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

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Message-ID: <03553755-6404-4bb3-badf-5091755f36b7@intel.com>
Date: Mon, 19 Aug 2024 15:58:35 +0800
From: Yi Liu <yi.l.liu@...el.com>
To: Baolu Lu <baolu.lu@...ux.intel.com>, Jacob Pan
	<jacob.pan@...ux.microsoft.com>, Tina Zhang <tina.zhang@...el.com>
CC: 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

On 2024/8/17 11:28, Baolu Lu 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.

Hi Jacob,

Do you mean the configuration that iommu.strict==0? :) As the above
explanation from Baolu, this patch is not for that although it uses
the term "batched". Also, it would reduce the VMExits that due to the
IOTLB/DevTLB invalidation a lot in the virtualization environment.

-- 
Regards,
Yi Liu