Re: [PATCH v2 23/30] vfio/pci: re-introduce CONFIG_VFIO_PCI_ZDEV

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On 1/18/22 12:45 PM, Pierre Morel wrote:


On 1/18/22 18:32, Matthew Rosato wrote:
On 1/18/22 12:20 PM, Pierre Morel wrote:


On 1/14/22 21:31, Matthew Rosato wrote:
This was previously removed as unnecessary; while that was true, subsequent changes will make KVM an additional required component for vfio-pci-zdev. Let's re-introduce CONFIG_VFIO_PCI_ZDEV as now there is actually a reason
to say 'n' for it (when not planning to CONFIG_KVM).

Signed-off-by: Matthew Rosato <mjrosato@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 860424ccda1b..fedd1d4cb592 100644
--- a/drivers/vfio/pci/Kconfig
+++ b/drivers/vfio/pci/Kconfig
@@ -42,5 +42,16 @@ config VFIO_PCI_IGD
        and LPC bridge config space.
        To enable Intel IGD assignment through vfio-pci, say Y.
+
+config VFIO_PCI_ZDEV
+    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.

In several patches we check on CONFIG_PCI (14,15,16,17 and 22) but we may have PCI without VFIO_PCI, wouldn't it be a problem?

Here we define a new CONFIG entry and I have two questions:

1- there is no dependency on VFIO_PCI while the functionality is obviously based on VFIO_PCI

It's not obvious from this diff, but this 'config VFIO_PCI_ZDEV' statement is within an 'if VFIO_PCI' statement, just like VFIO_PCI_IGD above -- so the dependency is there.

sorry, I remember now you already answered this to Christian last time.



2- Wouldn't it be possible to use this item and the single condition for the different checks we need through the new VFIO interpretation functionality.

Possibly, but 1) we'd have to make linking arch/s390/kvm/pci.o dependent on CONFIG_VFIO_PCI instead of CONFIG_PCI in patch 14 and 2) if the relationship between CONFIG_VFIO_PCI and CONFIG_PCI were to ever change (though I don't see why it would..), we would be broken because the symbols we are referencing really require CONFIG_PCI (as they are located in s390 PCI).


Yes but VFIO_PCI_ZDEV depends on KVM, PCI and on VFIO_PCI
Wouldn't a single config item for this new code be easier to manage and understand?


I guess my primary resistance is to abstracting/hiding the dependency. Yes, userspace will never setup for zPCI interpretation without CONFIG_VFIO_PCI{_ZDEV}, but that's not where the compilation dependency is -- it's on CONFIG_PCI specifically.

But I guess on the other hand you could argue why even bother building pci.o into kvm without CONFIG_VFIO_PCI_ZDEV as it will never be used.

OK, I will have a look at making this change. It will require a little reorganization, at least moving this patch up before patch 14.



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