RE: [PATCH] VT-d: prune super-page related capability macros

["Tian, Kevin" <kevin.tian@xxxxxxxxx>]

> From: Jan Beulich <jbeulich@xxxxxxxx>
> Sent: Friday, November 12, 2021 5:51 PM
> 
> cap_super_page_val() and cap_super_offset() are unused (apart from the
> latter using the former). I don't see how cap_super_offset() can be
> useful in its current shape: cap_super_page_val()'s result is not an
> lvalue and hence can't have its address taken. Plus a user would have
> to check the capability register field is non-zero, for
> find_first_bit() (or find_first_set_bit(), if suitably corrected) to be
> valid in the first place. Yet as per the spec when the field is non-zero
> the low bit would always be set, so the result would be independent of
> the actual value the field holds.
> 
> Further zap cap_sps_512gb() and cap_sps_1tb(). While earlier versions
> of the spec had things spelled out that way, the current version marks
> the two bits as reserved. And "48-bit offset to page frame" wasn't in
> line with 1Tb pages anyway - clearly 256Tb pages would have been meant
> here.
> 
> Finally properly parenthesize parameter uses in the remaining two
> macros.
> 
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

Reviewed-by: Kevin Tian <kevin.tian@xxxxxxxxx>

> 
> --- a/xen/drivers/passthrough/vtd/iommu.h
> +++ b/xen/drivers/passthrough/vtd/iommu.h
> @@ -68,13 +68,8 @@
>  #define cap_num_fault_regs(c)  ((((c) >> 40) & 0xff) + 1)
>  #define cap_pgsel_inv(c)       (((c) >> 39) & 1)
> 
> -#define cap_super_page_val(c)  (((c) >> 34) & 0xf)
> -#define cap_super_offset(c)    (((find_first_bit(&cap_super_page_val(c), 4)) 
> \
> -                                 * OFFSET_STRIDE) + 21)
> -#define cap_sps_2mb(c)         ((c >> 34) & 1)
> -#define cap_sps_1gb(c)         ((c >> 35) & 1)
> -#define cap_sps_512gb(c)       ((c >> 36) & 1)
> -#define cap_sps_1tb(c)         ((c >> 37) & 1)
> +#define cap_sps_2mb(c)         (((c) >> 34) & 1)
> +#define cap_sps_1gb(c)         (((c) >> 35) & 1)
> 
>  #define cap_fault_reg_offset(c)    ((((c) >> 24) & 0x3ff) * 16)
>