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

[Paul Durrant <xadimgnik@gmail.com>]

> -----Original Message-----
> From: Jan Beulich <jbeulich@suse.com>
> Sent: 06 August 2020 13:47
> To: Paul Durrant <paul@xen.org>
> Cc: xen-devel@lists.xenproject.org; Paul Durrant <pdurrant@amazon.com>; Kevin Tian
> <kevin.tian@intel.com>
> Subject: Re: [PATCH v4 13/14] vtd: use a bit field for context_entry
> 
> 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.

The problem with 'root' in the VT-d code is that it is ambiguous between this case and manipulations of 'root entries', which is why I moved away from it. The spec refers to 'slptptr' but I shortened it to 'slptp' for consistency with the root 'ctp'... I should really use the name from the spec. though.
I will add a comment above the macro stating what the 'slptptr' is too. 

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

Less useful. I was worried about ditching them causing the patches to balloon in size but maybe they won't... I'll see.

> > +#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.

I'd prefer to keep both with a barrier, particularly if I get rid of the macros.

  Paul

> 
> Also comments given on the previous patch apply respectively here.
> 
> Jan