Re: [RFC] Legacy Virtio Driver with Device Has Limited Memory Access

[Manivannan Sadhasivam <mani@kernel.org>]

On Wed, Jun 19, 2024 at 03:58:15AM -0400, Michael S. Tsirkin wrote:
> On Wed, Jun 19, 2024 at 01:09:29PM +0530, Manivannan Sadhasivam wrote:
> > On Wed, Jun 19, 2024 at 03:32:41AM -0400, Michael S. Tsirkin wrote:
> > > On Tue, Jun 18, 2024 at 09:51:41PM +0900, Shunsuke Mie wrote:
> > > > 2024年6月18日(火) 19:47 Michael S. Tsirkin <mst@redhat.com>:
> > > > >
> > > > > On Tue, Jun 18, 2024 at 07:40:34PM +0900, Shunsuke Mie wrote:
> > > > > > 2024年6月18日(火) 19:33 Michael S. Tsirkin <mst@redhat.com>:
> > > > > > >
> > > > > > > On Tue, Jun 18, 2024 at 07:15:47PM +0900, Shunsuke Mie wrote:
> > > > > > > > Thank you for your response.
> > > > > > > >
> > > > > > > > 2024年6月18日(火) 18:47 Michael S. Tsirkin <mst@redhat.com>:
> > > > > > > > >
> > > > > > > > > On Tue, Jun 18, 2024 at 08:41:09AM +0900, Shunsuke Mie wrote:
> > > > > > > > > > Let's clarify the situation.
> > > > > > > > > >
> > > > > > > > > > The Virtio device and driver are not working properly due to a
> > > > > > > > > > combination of the following reasons:
> > > > > > > > > >
> > > > > > > > > > 1. Regarding VIRTIO_F_ACCESS_PLATFORM:
> > > > > > > > > > - The modern spec includes VIRTIO_F_ACCESS_PLATFORM, which allows
> > > > > > > > > > Physical DMAC to be used.
> > > > > > > > > > - This feature is not available in the legacy spec.
> > > > > > > > >
> > > > > > > > > ... because legacy drivers don't set it
> > > > > > > > >
> > > > > > > > > > 2. Regarding Virtio PCIe Capability:
> > > > > > > > > > - The modern spec requires Virtio PCIe Capability.
> > > > > > > > >
> > > > > > > > > It's a PCI capability actually. People have been asking
> > > > > > > > > about option to make it a pcie extended capability,
> > > > > > > > > but no one did the spec, qemu and driver work, yet.
> > > > > > > > >
> > > > > > > > > > - In some environments, Virtio PCIe Capability cannot be provided.
> > > > > > > > >
> > > > > > > > > why not?
> > > > > > > > PCIe Endpoint Controller chips are available from several vendors and allow
> > > > > > > > software to describe the behavior of PCIe Endpoint functions. However, they
> > > > > > > > offer only limited functionality. Specifically, while PCIe bus communication is
> > > > > > > > programmable, PCIe Capabilities are fixed and cannot be made to show as
> > > > > > > > virtio's.
> > > > > > >
> > > > > > > Okay. So where could these structures live, if not in pci config?
> > > > > > What does "these structures" refer to? PCIe Capabilities? virtio configs?
> > > > >
> > > > > Virtio uses a bunch of read only structures that look like this:
> > > > >
> > > > > struct virtio_pci_cap {
> > > > >
> > > > >         ..... [skipped, specific to pci config caps] ...
> > > > >
> > > > >         u8 cfg_type;    /* Identifies the structure. */
> > > > >         u8 bar;         /* Where to find it. */
> > > > >         u8 id;          /* Multiple capabilities of the same type */
> > > > >         u8 padding[2];  /* Pad to full dword. */
> > > > >         le32 offset;    /* Offset within bar. */
> > > > >         le32 length;    /* Length of the structure, in bytes. */
> > > > > };
> > > > >
> > > > >
> > > > > The driver uses that to locate parts of device interface it
> > > > > later uses.
> > > > >
> > > > >
> > > > > Per spec, they are currently in pci config, you are saying some devices
> > > > > can't have this data in pci config - is that right? Where yes?
> > > > I understood. The configuration structure is in the space that is
> > > > indicated by BAR0.
> > > > Follows the instructions in the spec:
> > > > ```
> > > > 4.1.4.10 Legacy Interfaces: A Note on PCI Device Layout
> > > > Transitional devices MUST present part of configuration registers in a
> > > > legacy configuration structure in BAR0 in the first I/O region of the
> > > > PCI device, as documented below.
> > > > ...
> > > > ```
> > > 
> > > No, and it's best everyone stopped talking about legacy like we
> > > are going to add new features to it.
> > > 
> > > The configuration structure is in the space that is
> > > indicated by the vendor specific structure, which is currently
> > > in pci config space.
> > > 
> > > If use of a vendor specific structure in pci config space is
> > > problematic, we can try to fix that by extending the virtio spec.
> > > 
> > 
> > It is indeed problematic as the device vendors usually use those capability
> > registers themselves and wouldn't allow users to configure them.
> > 
> > So if the spec is ammended to support other way of finding the virtio config
> > structures, then it would really unblock us from supporting modern virtio
> > devices.
> > 
> > - Mani
> 
> Okay.
> Anyone from your side can join the virtio TC to champion this?
> We don't have anyone knowledgeable about this protocol ATM AFAIK.
> 

Sure. Myself/Shunsuke can join and help with this.

Shunsuke did the Virtio bringup for PCI Endpoint subsystem and I'm the
maintainer for it.

- Mani

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