linux-kernel - Re: [PATCH 1/1] iommu/vt-d: Omit devTLB invalidation requests when TES=0

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Date:   Tue, 14 Nov 2023 12:54:46 +0800
From:   Baolu Lu <baolu.lu@...ux.intel.com>
To:     "Tian, Kevin" <kevin.tian@...el.com>,
        Joerg Roedel <joro@...tes.org>, Will Deacon <will@...nel.org>,
        Robin Murphy <robin.murphy@....com>,
        Jason Gunthorpe <jgg@...pe.ca>
Cc:     baolu.lu@...ux.intel.com,
        "iommu@...ts.linux.dev" <iommu@...ts.linux.dev>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/1] iommu/vt-d: Omit devTLB invalidation requests when
 TES=0

On 11/14/23 12:45 PM, Tian, Kevin wrote:
>> From: Baolu Lu <baolu.lu@...ux.intel.com>
>> Sent: Tuesday, November 14, 2023 11:14 AM
>>
>> On 11/14/23 11:14 AM, Tian, Kevin wrote:
>>>> From: Lu Baolu<baolu.lu@...ux.intel.com>
>>>> Sent: Tuesday, November 14, 2023 9:11 AM
>>>>
>>>> The latest VT-d spec indicates that when remapping hardware is disabled
>>>> (TES=0 in Global Status Register), upstream ATS Invalidation Completion
>>>> requests are treated as UR (Unsupported Request).
>>>>
>>>> Consequently, the spec recommends in section 4.3 Handling of Device-TLB
>>>> Invalidations that software refrain from submitting any Device-TLB
>>>> invalidation requests when address remapping hardware is disabled.
>>>>
>>>> Verify address remapping hardware is enabled prior to submitting Device-
>>>> TLB invalidation requests.
>>>>
>>>> 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/dmar.c | 18 ++++++++++++++++++
>>>>    1 file changed, 18 insertions(+)
>>>>
>>>> diff --git a/drivers/iommu/intel/dmar.c b/drivers/iommu/intel/dmar.c
>>>> index a3414afe11b0..23cb80d62a9a 100644
>>>> --- a/drivers/iommu/intel/dmar.c
>>>> +++ b/drivers/iommu/intel/dmar.c
>>>> @@ -1522,6 +1522,15 @@ void qi_flush_dev_iotlb(struct intel_iommu
>>>> *iommu, u16 sid, u16 pfsid,
>>>>    {
>>>>    	struct qi_desc desc;
>>>>
>>>> +	/*
>>>> +	 * VT-d spec, section 4.3:
>>>> +	 *
>>>> +	 * Software is recommended to not submit any Device-TLB
>>>> invalidation
>>>> +	 * requests while address remapping hardware is disabled.
>>>> +	 */
>>>> +	if (!(iommu->gcmd & DMA_GCMD_TE))
>>>> +		return;
>>>> +
>>> Is it a valid case to see such request when the iommu is disabled?
>>> If not then let's add a WARN.
>>
>> There might be valid cases. The VT-d translation is turned on after all
>> devices get probed.
>>
> 
> but I didn't get why there will be actual mapping changes before
> vtd translation is enabled...

For an example, in iommu_create_device_direct_mappings(),
iommu_flush_iotlb_all() is called after direct mappings are created.

Best regards,
baolu