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

[Yi Liu]

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?

> > > 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/b7ac7ce3a2e21eb1b3172743ee6f73e80fe67b3a

good to know it. :) Will you fail the VM if the device's iommu does not
support FS1GP or just mask out the FS1GP?

--
Regards,
Yi Liu