Re: [PATCH v4 12/14] vtd: use a bit field for root_entry

[Jan Beulich <jbeulich@xxxxxxxx>]

On 04.08.2020 15:42, Paul Durrant wrote:
> --- a/xen/drivers/passthrough/vtd/iommu.h
> +++ b/xen/drivers/passthrough/vtd/iommu.h
> @@ -184,21 +184,28 @@
>  #define dma_frcd_source_id(c) (c & 0xffff)
>  #define dma_frcd_page_addr(d) (d & (((u64)-1) << 12)) /* low 64 bit */
>  
> -/*
> - * 0: Present
> - * 1-11: Reserved
> - * 12-63: Context Ptr (12 - (haw-1))
> - * 64-127: Reserved
> - */
>  struct root_entry {
> -    u64    val;
> -    u64    rsvd1;
> +    union {
> +        __uint128_t val;

I couldn't find a use of this field, and I also can't foresee any.
Could it be left out?

> +        struct { uint64_t lo, hi; };
> +        struct {
> +            /* 0 - 63 */
> +            uint64_t p:1;

bool?

> +            uint64_t reserved0:11;
> +            uint64_t ctp:52;
> +
> +            /* 64 - 127 */
> +            uint64_t reserved1;
> +        };
> +    };
>  };
> -#define root_present(root)    ((root).val & 1)
> -#define set_root_present(root) do {(root).val |= 1;} while(0)
> -#define get_context_addr(root) ((root).val & PAGE_MASK_4K)
> -#define set_root_value(root, value) \
> -    do {(root).val |= ((value) & PAGE_MASK_4K);} while(0)
> +
> +#define root_present(r) (r).p
> +#define set_root_present(r) do { (r).p = 1; } while (0)

And then "true" here?

> +#define root_ctp(r) ((r).ctp << PAGE_SHIFT_4K)
> +#define set_root_ctp(r, val) \
> +    do { (r).ctp = ((val) >> PAGE_SHIFT_4K); } while (0)

For documentation purposes, can the 2nd macro param be named maddr
or some such?

> --- a/xen/drivers/passthrough/vtd/x86/ats.c
> +++ b/xen/drivers/passthrough/vtd/x86/ats.c
> @@ -74,8 +74,8 @@ int ats_device(const struct pci_dev *pdev, const struct 
> acpi_drhd_unit *drhd)
>  static bool device_in_domain(const struct vtd_iommu *iommu,
>                               const struct pci_dev *pdev, uint16_t did)
>  {
> -    struct root_entry *root_entry;
> -    struct context_entry *ctxt_entry = NULL;
> +    struct root_entry *root_entry, *root_entries = NULL;
> +    struct context_entry *context_entry, *context_entries = NULL;

Just like root_entry, root_entries doesn't look to need an initializer.
I'm unconvinced anyway that you now need two variables each:
unmap_vtd_domain_page() does quite fine with the low 12 bits not all
being zero, afaict.

Jan