Re: [PATCH v2 4/4] x86/shadow: refactor shadow_vram_{get,put}_l1e()

["Roger Pau Monné" <roger.pau@citrix.com>]

On Thu, Oct 08, 2020 at 04:36:47PM +0100, Andrew Cooper wrote:
> On 08/10/2020 16:15, Roger Pau Monné wrote:
> > On Wed, Sep 16, 2020 at 03:08:40PM +0200, Jan Beulich wrote:
> >> By passing the functions an MFN and flags, only a single instance of
> >                            ^ a
> 
> 'an' is correct.
> 
> an MFN
> a Machine Frame Number
> 
> because the pronunciation changes.  "an" precedes anything with a vowel
> sound, not just vowels themselves.  (Isn't English great...)

Oh great, I think I've been misspelling this myself for a long time.

> >> each is needed; they were pretty large for being inline functions
> >> anyway.
> >>
> >> While moving the code, also adjust coding style and add const where
> >> sensible / possible.
> >>
> >> Signed-off-by: Jan Beulich <jbeulich@suse.com>
> >> ---
> >> v2: New.
> >>
> >> --- a/xen/arch/x86/mm/shadow/hvm.c
> >> +++ b/xen/arch/x86/mm/shadow/hvm.c
> >> @@ -903,6 +903,104 @@ int shadow_track_dirty_vram(struct domai
> >>      return rc;
> >>  }
> >>  
> >> +void shadow_vram_get_mfn(mfn_t mfn, unsigned int l1f,
> >> +                         mfn_t sl1mfn, const void *sl1e,
> >> +                         const struct domain *d)
> >> +{
> >> +    unsigned long gfn;
> >> +    struct sh_dirty_vram *dirty_vram = d->arch.hvm.dirty_vram;
> >> +
> >> +    ASSERT(is_hvm_domain(d));
> >> +
> >> +    if ( !dirty_vram /* tracking disabled? */ ||
> >> +         !(l1f & _PAGE_RW) /* read-only mapping? */ ||
> >> +         !mfn_valid(mfn) /* mfn can be invalid in mmio_direct */)
> >> +        return;
> >> +
> >> +    gfn = gfn_x(mfn_to_gfn(d, mfn));
> >> +    /* Page sharing not supported on shadow PTs */
> >> +    BUG_ON(SHARED_M2P(gfn));
> >> +
> >> +    if ( (gfn >= dirty_vram->begin_pfn) && (gfn < dirty_vram->end_pfn) )
> >> +    {
> >> +        unsigned long i = gfn - dirty_vram->begin_pfn;
> >> +        const struct page_info *page = mfn_to_page(mfn);
> >> +
> >> +        if ( (page->u.inuse.type_info & PGT_count_mask) == 1 )
> >> +            /* Initial guest reference, record it */
> >> +            dirty_vram->sl1ma[i] = mfn_to_maddr(sl1mfn) |
> >> +                                   PAGE_OFFSET(sl1e);
> >> +    }
> >> +}
> >> +
> >> +void shadow_vram_put_mfn(mfn_t mfn, unsigned int l1f,
> >> +                         mfn_t sl1mfn, const void *sl1e,
> >> +                         const struct domain *d)
> >> +{
> >> +    unsigned long gfn;
> >> +    struct sh_dirty_vram *dirty_vram = d->arch.hvm.dirty_vram;
> >> +
> >> +    ASSERT(is_hvm_domain(d));
> >> +
> >> +    if ( !dirty_vram /* tracking disabled? */ ||
> >> +         !(l1f & _PAGE_RW) /* read-only mapping? */ ||
> >> +         !mfn_valid(mfn) /* mfn can be invalid in mmio_direct */)
> >> +        return;
> >> +
> >> +    gfn = gfn_x(mfn_to_gfn(d, mfn));
> >> +    /* Page sharing not supported on shadow PTs */
> >> +    BUG_ON(SHARED_M2P(gfn));
> >> +
> >> +    if ( (gfn >= dirty_vram->begin_pfn) && (gfn < dirty_vram->end_pfn) )
> >> +    {
> >> +        unsigned long i = gfn - dirty_vram->begin_pfn;
> >> +        const struct page_info *page = mfn_to_page(mfn);
> >> +        bool dirty = false;
> >> +        paddr_t sl1ma = mfn_to_maddr(sl1mfn) | PAGE_OFFSET(sl1e);
> >> +
> >> +        if ( (page->u.inuse.type_info & PGT_count_mask) == 1 )
> >> +        {
> >> +            /* Last reference */
> >> +            if ( dirty_vram->sl1ma[i] == INVALID_PADDR )
> >> +            {
> >> +                /* We didn't know it was that one, let's say it is dirty */
> >> +                dirty = true;
> >> +            }
> >> +            else
> >> +            {
> >> +                ASSERT(dirty_vram->sl1ma[i] == sl1ma);
> >> +                dirty_vram->sl1ma[i] = INVALID_PADDR;
> >> +                if ( l1f & _PAGE_DIRTY )
> >> +                    dirty = true;
> >> +            }
> >> +        }
> >> +        else
> >> +        {
> >> +            /* We had more than one reference, just consider the page dirty. */
> >> +            dirty = true;
> >> +            /* Check that it's not the one we recorded. */
> >> +            if ( dirty_vram->sl1ma[i] == sl1ma )
> >> +            {
> >> +                /* Too bad, we remembered the wrong one... */
> >> +                dirty_vram->sl1ma[i] = INVALID_PADDR;
> >> +            }
> >> +            else
> >> +            {
> >> +                /*
> >> +                 * Ok, our recorded sl1e is still pointing to this page, let's
> >> +                 * just hope it will remain.
> >> +                 */
> >> +            }
> >> +        }
> >> +
> >> +        if ( dirty )
> >> +        {
> >> +            dirty_vram->dirty_bitmap[i / 8] |= 1 << (i % 8);
> > Could you use _set_bit here?
> 
> __set_bit() uses 4-byte accesses.  This uses 1-byte accesses.

Right, this is allocated using alloc directly, not the bitmap helper,
and the size if rounded to byte level, not unsigned int.

> Last I checked, there is a boundary issue at the end of the dirty_bitmap.
> 
> Both Julien and I have considered changing our bit infrastructure to use
> byte accesses, which would make them more generally useful.

Does indeed seem useful to me, as we could safely expand the usage of
the bitmap ops without risking introducing bugs.

Thanks, Roger.