On Tue, Dec 21, 2021 at 04:32:05AM +0000, Jag Raman wrote: > > On Dec 20, 2021, at 9:36 AM, Stefan Hajnoczi <stefa...@redhat.com> wrote: > > On Fri, Dec 17, 2021 at 08:00:35PM +0000, Jag Raman wrote: > >>> On Dec 16, 2021, at 9:40 AM, Stefan Hajnoczi <stefa...@redhat.com> wrote: > >>> On Wed, Dec 15, 2021 at 10:35:35AM -0500, Jagannathan Raman wrote: > >>> Also, why is PCI Memory Space isolated via VFUIOMMU but PCI IO Space is > >>> not? > >> > >> If I understand correctly, the IO address space translates sysmem address > >> to > >> direct device access (such as I2C). Once we are inside a device, we already > >> have access to all parts of the device (unlike RAM which sits outside the > >> device). > >> So didn’t think device would go via IOMMU to access IO. Also didn’t see any > >> other IOMMU translating IO address space accesses. > > > > I reviewed how BARs are configured with VFIO: > > > > 1. When the guest writes to the vfio-pci PCIDevice's Configuration Space > > the write is forwarded to the VFIO device (i.e. vfio-user or VFIO > > kernel ioctl). > > > > 2. The vfio-user server receives the Configuration Space write and > > forwards it to pci_dev (the PCIDevice we're serving up). BAR mappings > > are updated in the vfio-user server so the BAR MemoryRegions are > > mapped/unmapped at the locations given by the guest. > > > > This applies for both Memory and IO Space accesses. > > > > Because this patch series does not isolate IO Space between VfuObject > > instances the MemoryRegions will collide when two guests map IO Space > > BARs of different devices at the same IO Space address. In other words, > > vfu_object_bar_rw() uses the global address_space_io and that means > > collisions can occur. > > I agree that collision could occur from the CPU end. But I'm not if IOMMU > needs to translate IO space.
QEMU's IOMMUs do not translate IO Space addresses AFAIK. IO Space just needs to be isolated between vfio-user server instances so there is no collision when one client maps an IO Space BAR to the same address as another client. I think the cleanest way of achieving that is by creating a per-vfio-user server PCI bus with an address_space_io MemoryRegion. Stefan
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