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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

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...)

>> 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@xxxxxxxx>
>> ---
>> 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.

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.

~Andrew