linux-kernel - Re: [PATCH v1 3/9] iommu/arm-smmu-v3: Store ASIDs and VMID in arm_smmu

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Message-ID: <aUWkr5e+C11mbx0X@Asurada-Nvidia>
Date: Fri, 19 Dec 2025 11:17:03 -0800
From: Nicolin Chen <nicolinc@...dia.com>
To: Jason Gunthorpe <jgg@...dia.com>
CC: <will@...nel.org>, <robin.murphy@....com>, <joro@...tes.org>,
	<jpb@...nel.org>, <praan@...gle.com>, <miko.lenczewski@....com>,
	<linux-arm-kernel@...ts.infradead.org>, <iommu@...ts.linux.dev>,
	<linux-kernel@...r.kernel.org>, <patches@...ts.linux.dev>
Subject: Re: [PATCH v1 3/9] iommu/arm-smmu-v3: Store ASIDs and VMID in
 arm_smmu_master

On Fri, Dec 19, 2025 at 11:16:08AM -0400, Jason Gunthorpe wrote:
> On Thu, Dec 18, 2025 at 12:26:49PM -0800, Nicolin Chen wrote:
> > Currently, ASID is allocated per smmu_domain, stored in the domain, and
> > freed with the domain.
> > 
> > Practically, ASID is only used in a CD as an iotlb tag. Therefore, ASID
> > doesn't really follow the life cycle of a domain but domain attachment.
> > 
> > On the other hand, the CD carrying ASID is installed to a device's STE.
> > 
> > This applies to the VMID as well, which is installed in an STE directly.
> > 
> > Since a device can only have one ASID per SSID and one VMID per SID, add
> > an ASID array and VMID in the arm_smmu_master structure, to decouple the
> > ASID/VMID from the domain structure.
> 
> I don't think this is entirely right..
> 
> When a S1 is attached to a master the master needs to store the VMID
> it is using, as the VMID is global to the STE and effectively becomes
> global to the master.
>
> But the ASID should be stored in the invalidation list of the domain.

Hmm, you mean the nested case (host VMID + guest ASID), right?

But in that case, the guest-level ASID isn't needed anywhere in
the host driver? The ASID should be configured in a guest-level
CD. And the invalidation command directly coming from the guest
is already filled with the guest-level ASID.

Also, the invalidation array stores either ASID or VMID, there
is no case of storing both, right?

> Jus search the list for the (instance, vmid) pair of the master to get
> back the right ASID.
> 
> We don't need to store it again in another list, that's confusing.

Okay. I was trying to get rid of the extra for loop searching
for an ID in the array. That's why I had the alloc_id/free_id
ops too, so everything would be done in existing two loops in
the merge/unref functions.

Nicolin