linux-kernel - RE: [PATCH 2/2] iommu/vt-d: Remove caching mode check before devtlb flush

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Message-ID: <MW5PR11MB5881E15977B2987AE32510B289062@MW5PR11MB5881.namprd11.prod.outlook.com>
Date: Wed, 10 Apr 2024 23:49:01 +0000
From: "Zhang, Tina" <tina.zhang@...el.com>
To: "Tian, Kevin" <kevin.tian@...el.com>, Lu Baolu <baolu.lu@...ux.intel.com>,
	"iommu@...ts.linux.dev" <iommu@...ts.linux.dev>
CC: "Liu, Yi L" <yi.l.liu@...el.com>, Joerg Roedel <joro@...tes.org>, "Will
 Deacon" <will@...nel.org>, Robin Murphy <robin.murphy@....com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH 2/2] iommu/vt-d: Remove caching mode check before devtlb
 flush

Hi,

> -----Original Message-----
> From: Tian, Kevin <kevin.tian@...el.com>
> Sent: Tuesday, April 9, 2024 3:30 PM
> To: Lu Baolu <baolu.lu@...ux.intel.com>; iommu@...ts.linux.dev
> Cc: Liu, Yi L <yi.l.liu@...el.com>; Joerg Roedel <joro@...tes.org>; Will
> Deacon <will@...nel.org>; Robin Murphy <robin.murphy@....com>; linux-
> kernel@...r.kernel.org
> Subject: RE: [PATCH 2/2] iommu/vt-d: Remove caching mode check before
> devtlb flush
> 
> > From: Lu Baolu <baolu.lu@...ux.intel.com>
> > Sent: Sunday, April 7, 2024 10:43 PM
> >
> > The Caching Mode (CM) of the Intel IOMMU indicates if the hardware
> > implementation caches not-present or erroneous translation-structure
> > entries except the first-stage translation. The caching mode is
> > unrelated to the device TLB , therefore there is no need to check it
> > before a device TLB invalidation operation.
> >
> > Before the scalable mode is introduced, caching mode is treated as an
> > indication that the driver is running in a VM guest. This is just a
> > software contract as shadow page table is the only way to implement a
> > virtual IOMMU. But the VT-d spec doesn't state this anywhere. After
> > the scalable mode is introduced, this doesn't stand for anymore, as
> > caching mode is not relevant for the first-stage translation. A
> > virtual IOMMU implementation is free to support first-stage
> > translation only with caching mode cleared.
> 
> I didn't get the connection to the scalable mode.
> 
> if required we can still use caching mode to imply running as a guest.
> Just need to make sure its implementation conforming to the VT-d spec.
> 
> >
> > Remove the caching mode check before device TLB invalidation to ensure
> > compatibility with the scalable mode use cases.
> >
> > Fixes: 792fb43ce2c9 ("iommu/vt-d: Enable Intel IOMMU scalable mode by
> > default")
> > Signed-off-by: Lu Baolu <baolu.lu@...ux.intel.com>
> > ---
> >  drivers/iommu/intel/iommu.c | 5 ++---
> >  1 file changed, 2 insertions(+), 3 deletions(-)
> >
> > diff --git a/drivers/iommu/intel/iommu.c b/drivers/iommu/intel/iommu.c
> > index 493b6a600394..681789b1258d 100644
> > --- a/drivers/iommu/intel/iommu.c
> > +++ b/drivers/iommu/intel/iommu.c
> > @@ -1501,7 +1501,7 @@ static void iommu_flush_iotlb_psi(struct
> > intel_iommu *iommu,
> >  	else
> >  		__iommu_flush_iotlb_psi(iommu, did, pfn, pages, ih);
> >
> > -	if (!cap_caching_mode(iommu->cap) && !map)
> > +	if (!map)
> >  		iommu_flush_dev_iotlb(domain, addr, mask);
> 
> as commented earlier better squash this in patch1.
> 
> >  }
> >
> > @@ -1575,8 +1575,7 @@ static void intel_flush_iotlb_all(struct
> > iommu_domain *domain)
> >  			iommu->flush.flush_iotlb(iommu, did, 0, 0,
> >  						 DMA_TLB_DSI_FLUSH);
> >
> > -		if (!cap_caching_mode(iommu->cap))
> > -			iommu_flush_dev_iotlb(dmar_domain, 0,
> > MAX_AGAW_PFN_WIDTH);
> > +		iommu_flush_dev_iotlb(dmar_domain, 0,
> > MAX_AGAW_PFN_WIDTH);
> >  	}
> >
> 
> I'm hesitating to agree with this change. Strictly speaking it's correct.
> but w/o supporting batch invalidation this implies performance drop on
> viommu due to more VM-exits and there may incur user complaints when
> their VMs upgrade to a newer kernel version.
> 
> So it'd be better to keep this behavior and fix it together with batch
> invalidation support. Anyway none of the viommu implementations today
> (either shadow or nested translation) relies on the correct devtlb behavior
> from the guest otherwise it's already broken.
How about we split this change into a patch. I'm working on the batch invalidation patch-set now and I'm happy to include this code change into the batch invalidation series.

Regards,
-Tina