Re: [Qemu-devel] subpages with memory region aliases

[Avi Kivity <avi@redhat.com>]

On 02/09/2012 04:23 PM, Jan Kiszka wrote:
> On 2012-02-09 10:32, Avi Kivity wrote:
> > On 02/09/2012 10:35 AM, Jan Kiszka wrote:
> >> Avi,
> >>
> >> Before I forget:
> >>
> >> On 2012-02-05 13:39, Jan Kiszka wrote:
> >>> +static void vapic_map_rom_writable(VAPICROMState *s)
> >>> +{
> >>> +    target_phys_addr_t rom_paddr = s->rom_state_paddr & ROM_BLOCK_MASK;
> >>> +    MemoryRegionSection section;
> >>> +    MemoryRegion *as;
> >>> +    size_t rom_size;
> >>> +    uint8_t *ram;
> >>> +
> >>> +    as = sysbus_address_space(&s->busdev);
> >>> +
> >>> +    if (s->rom_mapped_writable) {
> >>> +        memory_region_del_subregion(as, &s->rom);
> >>> +        memory_region_destroy(&s->rom);
> >>> +    }
> >>> +
> >>> +    /* grab RAM memory region (region @rom_paddr may still be pc.rom) */
> >>> +    section = memory_region_find(as, 0, 1);
> >>> +
> >>> +    /* read ROM size from RAM region */
> >>> +    ram = memory_region_get_ram_ptr(section.mr);
> >>> +    rom_size = ram[rom_paddr + 2] * ROM_BLOCK_SIZE;
> >>> +    s->rom_size = rom_size;
> >>> +
> >>> +    /* FIXME: round up as everything underneath would fall apart otherwise
> >>> +     * (subpages are broken) */
> >>> +    rom_size = TARGET_PAGE_ALIGN(rom_size);
> >>
> >> Removing this alignment triggers an interesting bug in the memory layer.
> >> Haven't understood the details yet. Is subpage support supposed to work?
> >>
> >>
> > 
> > There are plenty of restrictions wrt page size in the memory API.  Some
> > will be removed, some are enforced by hardware (for example the low bits
> > of the guest address and host address must match).
> > 
> > Subpage of course works, but it can't be direct mapped.
> > 
>
> Does this mean you can't use them with RAM backing? Should probably be
> documented and caught gracefully in that case.

Since a few monthss ago, you can (56384e8b).  But it will be handled as
mmio, of course.

-- 
error compiling committee.c: too many arguments to function