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

[Daniel P . Berrangé]

On Fri, Feb 07, 2025 at 10:21:17AM +0000, Shameerali Kolothum Thodi wrote:
> 
> 
> > -----Original Message-----
> > From: Nicolin Chen <nicolinc@nvidia.com>
> > Sent: Thursday, February 6, 2025 8:33 PM
> > To: Shameerali Kolothum Thodi
> > <shameerali.kolothum.thodi@huawei.com>; Daniel P. Berrangé
> > <berrange@redhat.com>; Jason Gunthorpe <jgg@nvidia.com>
> > Cc: qemu-arm@nongnu.org; qemu-devel@nongnu.org;
> > eric.auger@redhat.com; peter.maydell@linaro.org; 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 02:22:01PM -0400, Jason Gunthorpe wrote:
> > > On Thu, Feb 06, 2025 at 06:18:14PM +0000, Shameerali Kolothum Thodi
> > wrote:
> > >
> > > > > So even if you invent an iommu ID we cannot accept it as a handle to
> > > > > create viommu in iommufd.
> > > >
> > > > Creating the vIOMMU only happens when the user does a  cold/hot
> > plug of
> > > > a VFIO device. At that time Qemu checks whether the assigned id
> > matches
> > > > with whatever the kernel tell it.
> > >
> > > This is not hard up until the guest is started. If you boot a guest
> > > without a backing viommu iommufd object then there will be some more
> > > complexities.
> > 
> > Yea, I imagined that things would be complicated with hotplugs..
> > 
> > On one hand, I got the part that we need some fixed link forehand
> > to ease migration/hotplugs.
> > 
> > On the other hand, all IOMMUFD ioctls need a VFIO device FD, which
> > brings the immediate attention that we cannot even decide vSMMU's
> > capabilities being reflected in its IDR/IIDR registers, without a
> > coldplug device -- if we boot a VM (one vSMMU<->pSMMU) with only a
> > hotplug device, the IOMMU_GET_HW_INFO cannot be done during guest
> 
> Right. I forgot about the call to smmu_dev_get_info() during the reset.
> That means we need at least one dev per Guest SMMU during Guest
> boot :(

That's pretty unpleasant as a usage restriction. It sounds like there
needs to be a way to configure & control the vIOMMU independantly of
attaching a specific VFIO device.

With regards,
Daniel
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