Re: [Xen-devel] [PATCH for-4.12] amd/iommu: fix present bit checking when clearing PTE

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----Original Message-----
> From: Roger Pau Monne [mailto:roger.pau@xxxxxxxxxx]
> Sent: 23 January 2019 09:48
> To: xen-devel@xxxxxxxxxxxxxxxxxxxx
> Cc: Roger Pau Monne <roger.pau@xxxxxxxxxx>; Suravee Suthikulpanit
> <suravee.suthikulpanit@xxxxxxx>; Brian Woods <brian.woods@xxxxxxx>;
> Juergen Gross <jgross@xxxxxxxx>; Paul Durrant <Paul.Durrant@xxxxxxxxxx>
> Subject: [PATCH for-4.12] amd/iommu: fix present bit checking when
> clearing PTE
> 
> The current check for the present bit is wrong, since the present bit
> is located in the low part of the entry.
> 
> Fixes: e8afe1124cc1 ("iommu: elide flushing for higher order map/unmap
> operations")
> Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>

No idea how I managed to get that wrong. I had a bad feeling at the time about 
not accessing the pte as a uint32_t[2] array.

Reviewed-by: Paul Durrant <paul.durrant@xxxxxxxxxx>

> ---
> Cc: Suravee Suthikulpanit <suravee.suthikulpanit@xxxxxxx>
> Cc: Brian Woods <brian.woods@xxxxxxx>
> Cc: Juergen Gross <jgross@xxxxxxxx>
> Cc: Paul Durrant <paul.durrant@xxxxxxxxxx>
> ---
>  xen/drivers/passthrough/amd/iommu_map.c | 4 +---
>  1 file changed, 1 insertion(+), 3 deletions(-)
> 
> diff --git a/xen/drivers/passthrough/amd/iommu_map.c
> b/xen/drivers/passthrough/amd/iommu_map.c
> index 99ac0a6862..67329b0c95 100644
> --- a/xen/drivers/passthrough/amd/iommu_map.c
> +++ b/xen/drivers/passthrough/amd/iommu_map.c
> @@ -39,15 +39,13 @@ static unsigned int clear_iommu_pte_present(unsigned
> long l1_mfn,
>                                              unsigned long dfn)
>  {
>      uint64_t *table, *pte;
> -    uint32_t entry;
>      unsigned int flush_flags;
> 
>      table = map_domain_page(_mfn(l1_mfn));
> 
>      pte = (table + pfn_to_pde_idx(dfn, 1));
> -    entry = *pte >> 32;
> 
> -    flush_flags = get_field_from_reg_u32(entry, IOMMU_PTE_PRESENT_MASK,
> +    flush_flags = get_field_from_reg_u32(*pte, IOMMU_PTE_PRESENT_MASK,
>                                           IOMMU_PTE_PRESENT_SHIFT) ?
>                                           IOMMU_FLUSHF_modified : 0;
> 
> --
> 2.20.1

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