Re: [PATCH v9 10/21] vfio/pci: introduce CONFIG_VFIO_PCI_ZDEV_KVM

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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@xxxxxxxxxxxxx>

Reviewed-by: Pierre Morel <pmorel@xxxxxxxxxxxxx>


---
  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;

----------^

+	}


So if you want to make this selectable by the user, I think you have to put some more #ifdefs in the following patches. But if this was not meant to be selectable by the user, I think it should not get a help text and rather be selected by the KVM switch in arch/s390/kvm/Kconfig instead of having a "default y".

  Thomas



--
Pierre Morel
IBM Lab Boeblingen



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