linux-kernel - Re: [PATCH v1 14/14] iommu/arm-smmu-v3: Add arm_smmu_cache_invalidate

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Message-ID: <ZBqOxNuHWPuhNy5b@Asurada-Nvidia>
Date:   Tue, 21 Mar 2023 22:14:44 -0700
From:   Nicolin Chen <nicolinc@...dia.com>
To:     Jason Gunthorpe <jgg@...dia.com>
CC:     Robin Murphy <robin.murphy@....com>, <will@...nel.org>,
        <eric.auger@...hat.com>, <kevin.tian@...el.com>,
        <baolu.lu@...ux.intel.com>, <joro@...tes.org>,
        <shameerali.kolothum.thodi@...wei.com>, <jean-philippe@...aro.org>,
        <linux-arm-kernel@...ts.infradead.org>, <iommu@...ts.linux.dev>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v1 14/14] iommu/arm-smmu-v3: Add
 arm_smmu_cache_invalidate_user

On Mon, Mar 20, 2023 at 07:14:17PM -0300, Jason Gunthorpe wrote:
> On Mon, Mar 20, 2023 at 01:46:52PM -0700, Nicolin Chen wrote:
> > On Mon, Mar 20, 2023 at 03:07:13PM -0300, Jason Gunthorpe wrote:
> > > On Mon, Mar 20, 2023 at 09:35:20AM -0700, Nicolin Chen wrote:
> > > 
> > > > > You need to know what devices the vSID is targetting ang issues
> > > > > invalidations only for those devices.
> > > > 
> > > > I agree with that, yet cannot think of a solution to achieve
> > > > that out of vSID. QEMU code by means of emulating a physical
> > > > SMMU only reads the commands from the queue, without knowing
> > > > which device (vSID) actually sent these commands.
> > > 
> > > Huh?
> > > 
> > > CMD_ATC_INV has the SID
> > > 
> > > Other commands have the ASID.
> > > 
> > > You never need to cross an ASID to a SID or vice versa.
> > > 
> > > If the guest is aware of ATS it will issue CMD_ATC_INV with vSIDs, and
> > > the hypervisor just needs to convert vSID to pSID.
> > > 
> > > Otherwise vSID doesn't matter because it isn't used in the invalidation
> > > API and you are just handling ASIDs that only need the VM_ID scope
> > > applied.
> > 
> > Yea, I was thinking of your point (at the top) how we could
> > ensure if an invalidation is targeting a correct vSID. So,
> > that narrative was only about CMD_ATC_INV...
> > 
> > Actually, we don't forward CMD_ATC_INV in QEMU. In another
> > thread, Kevin also remarked whether we need to support that
> > in the host or not. And I plan to drop CMD_ATC_INV from the
> > list of cache_invalidate_user(), following his comments and
> > the QEMU situation. Our uAPI, either forwarding the commands
> > or a package of queue info, should be able to cover this in
> > the future whenever we think it's required.
> 
> Something has to generate CMD_ATC_INV.
>
> How do you plan to generate this from the hypervisor based on ASID
> invalidations?
>
> The hypervisor doesn't know what ASIDs are connected to what SIDs to
> generate the ATC?
> 
> Intel is different, they know what devices the vDID is connected to,
> so when they get a vDID invalidation they can elaborate it into a ATC
> invalidation. ARM doesn't have that information.

I see. Perhaps vSMMU still needs to forward CMD_ATC_INV. And,
as you suggested, it should go through a vSID sanity check by
the host handler. We can find the corresponding pSID to check
if the device is associated with the iommu_domain?

Thanks
Nic