Re: [Qemu PATCH v2 9/9] hw/mem/cxl_type3: Add dpa range validation for accesses to dc regions

[Jonathan Cameron via <qemu-devel@nongnu.org>]

> > +
> > +/*
> > + * This function check whether a dpa range [dpa, dpa + len) has been backed
> > + * with dc extents, used when validating read/write to dc regions
> > + */
> > +static bool test_region_block_backed(CXLType3Dev *ct3d, uint64_t dpa,
> > +        uint64_t len)
> > +{
> > +    int i;
> > +    CXLDCD_Region *region = &ct3d->dc.regions[0];
> > +    uint64_t nbits;
> > +    long nr;
> > +
> > +    if (dpa < region->base
> > +            || dpa >= region->base + ct3d->dc.total_capacity)
> > +        return false;
> > +
> > +    /*
> > +     * spec 3.0 9.13.3: Regions are used in increasing-DPA order, with
> > +     * Region 0 being used for the lowest DPA of Dynamic Capacity and
> > +     * Region 7 for the highest DPA.
> > +     * So we check from the last region to find where the dpa belongs.
> > +     * access across multiple regions is not allowed.
> > +     */
> > +    for (i = ct3d->dc.num_regions - 1; i >= 0; i--) {
> > +        region = &ct3d->dc.regions[i];
> > +        if (dpa >= region->base) {
> > +            break;
> > +        }
> > +    }
> > +
> > +    nr = (dpa - region->base) / region->block_size;  
> 
> > +    nbits = len / region->block_size;
oops. Len is probably always smaller than block_size (typically 8 or less)
so nbits always 0.  Should be 1 in those cases.

> > +    return find_next_zero_bit(region->blk_bitmap, nbits, nr) >= nr + nbits;  
> 
> The 2nd parameter to find_next_zero_bit is the length of the bitmap, so 
> shouldn't this be something like (also considering that len is the 
> read/write len, and will be smaller than the region block size):
> 
>    nbits = DIV_ROUND_UP(len, region->block_size);

>  
> 
>    return find_next_zero_bit(region->blk_bitmap, nbits + nr, nr) ==
>           nbits + nr;

Agreed with your suggestion. I'll carry that in my forward port of this
series for now and update my tree at
gitlab.com/jic23/qemu branch will probably be cxl-2023-08-30
a bit later today.

Jonathan