linux-kernel - Re: [PATCH v2 04/19] iommufd: Allow pt_id to carry viommu_id for IOMMU_HWPT

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Message-ID: <ZvW/vS5/vulxw3co@Asurada-Nvidia>
Date: Thu, 26 Sep 2024 13:10:37 -0700
From: Nicolin Chen <nicolinc@...dia.com>
To: Yi Liu <yi.l.liu@...el.com>
CC: <jgg@...dia.com>, <kevin.tian@...el.com>, <will@...nel.org>,
	<joro@...tes.org>, <suravee.suthikulpanit@....com>, <robin.murphy@....com>,
	<dwmw2@...radead.org>, <baolu.lu@...ux.intel.com>, <shuah@...nel.org>,
	<linux-kernel@...r.kernel.org>, <iommu@...ts.linux.dev>,
	<linux-arm-kernel@...ts.infradead.org>, <linux-kselftest@...r.kernel.org>,
	<eric.auger@...hat.com>, <jean-philippe@...aro.org>, <mdf@...nel.org>,
	<mshavit@...gle.com>, <shameerali.kolothum.thodi@...wei.com>,
	<smostafa@...gle.com>
Subject: Re: [PATCH v2 04/19] iommufd: Allow pt_id to carry viommu_id for
 IOMMU_HWPT_ALLOC

On Thu, Sep 26, 2024 at 04:50:46PM +0800, Yi Liu wrote:
> On 2024/8/28 00:59, Nicolin Chen wrote:
> > Now a VIOMMU can wrap a shareable nested parent HWPT. So, it can act like
> > a nested parent HWPT to allocate a nested HWPT.
> > 
> > Support that in the IOMMU_HWPT_ALLOC ioctl handler, and update its kdoc.
> > 
> > Also, associate a viommu to an allocating nested HWPT.
> 
> it still not quite clear to me what vIOMMU obj stands for. Here, it is a
> wrapper of s2 hpwt IIUC. But in the cover letter, vIOMMU obj can instanced
> per the vIOMMU units in VM.

Yea, the implementation in this version is merely a wrapper. I
had a general introduction of vIOMMU in the other reply. And I
will put something similar in the next version of the series,
so the idea would be bigger than a wrapper.

> Does it mean each vIOMMU of VM can only have
> one s2 HWPT?

Giving some examples here:
 - If a VM has 1 vIOMMU, there will be 1 vIOMMU object in the
   kernel holding one S2 HWPT.
 - If a VM has 2 vIOMMUs, there will be 2 vIOMMU objects in the
   kernel that can hold two different S2 HWPTs, or share one S2
   HWPT (saving memory).

Thanks
Nic