Re: [PATCH RFCv2 15/15] xen/arm: mm: Re-implement setup_frame_table_mappings() with map_pages_to_xen()

[Stefano Stabellini <sstabellini@kernel.org>]

On Sat, 15 May 2021, Julien Grall wrote:
> Hi Stefano,
> 
> On 15/05/2021 01:02, Stefano Stabellini wrote:
> > On Sun, 25 Apr 2021, Julien Grall wrote:
> > > From: Julien Grall <julien.grall@arm.com>
> > > 
> > > Now that map_pages_to_xen() has been extended to support 2MB mappings,
> > > we can replace the create_mappings() call by map_pages_to_xen() call.
> > > 
> > > This has the advantage to remove the different between 32-bit and 64-bit
> > > code.
> > > 
> > > Lastly remove create_mappings() as there is no more callers.
> > > 
> > > Signed-off-by: Julien Grall <julien.grall@arm.com>
> > > Signed-off-by: Julien Grall <jgrall@amazon.com>
> > > 
> > > ---
> > >      Changes in v2:
> > >          - New patch
> > > 
> > >      TODO:
> > >          - Add support for setting the contiguous bit
> > > ---
> > >   xen/arch/arm/mm.c | 64 +++++------------------------------------------
> > >   1 file changed, 6 insertions(+), 58 deletions(-)
> > > 
> > > diff --git a/xen/arch/arm/mm.c b/xen/arch/arm/mm.c
> > > index c49403b687f5..5f8ae029dd6d 100644
> > > --- a/xen/arch/arm/mm.c
> > > +++ b/xen/arch/arm/mm.c
> > > @@ -359,40 +359,6 @@ void clear_fixmap(unsigned map)
> > >       BUG_ON(res != 0);
> > >   }
> > >   -/* Create Xen's mappings of memory.
> > > - * Mapping_size must be either 2MB or 32MB.
> > > - * Base and virt must be mapping_size aligned.
> > > - * Size must be a multiple of mapping_size.
> > > - * second must be a contiguous set of second level page tables
> > > - * covering the region starting at virt_offset. */
> > > -static void __init create_mappings(lpae_t *second,
> > > -                                   unsigned long virt_offset,
> > > -                                   unsigned long base_mfn,
> > > -                                   unsigned long nr_mfns,
> > > -                                   unsigned int mapping_size)
> > > -{
> > > -    unsigned long i, count;
> > > -    const unsigned long granularity = mapping_size >> PAGE_SHIFT;
> > > -    lpae_t pte, *p;
> > > -
> > > -    ASSERT((mapping_size == MB(2)) || (mapping_size == MB(32)));
> > > -    ASSERT(!((virt_offset >> PAGE_SHIFT) % granularity));
> > > -    ASSERT(!(base_mfn % granularity));
> > > -    ASSERT(!(nr_mfns % granularity));
> > > -
> > > -    count = nr_mfns / LPAE_ENTRIES;
> > > -    p = second + second_linear_offset(virt_offset);
> > > -    pte = mfn_to_xen_entry(_mfn(base_mfn), MT_NORMAL);
> > > -    if ( granularity == 16 * LPAE_ENTRIES )
> > > -        pte.pt.contig = 1;  /* These maps are in 16-entry contiguous
> > > chunks. */
> > > -    for ( i = 0; i < count; i++ )
> > > -    {
> > > -        write_pte(p + i, pte);
> > > -        pte.pt.base += 1 << LPAE_SHIFT;
> > > -    }
> > > -    flush_xen_tlb_local();
> > > -}
> > > -
> > >   #ifdef CONFIG_DOMAIN_PAGE
> > >   void *map_domain_page_global(mfn_t mfn)
> > >   {
> > > @@ -850,36 +816,18 @@ void __init setup_frametable_mappings(paddr_t ps,
> > > paddr_t pe)
> > >       unsigned long frametable_size = nr_pdxs * sizeof(struct page_info);
> > >       mfn_t base_mfn;
> > >       const unsigned long mapping_size = frametable_size < MB(32) ? MB(2)
> > > : MB(32);
> > > -#ifdef CONFIG_ARM_64
> > > -    lpae_t *second, pte;
> > > -    unsigned long nr_second;
> > > -    mfn_t second_base;
> > > -    int i;
> > > -#endif
> > > +    int rc;
> > >         frametable_base_pdx = mfn_to_pdx(maddr_to_mfn(ps));
> > >       /* Round up to 2M or 32M boundary, as appropriate. */
> > >       frametable_size = ROUNDUP(frametable_size, mapping_size);
> > >       base_mfn = alloc_boot_pages(frametable_size >> PAGE_SHIFT,
> > > 32<<(20-12));
> > >   -#ifdef CONFIG_ARM_64
> > > -    /* Compute the number of second level pages. */
> > > -    nr_second = ROUNDUP(frametable_size, FIRST_SIZE) >> FIRST_SHIFT;
> > > -    second_base = alloc_boot_pages(nr_second, 1);
> > > -    second = mfn_to_virt(second_base);
> > > -    for ( i = 0; i < nr_second; i++ )
> > > -    {
> > > -        clear_page(mfn_to_virt(mfn_add(second_base, i)));
> > > -        pte = mfn_to_xen_entry(mfn_add(second_base, i), MT_NORMAL);
> > > -        pte.pt.table = 1;
> > > -
> > > write_pte(&xen_first[first_table_offset(FRAMETABLE_VIRT_START)+i], pte);
> > > -    }
> > > -    create_mappings(second, 0, mfn_x(base_mfn), frametable_size >>
> > > PAGE_SHIFT,
> > > -                    mapping_size);
> > > -#else
> > > -    create_mappings(xen_second, FRAMETABLE_VIRT_START, mfn_x(base_mfn),
> > > -                    frametable_size >> PAGE_SHIFT, mapping_size);
> > > -#endif
> > > +    /* XXX: Handle contiguous bit */
> > > +    rc = map_pages_to_xen(FRAMETABLE_VIRT_START, base_mfn,
> > > +                          frametable_size >> PAGE_SHIFT,
> > > PAGE_HYPERVISOR_RW);
> > > +    if ( rc )
> > > +        panic("Unable to setup the frametable mappings.\n");
> > 
> > This is a lot better.
> > 
> > I take that "XXX: Handle contiguous bit" refers to the lack of
> > _PAGE_BLOCK. Why can't we just | _PAGE_BLOCK like in other places?
> 
> I forgot to add _PAGE_BLOCK, however this is unrelated to my comment.
> 
> Currently, the frametable is mapped using 2MB mapping and setting the
> contiguous bit for each entry if the mapping is 32MB aligned.
> 
> _PAGE_BLOCK will only create 2MB mapping but will not set the contiguous bit.
> This will increase the pressure on the TLBs (we would get 16 entry rather than
> 1) if on system where the TLBs can take advantange of it.
> 
> So map_pages_to_xen() needs to gain support for contiguous bit. I haven't yet
> looked at it (hence the RFC state).

I see, thanks for the explanation