linux-kernel - Re: Question about domain

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Date:   Fri, 27 Nov 2020 00:48:25 -0700
From:   Jerry Snitselaar <jsnitsel@...hat.com>
To:     Lu Baolu <baolu.lu@...ux.intel.com>
Cc:     Joerg Roedel <joro@...tes.org>, iommu@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org, stable@...nel.vger.org
Subject: Re: Question about domain_init (v5.3-v5.7)


Lu Baolu @ 2020-11-26 19:12 MST:

> Hi Jerry,
>
> On 11/27/20 5:35 AM, Jerry Snitselaar wrote:
>> Lu Baolu @ 2020-11-26 04:01 MST:
>> 
>>> Hi Jerry,
>>>
>>> On 2020/11/26 4:27, Jerry Snitselaar wrote:
>>>> Is there a reason we check the requested guest address width against
>>>> the
>>>> iommu's mgaw, instead of the agaw that we already know for the iommu?
>>>> I've run into a case with a new system where the mgaw reported is 57,
>>>> but if they set PAE to 46 instead of 52 in the bios, then sagaw reports
>>>> the highest supported agaw is 48 and the domain_init code fails here. In
>>>
>>> Isn't this a platform bug? If it's too late to fix it in the BIOS, you
>>> maybe have to add a platform specific quirk to set mgaw to the highest
>>> supported agaw?
>>>
>>> Best regards,
>>> baolu
>> Is there somewhere you can point me to that discusses how they
>> should be
>> setting the mgaw? I misunderstood when I previously asked you about
>> whether the mgaw could be a value that was greater than any of sagaw.
>> If it is a bios issue, then they should fix it there.
>
> MGAW indicates the max gpa width supported by 2nd translation. The VT-d
> spec requires that this value must be at least equal to the host
> physical addressibility. According to this, BIOS is good, right?
>

Yes, the host address width is 46. MGAW reports 57 (56+1), and highest
sagaw bit is for 48.


> For this failure case, domain_init() just wants to find a suitable agaw
> for the private domain. I think it makes sense to check against
> iommu->agaw instead of cap_mgaw.
>
> Best regards,
> baolu
>
>> 
>>>
>>>> other places like prepare_domain_attach_device, the dmar domain agaw
>>>> gets adjusted down to the iommu agaw. The agaw of the iommu gets
>>>> determined based off what is reported for sagaw. I'm wondering if it
>>>> can't instead do:
>>>> ---
>>>>    drivers/iommu/intel-iommu.c | 4 ++--
>>>>    1 file changed, 2 insertions(+), 2 deletions(-)
>>>> diff --git a/drivers/iommu/intel-iommu.c
>>>> b/drivers/iommu/intel-iommu.c
>>>> index 6ca5c92ef2e5..a8e41ec36d9e 100644
>>>> --- a/drivers/iommu/intel-iommu.c
>>>> +++ b/drivers/iommu/intel-iommu.c
>>>> @@ -1862,8 +1862,8 @@ static int domain_init(struct dmar_domain *domain, struct intel_iommu *iommu,
>>>>    	domain_reserve_special_ranges(domain);
>>>>    	/* calculate AGAW */
>>>> -	if (guest_width > cap_mgaw(iommu->cap))
>>>> -	        guest_width = cap_mgaw(iommu->cap);
>>>> +	if (guest_width > agaw_to_width(iommu->agaw))
>>>> +	        guest_width = agaw_to_width(iommu->agaw);
>>>>    	domain->gaw = guest_width;
>>>>    	adjust_width = guestwidth_to_adjustwidth(guest_width);
>>>>    	agaw = width_to_agaw(adjust_width);
>>>> --
>>>> 2.27.0
>>>>
>>>> Thoughts? With the former code the ehci device for the ilo fails when
>>>> trying to get a private domain.
>>>> Thanks,
>>>> Jerry
>>>>
>>