RE: [RFC PATCH 0/5] hw/arm/virt: Add support for user-creatable nested SMMUv3

[Shameerali Kolothum Thodi]

> -----Original Message-----
> From: Jason Gunthorpe <jgg@nvidia.com>
> Sent: Thursday, February 6, 2025 5:47 PM
> To: Daniel P. Berrangé <berrange@redhat.com>
> Cc: Shameerali Kolothum Thodi
> <shameerali.kolothum.thodi@huawei.com>; qemu-arm@nongnu.org;
> qemu-devel@nongnu.org; eric.auger@redhat.com;
> peter.maydell@linaro.org; nicolinc@nvidia.com; ddutile@redhat.com;
> Linuxarm <linuxarm@huawei.com>; Wangzhou (B)
> <wangzhou1@hisilicon.com>; jiangkunkun <jiangkunkun@huawei.com>;
> Jonathan Cameron <jonathan.cameron@huawei.com>;
> zhangfei.gao@linaro.org; nathanc@nvidia.com
> Subject: Re: [RFC PATCH 0/5] hw/arm/virt: Add support for user-creatable
> nested SMMUv3
> 
> On Thu, Feb 06, 2025 at 05:10:32PM +0000, Daniel P. Berrangé wrote:
> > On Thu, Feb 06, 2025 at 01:02:38PM -0400, Jason Gunthorpe wrote:
> > > On Thu, Feb 06, 2025 at 03:07:06PM +0000, Shameerali Kolothum Thodi
> wrote:
> > > > > If we set the physical/guest SMMU relationship directly, then at the
> > > > > time the VFIO device is plugged, we can diagnose the incorrectly
> > > > > placed VFIO device, and better reason about behaviour.
> > > >
> > > > Agree.
> > >
> > > Can you just take in a VFIO cdev FD reference on this command line:
> > >
> > >  -device arm-smmuv3-accel,id=smmuv2,bus=pcie.2
> > >
> > > And that will lock the pSMMU/vSMMU relationship?
> >
> > We shouldn't assume any VFIO device exists in the QEMU cnofig at the
> time
> > we realize the virtual ssmu. I expect the SMMU may be cold plugged,
> while
> > the VFIO devices may be hot plugged arbitrarly later, and we should have
> > the association initialized the SMMU is realized.
> 
> This is not supported kernel side, you can't instantiate a vIOMMU
> without a VFIO device that uses it. For security.

I think that is fine if Qemu knows about association beforehand. During 
vIOMMU instantiation it can cross check whether the user specified
pSMMU <->vSMMU is correct for the device.

Also how do we do it with multiple VF devices under a pSUMMU ? Which
cdev fd in that case? 

Thanks,
Shameer