On Fri, Dec 20, 2024 at 04:42:35PM +0100, David Hildenbrand wrote: > On 18.11.24 14:19, ankita@xxxxxxxxxx wrote: > > From: Ankit Agrawal <ankita@xxxxxxxxxx> > > > > Currently KVM determines if a VMA is pointing at IO memory by checking > > pfn_is_map_memory(). However, the MM already gives us a way to tell what > > kind of memory it is by inspecting the VMA. > > Do you primarily care about VM_PFNMAP/VM_MIXEDMAP VMAs, or also other VMA > types? I think this is exclusively about allowing cachable memory inside a VM_PFNMAP VMA (created by VFIO) remain cachable inside the guest VM. > > This patch solves the problems where it is possible for the kernel to > > have VMAs pointing at cachable memory without causing > > pfn_is_map_memory() to be true, eg DAX memremap cases and CXL/pre-CXL > > devices. This memory is now properly marked as cachable in KVM. > > Does this only imply in worse performance, or does this also affect > correctness? I suspect performance is the problem, correct? Correctness. Things like atomics don't work on non-cachable mappings. > Maybe one could just reject such cases (if KVM PFN lookup code not > already rejects them, which might just be that case IIRC). At least VFIO enforces SHARED or it won't create the VMA. drivers/vfio/pci/vfio_pci_core.c: if ((vma->vm_flags & VM_SHARED) == 0) This is pretty normal/essential for drivers.. Are you suggesting the VMA flags should be inspected more? VM_SHARED/PFNMAP before allowing this? Jason
