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[63.177.217.227]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-451f99198a9sm9586775e9.30.2025.06.04.20.33.58 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 04 Jun 2025 20:34:01 -0700 (PDT) Message-ID: Date: Thu, 5 Jun 2025 11:33:56 +0800 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v1] iommu/vt-d: Remove the redundant logic in first_level_by_default() To: "Wang, Wei W" , "Tian, Kevin" , Ethan Zhao , "baolu.lu@linux.intel.com" , "Liu, Yi L" , "dwmw2@infradead.org" , "jroedel@suse.de" , "linux-kernel@vger.kernel.org" , "iommu@lists.linux.dev" References: <20250523081056.223082-1-wei.w.wang@intel.com> <01e5d0a9-715a-48d4-a2a4-2a0b5d99149b@linux.intel.com> <5fed2452-ae2a-4fca-8483-85ecf645ec1b@gmail.com> Content-Language: en-US From: Ethan Zhao In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 6/5/2025 9:04 AM, Wang, Wei W wrote: > On Wednesday, June 4, 2025 3:26 PM, Ethan Zhao wrote: >> On 5/29/2025 2:11 PM, Tian, Kevin wrote: >>>> From: Ethan Zhao >>>> 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 >>>>> --- >>>>> 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". >> > > Probably not a fix (just code improvement). Your 'e' above is not possible > from the hardware side, and the old implementation doesn't have it too. > Is there any items in Intel VT-d spec to say "It is impossible to have both flts and slts to false" or you design code under an assumption that hardware always works perfect ? You made an assumption that the hardware never would reach there and then got a conclusion 'e' doesn't exist ? 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. >