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

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Message-ID: <435ea410-6c48-4b0c-9bd3-fa8d85c24668@linux.intel.com>
Date: Mon, 8 Apr 2024 15:43:04 +0800
From: Ethan Zhao <haifeng.zhao@...ux.intel.com>
To: Baolu Lu <baolu.lu@...ux.intel.com>, iommu@...ts.linux.dev
Cc: Kevin Tian <kevin.tian@...el.com>, Yi Liu <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

On 4/8/2024 3:23 PM, Baolu Lu wrote:
> On 2024/4/8 15:21, Ethan Zhao wrote:
>> On 4/7/2024 10:42 PM, Lu Baolu wrote:
>>> 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.
>>>
>>> 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)
>>
>> My understanding, we don't need patch[1/2] at all, and customer is 
>> just asking
>> about the CM & tlb flushing, it is great to have this commit [2/2].
>
> Actually they fix different problems.

Yup, progress view.

Thanks,
Ethan

>
> Best regards,
> baolu