Poirier <mathieu.poirier@...aro.org>,
        Arnd Bergmann <arnd@...nel.org>, linux-kernel@...r.kernel.org,
        xen-devel@...ts.xenproject.org
Subject: Re: [PATCH V4 4/4] xen: privcmd: Add support for ioeventfd

On 16.10.23 09:11, Viresh Kumar wrote:
> Virtio guests send VIRTIO_MMIO_QUEUE_NOTIFY notification when they need
> to notify the backend of an update to the status of the virtqueue. The
> backend or another entity, polls the MMIO address for updates to know
> when the notification is sent.
> 
> It works well if the backend does this polling by itself. But as we move
> towards generic backend implementations, we end up implementing this in
> a separate user-space program.
> 
> Generally, the Virtio backends are implemented to work with the Eventfd
> based mechanism. In order to make such backends work with Xen, another
> software layer needs to do the polling and send an event via eventfd to
> the backend once the notification from guest is received. This results
> in an extra context switch.
> 
> This is not a new problem in Linux though. It is present with other
> hypervisors like KVM, etc. as well. The generic solution implemented in
> the kernel for them is to provide an IOCTL call to pass the address to
> poll and eventfd, which lets the kernel take care of polling and raise
> an event on the eventfd, instead of handling this in user space (which
> involves an extra context switch).
> 
> This patch adds similar support for xen.
> 
> Inspired by existing implementations for KVM, etc..
> 
> This also copies ioreq.h header file (only struct ioreq and related
> macros) from Xen's source tree (Top commit 5d84f07fe6bf ("xen/pci: drop
> remaining uses of bool_t")).
> 
> Signed-off-by: Viresh Kumar <viresh.kumar@...aro.org>

Reviewed-by: Juergen Gross <jgross@...e.com>


Juergen


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