Re: [PATCH v2 16/17] intel_iommu: Introduce a property to control FS1GP cap bit setting

[Yi Liu]

On 2024/8/19 17:41, Duan, Zhenzhong wrote:
> > -----Original Message-----
> > From: Liu, Yi L <yi.l.liu@intel.com>
> > Subject: Re: [PATCH v2 16/17] intel_iommu: Introduce a property to control
> > FS1GP cap bit setting
> >
> > On 2024/8/15 11:46, Duan, Zhenzhong wrote:
> > > > -----Original Message-----
> > > > From: Liu, Yi L <yi.l.liu@intel.com>
> > > > Subject: Re: [PATCH v2 16/17] intel_iommu: Introduce a property to
> > control
> > > > FS1GP cap bit setting
> > > >
> > > > On 2024/8/5 14:27, Zhenzhong Duan wrote:
> > > > > When host IOMMU doesn't support FS1GP but vIOMMU does, host
> > > > IOMMU
> > > > > can't translate stage-1 page table from guest correctly.
> > > >
> > > > this series is for emulated devices, so the above statement does not
> > > > belong to this series. Is there any other reason to have this option?
> > >
> > > Good catch, will remove this comment.
> > > In fact, this patch is mainly for passthrough device where host IOMMU
> > doesn't support fs1gp.
> >
> > I see. To me, as long as the vIOMMU page walk logic supports 1GP large
> > pages, it's ok to report the FS1GP cap to VM. But it is still fine to
> > have this property to opt-out FS1GP if admin/orchestration layer(e.g. libvirt)
> > knows no hw iommu has this capability, so it is better to opt out it
> > before invoking QEMU.
> >
> > Is this your motivation for this property?
>
> Exactly.

ok, then let's keep it in this series after refining the commit message a
bit.

> > > > > Add a property x-cap-fs1gp for user to turn FS1GP off so that
> > > > > nested page table on host side works.
> > > >
> > > > I guess you would need to sync the FS1GP cap with host before reporting
> > it
> > > > in vIOMMU when comes to support passthrough devices.
> > >
> > > Yes, we already have this check, see
> > https://github.com/yiliu1765/qemu/commit/b7ac7ce3a2e21eb1b3172743
> > ee6f73e80fe67b3a
> >
> > good to know it. :) Will you fail the VM if the device's iommu does not
> > support FS1GP or just mask out the FS1GP?
>
> For cold plugged VFIO device, it will fail the VM with "Stage-1 1GB huge page is 
> unsupported by host IOMMU" error report.
> For hotplug VFIO device, only hotplug fails with "Stage-1 1GB huge page is 
> unsupported by host IOMMU".
>
> We don't update vIOMMU cap/ecap from host cap/ecap per Michael's suggestion, 
> only vIOMMU properties can control them.

I see. yeah, it makes sense.

--
Regards,
Yi Liu