Re: [PATCH V3 2/2] xen: privcmd: Add support for irqfd

[Juergen Gross <jgross@suse.com>]

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On 25.07.23 08:47, Viresh Kumar wrote:
> Xen provides support for injecting interrupts to the guests via the
> HYPERVISOR_dm_op() hypercall. The same is used by the Virtio based
> device backend implementations, in an inefficient manner currently.
> 
> 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 poll the Eventfds and raise an interrupt to the
> guest using the Xen based mechanism. 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 interrupt
> details and eventfd, which lets the kernel take care of polling the
> eventfd and raising of the interrupt, instead of handling this in user
> space (which involves an extra context switch).
> 
> This patch adds support to inject a specific interrupt to guest using
> the eventfd mechanism, by preventing the extra context switch.
> 
> Inspired by existing implementations for KVM, etc..
> 
> Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
> ---
> V2.1->V3
> - No changes
> 
> V2->V2.1
> - Select EVENTFD from Kconfig
> 
> V1->V2:
> - Improve error handling.
> - Remove the unnecessary usage of list_for_each_entry_safe().
> - Restrict the use of XEN_DMOP_set_irq_level to only ARM64.
> 
>   drivers/xen/Kconfig        |   1 +
>   drivers/xen/privcmd.c      | 276 ++++++++++++++++++++++++++++++++++++-
>   include/uapi/xen/privcmd.h |  14 ++
>   3 files changed, 289 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/xen/Kconfig b/drivers/xen/Kconfig
> index d5d7c402b651..7967393c55a4 100644
> --- a/drivers/xen/Kconfig
> +++ b/drivers/xen/Kconfig
> @@ -261,6 +261,7 @@ config XEN_SCSI_BACKEND
>   config XEN_PRIVCMD
>   	tristate "Xen hypercall passthrough driver"
>   	depends on XEN
> +	select EVENTFD

I don't like this. Can we maybe add another bool config item depending on
XEN_PRIVCMD, EVENTFD and XEN_VIRTIO, which can then be used to guard the
code additions to privcmd.c?

This would avoid adding additional code for everyone.


Juergen

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