Re: [PATCH v4 13/14] vtd: use a bit field for context_entry

[Jan Beulich <jbeulich@xxxxxxxx>]

On 04.08.2020 15:42, Paul Durrant wrote:
> @@ -208,35 +209,53 @@ struct root_entry {
>      do { (r).ctp = ((val) >> PAGE_SHIFT_4K); } while (0)
>  
>  struct context_entry {
> -    u64 lo;
> -    u64 hi;
> +    union {
> +        __uint128_t val;
> +        struct { uint64_t lo, hi; };
> +        struct {
> +            /* 0 - 63 */
> +            uint64_t p:1;
> +            uint64_t fpd:1;
> +            uint64_t tt:2;
> +            uint64_t reserved0:8;
> +            uint64_t slptp:52;
> +
> +            /* 64 - 127 */
> +            uint64_t aw:3;
> +            uint64_t ignored:4;
> +            uint64_t reserved1:1;
> +            uint64_t did:16;
> +            uint64_t reserved2:40;
> +        };
> +    };
>  };
> -#define ROOT_ENTRY_NR (PAGE_SIZE_4K/sizeof(struct root_entry))
> -#define context_present(c) ((c).lo & 1)
> -#define context_fault_disable(c) (((c).lo >> 1) & 1)
> -#define context_translation_type(c) (((c).lo >> 2) & 3)
> -#define context_address_root(c) ((c).lo & PAGE_MASK_4K)
> -#define context_address_width(c) ((c).hi &  7)
> -#define context_domain_id(c) (((c).hi >> 8) & ((1 << 16) - 1))
> -
> -#define context_set_present(c) do {(c).lo |= 1;} while(0)
> -#define context_clear_present(c) do {(c).lo &= ~1;} while(0)
> -#define context_set_fault_enable(c) \
> -    do {(c).lo &= (((u64)-1) << 2) | 1;} while(0)
> -
> -#define context_set_translation_type(c, val) do { \
> -        (c).lo &= (((u64)-1) << 4) | 3; \
> -        (c).lo |= (val & 3) << 2; \
> -    } while(0)
> +
> +#define context_present(c) (c).p
> +#define context_set_present(c) do { (c).p = 1; } while (0)
> +#define context_clear_present(c) do { (c).p = 0; } while (0)
> +
> +#define context_fault_disable(c) (c).fpd
> +#define context_set_fault_enable(c) do { (c).fpd = 1; } while (0)
> +
> +#define context_translation_type(c) (c).tt
> +#define context_set_translation_type(c, val) do { (c).tt = val; } while (0)
>  #define CONTEXT_TT_MULTI_LEVEL 0
>  #define CONTEXT_TT_DEV_IOTLB   1
>  #define CONTEXT_TT_PASS_THRU   2
>  
> -#define context_set_address_root(c, val) \
> -    do {(c).lo &= 0xfff; (c).lo |= (val) & PAGE_MASK_4K ;} while(0)
> +#define context_slptp(c) ((c).slptp << PAGE_SHIFT_4K)
> +#define context_set_slptp(c, val) \
> +    do { (c).slptp = (val) >> PAGE_SHIFT_4K; } while (0)

Presumably "slptp" is in line with the doc, but "address_root" is
quite a bit more readable. I wonder if I could talk you into
restoring the old (or some similar) names.

More generally, and more so here than perhaps already on the previous
patch - are these helper macros useful to have anymore?

> +#define context_address_width(c) (c).aw
>  #define context_set_address_width(c, val) \
> -    do {(c).hi &= 0xfffffff8; (c).hi |= (val) & 7;} while(0)
> -#define context_clear_entry(c) do {(c).lo = 0; (c).hi = 0;} while(0)
> +    do { (c).aw = (val); } while (0)
> +
> +#define context_did(c) (c).did
> +#define context_set_did(c, val) \
> +    do { (c).did = (val); } while (0)
> +
> +#define context_clear_entry(c) do { (c).val = 0; } while (0)

While this is in line with previous code, I'm concerned:
domain_context_unmap_one() has

    context_clear_present(*context);
    context_clear_entry(*context);

No barrier means no guarantee of ordering. I'd drop clear_present()
here and make clear_entry() properly ordered. This, I think, will at
the same time render the __uint128_t field unused and hence
unnecessary again.

Also comments given on the previous patch apply respectively here.

Jan