linux-kernel - Re: [PATCH] xen: Remove config dependency in XEN

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Message-ID: <73174eb0-380d-4f95-a2c3-097b86fac8db@suse.com>
Date: Wed, 9 Oct 2024 09:05:40 +0200
From: Jan Beulich <jbeulich@...e.com>
To: Jiqian Chen <Jiqian.Chen@....com>
Cc: xen-devel@...ts.xenproject.org, linux-kernel@...r.kernel.org,
 Juergen Gross <jgross@...e.com>, Stefano Stabellini
 <sstabellini@...nel.org>,
 Oleksandr Tyshchenko <oleksandr_tyshchenko@...m.com>,
 Marek Marczykowski <marmarek@...isiblethingslab.com>
Subject: Re: [PATCH] xen: Remove config dependency in XEN_PRIVCMD definition

On 09.10.2024 08:20, Jiqian Chen wrote:
> Commit 2fae6bb7be32 ("xen/privcmd: Add new syscall to get gsi from dev")
> adds a weak reverse dependency to the config XEN_PRIVCMD definition, its
> purpose is to pass the combination of compilation that CONFIG_XEN_PRIVCMD=y
> and CONFIG_XEN_PCIDEV_BACKEND=m, because in that combination, xen-pciback
> is compiled as a module but xen-privcmd is built-in, so xen-privcmd can't
> find the implementation of pcistub_get_gsi_from_sbdf.
> 
> But that dependency causes xen-privcmd can't be loaded on domU, because
> dependent xen-pciback is always not be loaded successfully on domU.
> 
> To solve above problem and cover original commit's requirement, just remove
> that dependency, because the code "IS_REACHABLE(CONFIG_XEN_PCIDEV_BACKEND)"
> of original commit is enough to meet the requirement.
> 
> Fixes: 2fae6bb7be32 ("xen/privcmd: Add new syscall to get gsi from dev")
> Signed-off-by: Jiqian Chen <Jiqian.Chen@....com>

This lacks a Reported-by:.

> --- a/drivers/xen/Kconfig
> +++ b/drivers/xen/Kconfig
> @@ -261,7 +261,6 @@ config XEN_SCSI_BACKEND
>  config XEN_PRIVCMD
>  	tristate "Xen hypercall passthrough driver"
>  	depends on XEN
> -	imply XEN_PCIDEV_BACKEND
>  	default m
>  	help
>  	  The hypercall passthrough driver allows privileged user programs to

The report wasn't about a build problem, but a runtime one. Removing the
dependency here doesn't change anything in the dependency of xen-privcmd
on xen-pciback, as the use of pcistub_get_gsi_from_sbdf() continues to
exist. Consider the case of XEN_PCIDEV_BACKEND=m and XEN_PRIVCMD=m, which
I guess is what Marek is using in his config. Both drivers are available
in such a configuration, yet loading of xen-privcmd then requires to
load xen-pciback first. And that latter load attempt will fail in a DomU.
The two drivers simply may not have any dependency in either direction.

Jan