Re: [Xen-devel] [PATCH 10 of 16] amd iommu: Enable FC bit in iommu host level PTE

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 03.01.12 at 11:05, Wei Wang2 <wei.wang2@xxxxxxx> wrote:
> On Monday 02 January 2012 12:36:08 Jan Beulich wrote:
>> >>> On 23.12.11 at 12:29, Wei Wang <wei.wang2@xxxxxxx> wrote:
>> >
>> > # HG changeset patch
>> > # User Wei Wang <wei.wang2@xxxxxxx>
>> > # Date 1324569401 -3600
>> > # Node ID 30b1f434160d989be5e0bb6c6956bb7e3985db59
>> > # Parent  dd808bdd61c581b041d5b7e816b18674de51da6f
>> > amd iommu: Enable FC bit in iommu host level PTE
>> >
>> > Signed-off-by: Wei Wang <wei.wang2@xxxxxxx>
>> >
>> > diff -r dd808bdd61c5 -r 30b1f434160d
>> > xen/drivers/passthrough/amd/iommu_map.c ---
>> > a/xen/drivers/passthrough/amd/iommu_map.c  Thu Dec 22 16:56:38 2011 +0100
>> > +++ b/xen/drivers/passthrough/amd/iommu_map.c      Thu Dec 22 16:56:41 2011
>> > +0100 @@ -83,6 +83,11 @@ static bool_t set_iommu_pde_present(u32
>> >      set_field_in_reg_u32(ir, entry,
>> >                           IOMMU_PDE_IO_READ_PERMISSION_MASK,
>> >                           IOMMU_PDE_IO_READ_PERMISSION_SHIFT, &entry);
>> > +
>> > +    /* IOMMUv2 needs FC bit enabled  */
>>
>> This comment suggests that the patches prior to that aren't consistent.
>> Is this really a proper standalone patch, or is the word "needs" too
>> strict, or should it really be moved ahead in the series?
>>
>> > +    if ( next_level == IOMMU_PAGING_MODE_LEVEL_0 )
>> > +        set_field_in_reg_u32(IOMMU_CONTROL_ENABLED, entry,
>> > +                             IOMMU_PTE_FC_MASK, IOMMU_PTE_FC_SHIFT,
>> > &entry);
>>
>> This is being done no matter whether it actually is a v2 IOMMU that
>> you deal with here - if that's correct, the comment above should be
>> adjusted accordingly.
> 
> This bit forces pci-defined no snoop bit to be cleared. This helps to solve 
> potential issues in ATS devices with early drivers. I did not see any breaks 
> 
> on legacy devices and iommuv1 with FC = 1. But if you like I could make this 
> 
> only for v2 or change the comment a bit.

Whatever is the most appropriate thing to do here (you definitely
know better than I do).

Jan


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