linux-kernel - Re: [PATCH v3 4/4] iommu/vt-d: Introduce batched cache invalidation

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Message-ID: <20240820160437.76fb4d34@DESKTOP-0403QTC.>
Date: Tue, 20 Aug 2024 16:04:37 -0700
From: Jacob Pan <jacob.pan@...ux.microsoft.com>
To: Baolu Lu <baolu.lu@...ux.intel.com>
Cc: Tina Zhang <tina.zhang@...el.com>, Kevin Tian <kevin.tian@...el.com>,
 iommu@...ts.linux.dev, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 4/4] iommu/vt-d: Introduce batched cache invalidation

Hi Baolu,

On Tue, 20 Aug 2024 10:06:05 +0800
Baolu Lu <baolu.lu@...ux.intel.com> wrote:

> On 8/19/24 11:40 PM, Jacob Pan wrote:
> > On Sat, 17 Aug 2024 11:28:21 +0800
> > Baolu Lu<baolu.lu@...ux.intel.com>  wrote:
> >   
> >> On 2024/8/17 0:38, Jacob Pan wrote:  
> >>> On Thu, 15 Aug 2024 14:52:21 +0800
> >>> Tina Zhang<tina.zhang@...el.com>  wrote:
> >>>      
> >>>> @@ -270,7 +343,8 @@ static void cache_tag_flush_iotlb(struct
> >>>> dmar_domain *domain, struct cache_tag * u64 type =
> >>>> DMA_TLB_PSI_FLUSH;
> >>>>    	if (domain->use_first_level) {
> >>>> -		qi_flush_piotlb(iommu, tag->domain_id,
> >>>> tag->pasid, addr, pages, ih);
> >>>> +		qi_batch_add_piotlb(iommu, tag->domain_id,
> >>>> tag->pasid, addr,
> >>>> +				    pages, ih,
> >>>> domain->qi_batch); return;
> >>>>    	}
> >>>>    
> >>>> @@ -287,7 +361,8 @@ static void cache_tag_flush_iotlb(struct
> >>>> dmar_domain *domain, struct cache_tag * }
> >>>>    
> >>>>    	if (ecap_qis(iommu->ecap))
> >>>> -		qi_flush_iotlb(iommu, tag->domain_id, addr | ih,
> >>>> mask, type);
> >>>> +		qi_batch_add_iotlb(iommu, tag->domain_id, addr |
> >>>> ih, mask, type,
> >>>> +				   domain->qi_batch);
> >>>>        
> >>> If I understand this correctly, IOTLB flush maybe deferred until
> >>> the batch array is full, right? If so, is there a security gap
> >>> where callers think the mapping is gone after the call returns?  
> >> No. All related caches are flushed before function return. A domain
> >> can have multiple cache tags. Previously, we sent individual cache
> >> invalidation requests to hardware. This change combines all
> >> necessary invalidation requests into a single batch and raise them
> >> to hardware together to make it more efficient.  
> > I was looking at the code below, if the index does not reach
> > QI_MAX_BATCHED_DESC_COUNT. There will be no flush after
> > cache_tag_flush_iotlb() returns, right?  
> 
> No. qi_batch_flush_descs() is called explicitly before return.
I see, cache_tag_flush_iotlb() is really just adding descriptors to the
batch. Not doing any flush for most cases. IMHO, the name is a little
confusing.

Thanks,

Jacob