linux-kernel - Re: [PATCH v1] iommu/vt-d: Remove the redundant logic in first_level_by

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Message-ID: <5fed2452-ae2a-4fca-8483-85ecf645ec1b@gmail.com>
Date: Wed, 4 Jun 2025 15:25:42 +0800
From: Ethan Zhao <etzhao1900@...il.com>
To: "Tian, Kevin" <kevin.tian@...el.com>,
 Ethan Zhao <haifeng.zhao@...ux.intel.com>, "Wang, Wei W"
 <wei.w.wang@...el.com>, "baolu.lu@...ux.intel.com"
 <baolu.lu@...ux.intel.com>, "Liu, Yi L" <yi.l.liu@...el.com>,
 "dwmw2@...radead.org" <dwmw2@...radead.org>,
 "jroedel@...e.de" <jroedel@...e.de>,
 "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
 "iommu@...ts.linux.dev" <iommu@...ts.linux.dev>
Subject: Re: [PATCH v1] iommu/vt-d: Remove the redundant logic in
 first_level_by_default()



On 5/29/2025 2:11 PM, Tian, Kevin wrote:
>> From: Ethan Zhao <haifeng.zhao@...ux.intel.com>
>> Sent: Thursday, May 29, 2025 1:48 PM
>>
>> 在 2025/5/23 16:10, Wei Wang 写道:
>>> This original implementation included redundant logic to determine
>> whether
>>> first-stage translation should be used by default. Simplify it and
>>> preserve the original behavior:
>>> - Returns false in legacy mode (no scalable mode support).
>>> - Defaults to first-level translation when both FLTS and SLTS are
>>>     supported.
>>>
>>> Signed-off-by: Wei Wang <wei.w.wang@...el.com>
>>> ---
>>>    drivers/iommu/intel/iommu.c | 10 +---------
>>>    1 file changed, 1 insertion(+), 9 deletions(-)
>>>
>>> diff --git a/drivers/iommu/intel/iommu.c b/drivers/iommu/intel/iommu.c
>>> index cb0b993bebb4..228da47ab7cd 100644
>>> --- a/drivers/iommu/intel/iommu.c
>>> +++ b/drivers/iommu/intel/iommu.c
>>> @@ -1366,15 +1366,7 @@ static void free_dmar_iommu(struct
>> intel_iommu *iommu)
>>>     */
>>>    static bool first_level_by_default(struct intel_iommu *iommu)
>>>    {
>>> -	/* Only SL is available in legacy mode */
>>> -	if (!sm_supported(iommu))
>>> -		return false;
>>> -
>>> -	/* Only level (either FL or SL) is available, just use it */
>>> -	if (ecap_flts(iommu->ecap) ^ ecap_slts(iommu->ecap))
>>> -		return ecap_flts(iommu->ecap);
>>> -
>>> -	return true;
>>
>> The function works like a digital circurt has 3 single bit inputs  sm,
>> flts, slts and one bit output ret.
>>
>> so the true value table of the orignal function looks like
>>
>>      sm   flts   slts    ret
>> a   0     x     x      false
>> b   1     1     0      true
>> c   1     0     1      false
>> d   1     1     1      true
>> e   1     0     0      true
> 
> 'e' is actually wrong. We should not return true when the 1st level
> cap doesn't exist.

If so, this patch should mention it fixes such case with fix tag, not
  "removing the redundant logic with the same behavior".


Thanks,
Ethan
> 
>>
>>> +	return sm_supported(iommu) && ecap_flts(iommu->ecap);
>>
>> And the true value table of this new one looks like
>>
>>      sm  flts slts    ret
>>
>> f   1     1     x      true
>> g   1     0     x      false
>>
>> h   0     1     x      false
>> i    0     0     x      false
> 
> so this table is correct.