linux-kernel - Re: [PATCH v2 3/7] iommu/vt-d: Expose ISA direct mapping region via iommu_get_resv

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Message-Id: <6C211BF1-B5A0-4821-AB42-092B573DE667@arista.com>
Date:   Thu, 28 Mar 2019 18:37:31 +0000
From:   James Sewart <jamessewart@...sta.com>
To:     Lu Baolu <baolu.lu@...ux.intel.com>
Cc:     iommu@...ts.linux-foundation.org, Tom Murphy <tmurphy@...sta.com>,
        Dmitry Safonov <dima@...sta.com>,
        Jacob Pan <jacob.jun.pan@...ux.intel.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 3/7] iommu/vt-d: Expose ISA direct mapping region via
 iommu_get_resv_regions

Hey Lu,

> On 26 Mar 2019, at 01:24, Lu Baolu <baolu.lu@...ux.intel.com> wrote:
> 
> Hi James,
> 
> On 3/25/19 8:57 PM, James Sewart wrote:
>>>> Theres an issue that if we choose to alloc a new resv_region with type
>>>> IOMMU_RESV_DIRECT, we will need to refactor intel_iommu_put_resv_regions
>>>> to free this entry type which means refactoring the rmrr regions in
>>>> get_resv_regions. Should this work be in this patchset?
>>> Do you mean the rmrr regions are not allocated in get_resv_regions, but
>>> are freed in put_resv_regions? I think we should fix this in this patch
>>> set since this might impact the device passthrough if we don't do it.
>> They’re not allocated and not freed currently, only type IOMMU_RESV_MSI is
>> freed in put_resv_regions. If we allocate a new resv_region with type
>> IOMMU_RESV_DIRECT for the isa region, then it won’t be freed. If we modify
>> put_resv_regions to free type IOMMU_RESV_DIRECT, then we will try to free
>> the static RMRR regions.
>> Either the ISA region is static and not freed as with my implementation,
>> or the RMRR regions are converted to be allocated on each call to
>> get_resv_regions and freed in put_resv_regions.
> 
> By the way, there's another way in my mind. Let's add a new region type
> for LPC devices, e.x. IOMMU_RESV_LPC, and then handle it in the same way
> as those MSI regions. Just FYI.

This solution would require adding some extra code to 
iommu_group_create_direct_mappings as currently only type 
IOMMU_RESV_DIRECT is identity mapped, other types are only reserved.


> 
> Best regards,
> Lu Baolu

Cheers,
James.