Re: [Xen-devel] [PATCH RESEND v5 4/6] xen/arm: Implement virtual-linear page table for guest p2m mapping in live migration

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

> > > +{
> > > +    int i;
> > > +    dsb();
> > > +    for ( i = d->arch.dirty.second_lvl_start;
> > > +          i < d->arch.dirty.second_lvl_end;
> > > +          ++i )
> > > +    {
> > > +        int k = i % LPAE_ENTRIES;
> > > +        int l = i / LPAE_ENTRIES;
> > > +
> > > +        if ( xen_second[i].bits != d->arch.dirty.second_lvl[l][k].bits )
> > > +        {
> > > +            __write_pte(&xen_second[i], d->arch.dirty.second_lvl[l][k]);
> > > +            __flush_xen_data_tlb_range_va(i << SECOND_SHIFT,
> > > +                                          1 << SECOND_SHIFT);
> > > +        }
> > > +    }
> > > +    dsb();
> > > +    isb();
> > > +}
> > > +
> > > +/* setting up the xen page table for vlpt mapping for domain d */ int
> > > +prepare_vlpt(struct domain *d) {
> > > +    int xen_second_linear_base;
> > > +    int gp2m_start_index, gp2m_end_index;
> > > +    struct p2m_domain *p2m = &d->arch.p2m;
> > > +    struct page_info *second_lvl_page;
> > > +    paddr_t gma_start = 0;
> > > +    paddr_t gma_end = 0;
> > > +    lpae_t *first[2];
> > > +    int i;
> > > +    uint64_t required, avail = VIRT_LIN_P2M_END - VIRT_LIN_P2M_START;
> > > +
> > > +    get_gma_start_end(d, &gma_start, &gma_end);
> > > +    required = (gma_end - gma_start) >> LPAE_SHIFT;
> > > +
> > > +
> > > +    if ( required > avail )
> > 
> > avail is the number of bytes of virtual address space in the linear p2m
> > area.
> > 
> > required is the number of pages which the guest has. Does this comparison
> > correctly account for each page being represented by an 8-byte lpae_t
> > entry?
> 
> Yes, it does. See, LPAE_SHIFT only shifts 9 bits. Shifting PAGE_SHIFT (12 
> bits)
> gives the number of pages (every bit), and 9-bit shift gives the required
> memory for storing third-level PTE. Since this one-shifting of LPAE_SHIFT
> is confusing enough, we can make it explicit by using something like  
> required = ((gma_end - gma_start) >> PAGE_SHIFT ) * (sizeof lpae_t).

I think this is clearer than requiring the reader to have to think about
the effect of shift by N-M and the reasons for it, thanks. The
alternative would be a comment explaining what is going on.

Ian.


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