[virtio-dev] Re: [PATCH v1 1/2] transport-pci: Introduce legacy registers access commands

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

On Wed, May 03, 2023 at 01:21:39PM -0400, Parav Pandit wrote:
> 
> 
> On 5/3/2023 12:48 PM, Michael S. Tsirkin wrote:
> 
> > > > As simple as it is, I feel this falls far short of describing how
> > > > a device should operate.
> > > > Some issues:
> > > > 	- legacy device config offset changes as msi is enabled/disabled
> > > > 	  suggest separate commands for device/common config
> > > This is fine and covered here. The one who is making msix enable/disable
> > > knows which registers to access before/after disable/enable and device also
> > > knows it as it is supplying the register.
> > > So they just follow the standard legacy register access behavior.
> > 
> > But do VFs support INT#x? I will have to re-read the spec.
> > 
> VFs do not support INTx.
> When hypervisor knows that it cannot support msix for the guest, it can
> avoid using the VF for the guest.

it's the guest that does not support msix. Not host.

> > > > 	- legacy device endian-ness changes with guest
> > > > 	  suggest commands to enable LE and BE mode
> > > guest endianeness is not known to the device.
> > 
> > But is known to hypervisor.
> > 
> It can be an extension command in future as part of the VF administration
> command to set it by the hypervisor PF.
> 
> > > Currently it is only for the
> > > LE guests who had some legacy requirement.
> > 
> > I don't like tying this to LE implicitly some devices might be BE only.
> > With my idea device can support command to set LE, command to set BE or
> > both.
> > 
> It can be an addition in future if needed.
> 
> > > and PCIe is LE anyway.
> > 
> > PCIE config endian-ness does not matter heere.
> > 
> > > > 	- legacy guests often assume INT#x support
> > > > 	  suggest a way to tunnel that too;
> > > INT#x is present on the PCI device itself. So no need to tunnel it.
> > > Also INT#x is very narrow case. When MSI-X is not supported, a hypervisor
> > > can choose not to even connect such device to the guest VM.
> > 
> > devices will support MSI. But *guest* might not support MSIX you only
> > find out late when it is driving the device.
> > 
> It is a pathological case that may not exist because legacy drivers and
> linux kernels all the way upto 2.6.32 have msix support.
> 
> And in case if hypervisor sw wants to support for unknown scenario, it can
> use the one msix based interrupt to emulate intx.
> 
> > > > I expected to see more statements along the lines of
> > > >           command ABC has the same effect as access
> > > >           to register DEF of the member through the legacy pci interface
> > > > 
> > > Yes, good point. I will add it in the theory of operation section for this
> > > mapping detail.
> > 
> > OK, and overall if you see an existing statement about legacy do not
> > copy it, just explain how it is mapped.
> > 
> Yes, will not copy, only the mapping part to add.


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