From mboxrd@z Thu Jan 1 00:00:00 1970 From: Lu Baolu Subject: Re: What is the meaning of PASID_MIN? Date: Tue, 12 Feb 2019 11:13:28 +0800 Message-ID: References: <20190211192804.GI12668@bombadil.infradead.org> <0d86f3e8-60ec-5433-8602-487c8dc1ed83@linux.intel.com> <20190212030232.GI1011@xz-x1> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii"; Format="flowed" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <20190212030232.GI1011@xz-x1> Content-Language: en-US List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: iommu-bounces-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org Errors-To: iommu-bounces-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org To: Peter Xu Cc: Kevin Tian , Joerg Roedel , Ashok Raj , Matthew Wilcox , iommu-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org List-Id: iommu@lists.linux-foundation.org On 2/12/19 11:02 AM, Peter Xu wrote: > On Tue, Feb 12, 2019 at 10:44:23AM +0800, Lu Baolu wrote: >> Hi, >> >> On 2/12/19 3:28 AM, Matthew Wilcox wrote: >>> >>> I'm looking at commit 562831747f6299abd481b5b00bd4fa19d5c8a259 >>> which fails to adequately explain why we can't use PASID 0. Commit >>> af39507305fb83a5d3c475c2851f4d59545d8a18 also doesn't explain why PASID >>> 0 is no longer usable for the intel-svm driver. >> >> Sorry that we didn't make it clear. >> >>> >>> There are a load of simplifications that could be made to this, but I >>> don't know which ones to suggest without a clear understanding of the >>> problem you're actually trying to solve. >>> >> >> PASID 0 has been reserved by Intel IOMMU driver for RID_PASID purpose. >> >> VT-d scalable mode treats all address translation as PASID granularity. >> For DMA requests-with-PASID, the PASID value in the DMA request will be >> used. For DMA requests-without-PASID, VT-d will use a static PASID value >> specified in the RID_PASID field of the context entry. PASID 0 has been >> reserved for this usage for all devices. >> >> (Please refer to 9.4 of the spec 3.0 for more details.) > > Hi, Baolu, Hi Peter, > > I have a similar confusion. > > If PASID==0 is reserved for requests-without-PASID, then does it mean > that for each scalable mode context entry the RID_PASID field will > always be zero? Yes. > Or say, since we already have the per-context-entry > RID_PASID field which seems to be configurable, why we still need to > reserve the PASID==0? We decided to set RID_PASID always to 0. This will make things simple especially for virtual IOMMU case. It will also be compatible with other arch's which reserves PASID 0 for legacy translation. Best regards, Lu Baolu