Re: [PATCH v2 1/1] KVM: arm64: allow the VM to select DEVICE_* and NORMAL_NC for IO memory

[Catalin Marinas <catalin.marinas@arm.com>]

On Wed, Dec 06, 2023 at 01:20:35PM -0400, Jason Gunthorpe wrote:
> On Wed, Dec 06, 2023 at 04:31:48PM +0000, Catalin Marinas wrote:
> > > This would be fine, as would a VMA flag. Please pick one :)
> > > 
> > > I think a VMA flag is simpler than messing with pgprot.
> > 
> > I guess one could write a patch and see how it goes ;).
> 
> A lot of patches have been sent on this already :(

But not one with a VM_* flag. I guess we could also add a VM_VFIO flag
which implies KVM has less restrictions on the memory type. I think
that's more bike-shedding.

The key point is that we don't want to relax this for whatever KVM may
map in the guest but only for certain devices. Just having a vma may not
be sufficient, we can't tell where that vma came from.

So for the vfio bits, completely untested:

-------------8<----------------------------
diff --git a/drivers/vfio/pci/vfio_pci_core.c b/drivers/vfio/pci/vfio_pci_core.c
index 1929103ee59a..b89d2dfcd534 100644
--- a/drivers/vfio/pci/vfio_pci_core.c
+++ b/drivers/vfio/pci/vfio_pci_core.c
@@ -1863,7 +1863,7 @@ int vfio_pci_core_mmap(struct vfio_device *core_vdev, struct vm_area_struct *vma
 	 * See remap_pfn_range(), called from vfio_pci_fault() but we can't
 	 * change vm_flags within the fault handler.  Set them now.
 	 */
-	vm_flags_set(vma, VM_IO | VM_PFNMAP | VM_DONTEXPAND | VM_DONTDUMP);
+	vm_flags_set(vma, VM_VFIO | VM_IO | VM_PFNMAP | VM_DONTEXPAND | VM_DONTDUMP);
 	vma->vm_ops = &vfio_pci_mmap_ops;

 	return 0;
diff --git a/include/linux/mm.h b/include/linux/mm.h
index 418d26608ece..6df46fd7836a 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -391,6 +391,13 @@ extern unsigned int kobjsize(const void *objp);
 # define VM_UFFD_MINOR		VM_NONE
 #endif /* CONFIG_HAVE_ARCH_USERFAULTFD_MINOR */

+#ifdef CONFIG_64BIT
+#define VM_VFIO_BIT		39
+#define VM_VFIO			BIT(VM_VFIO_BIT)
+#else
+#define VM_VFIO			VM_NONE
+#endif
+
 /* Bits set in the VMA until the stack is in its final location */
 #define VM_STACK_INCOMPLETE_SETUP (VM_RAND_READ | VM_SEQ_READ | VM_STACK_EARLY)
-------------8<----------------------------

In KVM, Akita's patch would take this into account, not just rely on
"device==true".

> > > > If we want the VMM to drive this entirely, we could add a new mmap()
> > > > flag like MAP_WRITECOMBINE or PROT_WRITECOMBINE. They do feel a bit
> > > 
> > > As in the other thread, we cannot unconditionally map NORMAL_NC into
> > > the VMM.
> > 
> > I'm not suggesting this but rather the VMM map portions of the BAR with
> > either Device or Normal-NC, concatenate them (MAP_FIXED) and pass this
> > range as a memory slot (or multiple if a slot doesn't allow multiple
> > vmas).
> 
> The VMM can't know what to do. We already talked about this. The VMM
> cannot be involved in the decision to make pages NORMAL_NC or
> not. That idea ignores how actual devices work.
[...]
> > Are the Device/Normal offsets within a BAR fixed, documented in e.g. the
> > spec or this is something configurable via some MMIO that the guest
> > does.
> 
> No, it is fully dynamic on demand with firmware RPCs.

I think that's a key argument. The VMM cannot, on its own, configure the
BAR and figure a way to communicate this to the guest. We could invent
some para-virtualisation/trapping mechanism but that's unnecessarily
complicated. In the DPDK case, DPDK both configures and interacts with
the device. In the VMM/VM case, we need the VM to do this, we can't
split the configuration in VMM and interaction with the device in the
VM.

-- 
Catalin

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