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Date:   Fri, 17 Jun 2022 18:15:39 +0200
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Subject: Re: [PATCH v9 10/21] vfio/pci: introduce CONFIG_VFIO_PCI_ZDEV_KVM
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To:     Pierre Morel <pmorel@linux.ibm.com>,
        Matthew Rosato <mjrosato@linux.ibm.com>,
        linux-s390@vger.kernel.org
Cc:     alex.williamson@redhat.com, cohuck@redhat.com,
        schnelle@linux.ibm.com, farman@linux.ibm.com,
        borntraeger@linux.ibm.com, hca@linux.ibm.com, gor@linux.ibm.com,
        gerald.schaefer@linux.ibm.com, agordeev@linux.ibm.com,
        svens@linux.ibm.com, frankja@linux.ibm.com, david@redhat.com,
        imbrenda@linux.ibm.com, vneethv@linux.ibm.com,
        oberpar@linux.ibm.com, freude@linux.ibm.com, pasic@linux.ibm.com,
        pbonzini@redhat.com, corbet@lwn.net, jgg@nvidia.com,
        kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-doc@vger.kernel.org
References: <20220606203325.110625-1-mjrosato@linux.ibm.com>
 <20220606203325.110625-11-mjrosato@linux.ibm.com>
 <025699e6-b870-2648-d4a4-ffbc5fff22e8@redhat.com>
 <ac5cd90a-c92b-1bad-fbec-d1ca6287e826@linux.ibm.com>
 <69ec8abd-c579-46d0-08cd-2714de91b6cb@linux.ibm.com>
From:   Thomas Huth <thuth@redhat.com>
In-Reply-To: <69ec8abd-c579-46d0-08cd-2714de91b6cb@linux.ibm.com>
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On 14/06/2022 10.56, Pierre Morel wrote:
> 
> 
> On 6/8/22 15:15, Matthew Rosato wrote:
>> On 6/8/22 2:19 AM, Thomas Huth wrote:
>>> On 06/06/2022 22.33, Matthew Rosato wrote:
>>>> The current contents of vfio-pci-zdev are today only useful in a KVM
>>>> environment; let's tie everything currently under vfio-pci-zdev to
>>>> this Kconfig statement and require KVM in this case, reducing complexity
>>>> (e.g. symbol lookups).
>>>>
>>>> Signed-off-by: Matthew Rosato <mjrosato@linux.ibm.com>
> 
> Reviewed-by: Pierre Morel <pmorel@linux.ibm.com>
> 
> 
>>>> ---
>>>>   drivers/vfio/pci/Kconfig      | 11 +++++++++++
>>>>   drivers/vfio/pci/Makefile     |  2 +-
>>>>   include/linux/vfio_pci_core.h |  2 +-
>>>>   3 files changed, 13 insertions(+), 2 deletions(-)
>>>>
>>>> diff --git a/drivers/vfio/pci/Kconfig b/drivers/vfio/pci/Kconfig
>>>> index 4da1914425e1..f9d0c908e738 100644
>>>> --- a/drivers/vfio/pci/Kconfig
>>>> +++ b/drivers/vfio/pci/Kconfig
>>>> @@ -44,6 +44,17 @@ config VFIO_PCI_IGD
>>>>         To enable Intel IGD assignment through vfio-pci, say Y.
>>>>   endif
>>>> +config VFIO_PCI_ZDEV_KVM
>>>> +    bool "VFIO PCI extensions for s390x KVM passthrough"
>>>> +    depends on S390 && KVM
>>>> +    default y
>>>> +    help
>>>> +      Support s390x-specific extensions to enable support for enhancements
>>>> +      to KVM passthrough capabilities, such as interpretive execution of
>>>> +      zPCI instructions.
>>>> +
>>>> +      To enable s390x KVM vfio-pci extensions, say Y.
>>>
>>> Is it still possible to disable CONFIG_VFIO_PCI_ZDEV_KVM ? Looking at the 
>>> later patches (e.g. 20/21 where you call kvm_s390_pci_zpci_op() from 
>>> kvm-s390.c), it rather seems to me that it currently cannot be disabled 
>>> independently (as long as KVM is enabled).
>>
>> Yes, you can build with, for example, CONFIG_VFIO_PCI_ZDEV_KVM=n and 
>> CONFIG_KVM=m -- I tested it again just now.  The result is kvm and 
>> vfio-pci are built and vfio-pci works, but none of the vfio-pci-zdev 
>> extensions are available (including zPCI interpretation).
>>
>> This is accomplished via the placement of some IS_ENABLED checks.  Some 
>> calls (e.g. AEN init) are fenced by IS_ENABLED(CONFIG_VFIO_PCI_ZDEV_KVM).  
>> There are also some areas that are fenced off via a call to 
>> kvm_s390_pci_interp_allowed() which also includes an IS_ENABLED check 
>> along with checks for facility and cpu id.
>>
>> Using patch 20 as an example, KVM_CAP_S390_ZPCI_OP will always be reported 
>> as unavailable to userspace if CONFIG_VFIO_PCI_ZDEV_KVM=n due to the call 
>> to kvm_s390_pci_interp_allowed().  If userspace sends us the ioctl anyway, 
>> we will return -EINVAL because there is again a 
>> IS_ENABLED(CONFIG_VFIO_PCI_ZDEV_KVM) check before we read the ioctl args 
>> from userspace.
> 
> Yes and the code will not be generated by the compiler in patch 20 after the 
> break if CONFIG_VFIO_PCI_ZDEV_KVM is not enabled.
> 
> +    case KVM_S390_ZPCI_OP: {
> +        struct kvm_s390_zpci_op args;
> +
> +        r = -EINVAL;
> +        if (!IS_ENABLED(CONFIG_VFIO_PCI_ZDEV_KVM))
> +            break;
> 
> Code not generated----v
> 
> +        if (copy_from_user(&args, argp, sizeof(args))) {
> +            r = -EFAULT;
> +            break;
> +        }
> +        r = kvm_s390_pci_zpci_op(kvm, &args);
> +        break;
> 
> ----------^

OK, good to know, thanks for the clarification!

  Thomas