Re: [PATCH] xen: Remove config dependency in XEN_PRIVCMD definition

[Jan Beulich <jbeulich@suse.com>]

On 10.10.2024 07:39, Jürgen Groß wrote:
> On 10.10.24 00:46, Stefano Stabellini wrote:
>> On Wed, 9 Oct 2024, Jan Beulich wrote:
>>> 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@amd.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.
>>
>> The idea is that there should be no hard dependency on
>> pcistub_get_gsi_from_sbdf(). If it is available, the service will be
>> used, otherwise an error will be reported.
>>
>> The problem is that IS_REACHABLE is a compile-time check. What we need
>> is a runtime check instead. Maybe symbol_get or try_module_get ?
> 
> This is a rather clumsy solution IMO.
> 
> I'm seeing the following solutions:
> 
> 1. Don't fail to to load the pciback driver in a DomU, but only disable
>     any functionality.
> 
> 2. Move the drivers/xen/xen-pciback/pci_stub.c functionality in a module
>     of its own, allowing the privcmd driver to be loaded without needing
>     the rest of pciback.
> 
> 3. Add a hook to e.g. drivers/xen/pci.c instead for calling of
>     pcistub_get_gsi_from_sbdf() directly. pciback could register the real
>     call address. If pciback isn't loaded, the hook can return an error.
>     This would remove the explicit dependency of privcmd on pciback.
> 
> I'd prefer the 3rd variant.

Same here - order of preference backwards in the set presented above.
In the meantime the original change may simply need reverting?

Jan