linux-kernel - RE: [PATCH 06/12] iommu/vt-d: Use cache_tag_flush_cm_range() in iotlb_sync

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Message-ID: <BN9PR11MB5276FDB6B86B8E49959236008C002@BN9PR11MB5276.namprd11.prod.outlook.com>
Date: Mon, 8 Apr 2024 02:51:46 +0000
From: "Tian, Kevin" <kevin.tian@...el.com>
To: Baolu Lu <baolu.lu@...ux.intel.com>, Joerg Roedel <joro@...tes.org>, "Will
 Deacon" <will@...nel.org>, Robin Murphy <robin.murphy@....com>, "Jason
 Gunthorpe" <jgg@...pe.ca>
CC: "Zhang, Tina" <tina.zhang@...el.com>, "Liu, Yi L" <yi.l.liu@...el.com>,
	"iommu@...ts.linux.dev" <iommu@...ts.linux.dev>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH 06/12] iommu/vt-d: Use cache_tag_flush_cm_range() in
 iotlb_sync_map

> From: Baolu Lu <baolu.lu@...ux.intel.com>
> Sent: Sunday, April 7, 2024 2:41 PM
> 
> On 3/28/24 3:48 PM, Tian, Kevin wrote:
> >> From: Lu Baolu <baolu.lu@...ux.intel.com>
> >> Sent: Monday, March 25, 2024 10:17 AM
> >>
> >> The iotlb_sync_map callback is called by the iommu core after non-
> present
> >> to present mappings are created. The iommu driver uses this callback to
> >> invalidate caches if IOMMU is working in caching mode and second-only
> >> translation is used for the domain. Use cache_tag_flush_cm_range() in
> this
> >> callback.
> >>
> >> Signed-off-by: Lu Baolu <baolu.lu@...ux.intel.com>
> >> ---
> >>   drivers/iommu/intel/iommu.c | 22 +---------------------
> >>   1 file changed, 1 insertion(+), 21 deletions(-)
> >>
> >> diff --git a/drivers/iommu/intel/iommu.c b/drivers/iommu/intel/iommu.c
> >> index 1c03d2dafb9d..2dcab1e5cd4d 100644
> >> --- a/drivers/iommu/intel/iommu.c
> >> +++ b/drivers/iommu/intel/iommu.c
> >> @@ -1504,20 +1504,6 @@ static void iommu_flush_iotlb_psi(struct
> >> intel_iommu *iommu,
> >>   		iommu_flush_dev_iotlb(domain, addr, mask);
> >>   }
> >>
> >> -/* Notification for newly created mappings */
> >> -static void __mapping_notify_one(struct intel_iommu *iommu, struct
> >> dmar_domain *domain,
> >> -				 unsigned long pfn, unsigned int pages)
> >> -{
> >> -	/*
> >> -	 * It's a non-present to present mapping. Only flush if caching mode
> >> -	 * and second level.
> >> -	 */
> >> -	if (cap_caching_mode(iommu->cap) && !domain->use_first_level)
> >> -		iommu_flush_iotlb_psi(iommu, domain, pfn, pages, 0, 1);
> >> -	else
> >> -		iommu_flush_write_buffer(iommu);
> >> -}
> >
> > iommu_flush_write_buffer is for a quite different issue. it's clearer to
> > keep it separated from the iotlb helpers.
> 
> The VT-d spec describes the write buffer flushing in section 6.8. It
> states,
> 
> 1. Write buffer flushing is required only for earlier VT-d hardware
>     implementations. Newer hardware implementations are expected to NOT
>     require this.
> 2. Write buffer flushing is related to cache invalidation. The hardware
>     performs an implicit write-buffer-flushing as a pre-condition to
>     the invalidation operations.
> 
> According to this, write buffer flushing is only required in the map
> path when IOMMU is not in caching mode and the RWBF iommu capability is
> set, which perfectly matches the case of cache_tag_flush_cm_range().
> 
> By doing it in cache_tag_flush_cm_range(), the code will be simpler and
> easier for maintenance.
> 

but above applies actually when caching mode is false or when 1st
stage is being used. this is the opposite of what flush_cm_range()
implies.

if we do it through a general flush_range() helper which accepts
a parameter to indicate the map path, then it's reasonable to move
this special handling to that helper as that is another trick which
the caller doesn't need to know.