Re: [PATCH v5 03/18] pci: isolated address space for PCI bus

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Feb 10, 2022 at 10:23:01PM +0000, Jag Raman wrote:
> 
> 
> > On Feb 10, 2022, at 3:02 AM, Michael S. Tsirkin <mst@redhat.com> wrote:
> > 
> > On Thu, Feb 10, 2022 at 12:08:27AM +0000, Jag Raman wrote:
> >> 
> >> 
> >>> On Feb 2, 2022, at 12:34 AM, Alex Williamson <alex.williamson@redhat.com> wrote:
> >>> 
> >>> On Wed, 2 Feb 2022 01:13:22 +0000
> >>> Jag Raman <jag.raman@oracle.com> wrote:
> >>> 
> >>>>> On Feb 1, 2022, at 5:47 PM, Alex Williamson <alex.williamson@redhat.com> wrote:
> >>>>> 
> >>>>> On Tue, 1 Feb 2022 21:24:08 +0000
> >>>>> Jag Raman <jag.raman@oracle.com> wrote:
> >>>>> 
> >>>>>>> On Feb 1, 2022, at 10:24 AM, Alex Williamson <alex.williamson@redhat.com> wrote:
> >>>>>>> 
> >>>>>>> On Tue, 1 Feb 2022 09:30:35 +0000
> >>>>>>> Stefan Hajnoczi <stefanha@redhat.com> wrote:
> >>>>>>> 
> >>>>>>>> On Mon, Jan 31, 2022 at 09:16:23AM -0700, Alex Williamson wrote:    
> >>>>>>>>> On Fri, 28 Jan 2022 09:18:08 +0000
> >>>>>>>>> Stefan Hajnoczi <stefanha@redhat.com> wrote:
> >>>>>>>>> 
> >>>>>>>>>> On Thu, Jan 27, 2022 at 02:22:53PM -0700, Alex Williamson wrote:      
> >>>>>>>>>>> If the goal here is to restrict DMA between devices, ie. peer-to-peer
> >>>>>>>>>>> (p2p), why are we trying to re-invent what an IOMMU already does?        
> >>>>>>>>>> 
> >>>>>>>>>> The issue Dave raised is that vfio-user servers run in separate
> >>>>>>>>>> processses from QEMU with shared memory access to RAM but no direct
> >>>>>>>>>> access to non-RAM MemoryRegions. The virtiofs DAX Window BAR is one
> >>>>>>>>>> example of a non-RAM MemoryRegion that can be the source/target of DMA
> >>>>>>>>>> requests.
> >>>>>>>>>> 
> >>>>>>>>>> I don't think IOMMUs solve this problem but luckily the vfio-user
> >>>>>>>>>> protocol already has messages that vfio-user servers can use as a
> >>>>>>>>>> fallback when DMA cannot be completed through the shared memory RAM
> >>>>>>>>>> accesses.
> >>>>>>>>>> 
> >>>>>>>>>>> In
> >>>>>>>>>>> fact, it seems like an IOMMU does this better in providing an IOVA
> >>>>>>>>>>> address space per BDF.  Is the dynamic mapping overhead too much?  What
> >>>>>>>>>>> physical hardware properties or specifications could we leverage to
> >>>>>>>>>>> restrict p2p mappings to a device?  Should it be governed by machine
> >>>>>>>>>>> type to provide consistency between devices?  Should each "isolated"
> >>>>>>>>>>> bus be in a separate root complex?  Thanks,        
> >>>>>>>>>> 
> >>>>>>>>>> There is a separate issue in this patch series regarding isolating the
> >>>>>>>>>> address space where BAR accesses are made (i.e. the global
> >>>>>>>>>> address_space_memory/io). When one process hosts multiple vfio-user
> >>>>>>>>>> server instances (e.g. a software-defined network switch with multiple
> >>>>>>>>>> ethernet devices) then each instance needs isolated memory and io address
> >>>>>>>>>> spaces so that vfio-user clients don't cause collisions when they map
> >>>>>>>>>> BARs to the same address.
> >>>>>>>>>> 
> >>>>>>>>>> I think the the separate root complex idea is a good solution. This
> >>>>>>>>>> patch series takes a different approach by adding the concept of
> >>>>>>>>>> isolated address spaces into hw/pci/.      
> >>>>>>>>> 
> >>>>>>>>> This all still seems pretty sketchy, BARs cannot overlap within the
> >>>>>>>>> same vCPU address space, perhaps with the exception of when they're
> >>>>>>>>> being sized, but DMA should be disabled during sizing.
> >>>>>>>>> 
> >>>>>>>>> Devices within the same VM context with identical BARs would need to
> >>>>>>>>> operate in different address spaces.  For example a translation offset
> >>>>>>>>> in the vCPU address space would allow unique addressing to the devices,
> >>>>>>>>> perhaps using the translation offset bits to address a root complex and
> >>>>>>>>> masking those bits for downstream transactions.
> >>>>>>>>> 
> >>>>>>>>> In general, the device simply operates in an address space, ie. an
> >>>>>>>>> IOVA.  When a mapping is made within that address space, we perform a
> >>>>>>>>> translation as necessary to generate a guest physical address.  The
> >>>>>>>>> IOVA itself is only meaningful within the context of the address space,
> >>>>>>>>> there is no requirement or expectation for it to be globally unique.
> >>>>>>>>> 
> >>>>>>>>> If the vfio-user server is making some sort of requirement that IOVAs
> >>>>>>>>> are unique across all devices, that seems very, very wrong.  Thanks,      
> >>>>>>>> 
> >>>>>>>> Yes, BARs and IOVAs don't need to be unique across all devices.
> >>>>>>>> 
> >>>>>>>> The issue is that there can be as many guest physical address spaces as
> >>>>>>>> there are vfio-user clients connected, so per-client isolated address
> >>>>>>>> spaces are required. This patch series has a solution to that problem
> >>>>>>>> with the new pci_isol_as_mem/io() API.    
> >>>>>>> 
> >>>>>>> Sorry, this still doesn't follow for me.  A server that hosts multiple
> >>>>>>> devices across many VMs (I'm not sure if you're referring to the device
> >>>>>>> or the VM as a client) needs to deal with different address spaces per
> >>>>>>> device.  The server needs to be able to uniquely identify every DMA,
> >>>>>>> which must be part of the interface protocol.  But I don't see how that
> >>>>>>> imposes a requirement of an isolated address space.  If we want the
> >>>>>>> device isolated because we don't trust the server, that's where an IOMMU
> >>>>>>> provides per device isolation.  What is the restriction of the
> >>>>>>> per-client isolated address space and why do we need it?  The server
> >>>>>>> needing to support multiple clients is not a sufficient answer to
> >>>>>>> impose new PCI bus types with an implicit restriction on the VM.    
> >>>>>> 
> >>>>>> Hi Alex,
> >>>>>> 
> >>>>>> I believe there are two separate problems with running PCI devices in
> >>>>>> the vfio-user server. The first one is concerning memory isolation and
> >>>>>> second one is vectoring of BAR accesses (as explained below).
> >>>>>> 
> >>>>>> In our previous patches (v3), we used an IOMMU to isolate memory
> >>>>>> spaces. But we still had trouble with the vectoring. So we implemented
> >>>>>> separate address spaces for each PCIBus to tackle both problems
> >>>>>> simultaneously, based on the feedback we got.
> >>>>>> 
> >>>>>> The following gives an overview of issues concerning vectoring of
> >>>>>> BAR accesses.
> >>>>>> 
> >>>>>> The device’s BAR regions are mapped into the guest physical address
> >>>>>> space. The guest writes the guest PA of each BAR into the device’s BAR
> >>>>>> registers. To access the BAR regions of the device, QEMU uses
> >>>>>> address_space_rw() which vectors the physical address access to the
> >>>>>> device BAR region handlers.  
> >>>>> 
> >>>>> The guest physical address written to the BAR is irrelevant from the
> >>>>> device perspective, this only serves to assign the BAR an offset within
> >>>>> the address_space_mem, which is used by the vCPU (and possibly other
> >>>>> devices depending on their address space).  There is no reason for the
> >>>>> device itself to care about this address.  
> >>>> 
> >>>> Thank you for the explanation, Alex!
> >>>> 
> >>>> The confusion at my part is whether we are inside the device already when
> >>>> the server receives a request to access BAR region of a device. Based on
> >>>> your explanation, I get that your view is the BAR access request has
> >>>> propagated into the device already, whereas I was under the impression
> >>>> that the request is still on the CPU side of the PCI root complex.
> >>> 
> >>> If you are getting an access through your MemoryRegionOps, all the
> >>> translations have been made, you simply need to use the hwaddr as the
> >>> offset into the MemoryRegion for the access.  Perform the read/write to
> >>> your device, no further translations required.
> >>> 
> >>>> Your view makes sense to me - once the BAR access request reaches the
> >>>> client (on the other side), we could consider that the request has reached
> >>>> the device.
> >>>> 
> >>>> On a separate note, if devices don’t care about the values in BAR
> >>>> registers, why do the default PCI config handlers intercept and map
> >>>> the BAR region into address_space_mem?
> >>>> (pci_default_write_config() -> pci_update_mappings())
> >>> 
> >>> This is the part that's actually placing the BAR MemoryRegion as a
> >>> sub-region into the vCPU address space.  I think if you track it,
> >>> you'll see PCIDevice.io_regions[i].address_space is actually
> >>> system_memory, which is used to initialize address_space_system.
> >>> 
> >>> The machine assembles PCI devices onto buses as instructed by the
> >>> command line or hot plug operations.  It's the responsibility of the
> >>> guest firmware and guest OS to probe those devices, size the BARs, and
> >>> place the BARs into the memory hierarchy of the PCI bus, ie. system
> >>> memory.  The BARs are necessarily in the "guest physical memory" for
> >>> vCPU access, but it's essentially only coincidental that PCI devices
> >>> might be in an address space that provides a mapping to their own BAR.
> >>> There's no reason to ever use it.
> >>> 
> >>> In the vIOMMU case, we can't know that the device address space
> >>> includes those BAR mappings or if they do, that they're identity mapped
> >>> to the physical address.  Devices really need to not infer anything
> >>> about an address.  Think about real hardware, a device is told by
> >>> driver programming to perform a DMA operation.  The device doesn't know
> >>> the target of that operation, it's the guest driver's responsibility to
> >>> make sure the IOVA within the device address space is valid and maps to
> >>> the desired target.  Thanks,
> >> 
> >> Thanks for the explanation, Alex. Thanks to everyone else in the thread who
> >> helped to clarify this problem.
> >> 
> >> We have implemented the memory isolation based on the discussion in the
> >> thread. We will send the patches out shortly.
> >> 
> >> Devices such as “name" and “e1000” worked fine. But I’d like to note that
> >> the LSI device (TYPE_LSI53C895A) had some problems - it doesn’t seem
> >> to be IOMMU aware. In LSI’s case, the kernel driver is asking the device to
> >> read instructions from the CPU VA (lsi_execute_script() -> read_dword()),
> >> which is forbidden when IOMMU is enabled. Specifically, the driver is asking
> >> the device to access other BAR regions by using the BAR address programmed
> >> in the PCI config space. This happens even without vfio-user patches. For example,
> >> we could enable IOMMU using “-device intel-iommu” QEMU option and also
> >> adding the following to the kernel command-line: “intel_iommu=on iommu=nopt”.
> >> In this case, we could see an IOMMU fault.
> > 
> > So, device accessing its own BAR is different. Basically, these
> > transactions never go on the bus at all, never mind get to the IOMMU.
> 
> Hi Michael,
> 
> In LSI case, I did notice that it went to the IOMMU.

Hmm do you mean you analyzed how a physical device works?
Or do you mean in QEMU?

> The device is reading the BAR
> address as if it was a DMA address.

I got that, my understanding of PCI was that a device can
not be both a master and a target of a transaction at
the same time though. Could not find this in the spec though,
maybe I remember incorrectly.

> > I think it's just used as a handle to address internal device memory.
> > This kind of trick is not universal, but not terribly unusual.
> > 
> > 
> >> Unfortunately, we started off our project with the LSI device. So that lead to all the
> >> confusion about what is expected at the server end in-terms of
> >> vectoring/address-translation. It gave an impression as if the request was still on
> >> the CPU side of the PCI root complex, but the actual problem was with the
> >> device driver itself.
> >> 
> >> I’m wondering how to deal with this problem. Would it be OK if we mapped the
> >> device’s BAR into the IOVA, at the same CPU VA programmed in the BAR registers?
> >> This would help devices such as LSI to circumvent this problem. One problem
> >> with this approach is that it has the potential to collide with another legitimate
> >> IOVA address. Kindly share your thought on this.
> >> 
> >> Thank you!
> > 
> > I am not 100% sure what do you plan to do but it sounds fine since even
> > if it collides, with traditional PCI device must never initiate cycles
> 
> OK sounds good, I’ll create a mapping of the device BARs in the IOVA.
> 
> Thank you!
> --
> Jag
> 
> > within their own BAR range, and PCIe is software-compatible with PCI. So
> > devices won't be able to access this IOVA even if it was programmed in
> > the IOMMU.
> > 
> > As was mentioned elsewhere on this thread, devices accessing each
> > other's BAR is a different matter.
> > 
> > I do not remember which rules apply to multiple functions of a
> > multi-function device though. I think in a traditional PCI
> > they will never go out on the bus, but with e.g. SRIOV they
> > would probably do go out? Alex, any idea?
> > 
> > 
> >> --
> >> Jag
> >> 
> >>> 
> >>> Alex
> >>> 
> >> 
> > 
>