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

["Jürgen Groß" <jgross@suse.com>]

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On 10.10.24 09:07, Chen, Jiqian wrote:
> On 2024/10/10 13:39, 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.
> Thanks, I will send changes in v2 soon.
> And I prefer to add the hook in drivers/xen/acpi.c

Agreed.


Juergen


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