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

[Shameerali Kolothum Thodi via]

> -----Original Message-----
> From: Nicolin Chen <nicol...@nvidia.com>
> Sent: Thursday, February 6, 2025 8:33 PM
> To: Shameerali Kolothum Thodi
> <shameerali.kolothum.th...@huawei.com>; Daniel P. Berrangé
> <berra...@redhat.com>; Jason Gunthorpe <j...@nvidia.com>
> Cc: qemu-...@nongnu.org; qemu-devel@nongnu.org;
> eric.au...@redhat.com; peter.mayd...@linaro.org; ddut...@redhat.com;
> Linuxarm <linux...@huawei.com>; Wangzhou (B)
> <wangzh...@hisilicon.com>; jiangkunkun <jiangkun...@huawei.com>;
> Jonathan Cameron <jonathan.came...@huawei.com>;
> zhangfei....@linaro.org; nath...@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 :(

Thanks,
Shameer