Re: [PATCH RFC 04/19] vfio-user: Define type vfio_user_pci_dev_info

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Thu, Jul 29, 2021 at 12:55:08AM +0000, John Johnson wrote:
> 
> 
> > On Jul 28, 2021, at 3:16 AM, Stefan Hajnoczi <stefanha@redhat.com> wrote:
> > 
> > On Sun, Jul 18, 2021 at 11:27:43PM -0700, Elena Ufimtseva wrote:
> >> From: John G Johnson <john.g.johnson@oracle.com>
> >> 
> >> New class for vfio-user with its class and instance
> >> constructors and destructors.
> >> 
> >> Signed-off-by: Elena Ufimtseva <elena.ufimtseva@oracle.com>
> >> Signed-off-by: John G Johnson <john.g.johnson@oracle.com>
> >> Signed-off-by: Jagannathan Raman <jag.raman@oracle.com>
> >> ---
> >> hw/vfio/pci.c | 49 +++++++++++++++++++++++++++++++++++++++++++++++++
> >> 1 file changed, 49 insertions(+)
> >> 
> >> diff --git a/hw/vfio/pci.c b/hw/vfio/pci.c
> >> index bea95efc33..554b562769 100644
> >> --- a/hw/vfio/pci.c
> >> +++ b/hw/vfio/pci.c
> >> @@ -42,6 +42,7 @@
> >> #include "qapi/error.h"
> >> #include "migration/blocker.h"
> >> #include "migration/qemu-file.h"
> >> +#include "hw/vfio/user.h"
> >> 
> >> #define TYPE_VFIO_PCI_NOHOTPLUG "vfio-pci-nohotplug"
> >> 
> >> @@ -3326,3 +3327,51 @@ static void register_vfio_pci_dev_type(void)
> >> }
> >> 
> >> type_init(register_vfio_pci_dev_type)
> >> +
> >> +static void vfio_user_pci_realize(PCIDevice *pdev, Error **errp)
> >> +{
> >> +    ERRP_GUARD();
> >> +    VFIOUserPCIDevice *udev = VFIO_USER_PCI(pdev);
> >> +
> >> +    if (!udev->sock_name) {
> >> +        error_setg(errp, "No socket specified");
> >> +        error_append_hint(errp, "Use -device vfio-user-pci,socket=<name>\n");
> >> +        return;
> >> +    }
> >> +}
> >> +
> >> +static void vfio_user_instance_finalize(Object *obj)
> >> +{
> >> +}
> >> +
> >> +static Property vfio_user_pci_dev_properties[] = {
> >> +    DEFINE_PROP_STRING("socket", VFIOUserPCIDevice, sock_name),
> > 
> > Please use SocketAddress so that alternative socket connection details
> > can be supported without inventing custom syntax for vfio-user-pci. For
> > example, file descriptor passing should be possible.
> > 
> > I think this requires a bit of command-line parsing work, so don't worry
> > about it for now, but please add a TODO comment. When the -device
> > vfio-user-pci syntax is finalized (i.e. when the code is merged and the
> > device name doesn't start with the experimental x- prefix), then it
> > needs to be solved.
> > 
> 
> 	What do you want the options to look like at the endgame?  I’d
> rather work backward from that than have several different flavors of
> options as new socket options are added.  I did look at -chardev socket,
> and it was confusing enough that I went for the simple string.

The standard socket syntax is present in qemu-storage-daemon's --export
and --nbd-server options:

  addr.type=inet,addr.host=<host>,addr.port=<port>
  addr.type=unix,addr.path=<socket-path>
  addr.type=fd,addr.str=<fd>

--export and --nbd-server use QAPI to generate parsers for these options
(they use 'SocketAddress' from qapi/sockets.json). I'm not sure whether
it's easier to reuse the QAPI parser or to simply add qdev properties
mimicking the same syntax. Either way, there should probably be a common
qdev property API for SocketAddress values.

> >> +    DEFINE_PROP_BOOL("secure-dma", VFIOUserPCIDevice, secure, false),
> > 
> > I'm not sure what "secure-dma" means and the "secure" variable name is
> > even more inscrutable. Does this mean don't share memory so that each
> > DMA access is checked individually?
> > 
> 
> 	Yes.  Do you have another name you’d prefer? “no-shared-mem”?

I'm not sure other property names are much clearer, so feel free to
stick with "secure-dma". Renaming the "secure" field to "secure_dma" and
adding a comment that clarifies its purpose would be enough.

Here are some options:
- The vfio-user protocol message for sharing memory is called
  VFIO_USER_DMA_MAP. The option could be dma-map=on|off (default on).
  But this is based on protocol internals and may not be clear to users.
- shared-mem=on|off
- shared-ram=on|off

Thanks,
Stefan

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