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
