Re: [PATCH 2/5] efi/x86: Implement support for unaccepted memory

Kirill A. Shutemov
2021-08-12T21:14:41Z

On Tue, Aug 10, 2021 at 10:50:33AM -0700, Dave Hansen wrote:
> ...
> > +void mark_unaccepted(struct boot_params *params, u64 start, u64 num)
> > +{
>
> Some of these interfaces like accept_memory() take a start/end physical
> address. Having this take a "num pages" is bound to cause confusion.
> Could you make these all consistently take start/end physical addresses?

Okay.

>
> > + u64 end = start + num * PAGE_SIZE;
> > + unsigned int npages;
>
>
> Could you comment those, please?
>
> /*
> * The accepted memory bitmap only works at PMD_SIZE
> * granularity. If a request comes in to mark memory
> * as unaccepted which is not PMD_SIZE-aligned, simply
> * accept the memory now since it can not be *marked* as
> * unaccepted.
> */
>
> Then go on to comment the three cases:
>
> /* Check for ranges which do not span a whole PMD_SIZE area: */

Okay.

> > + if ((start & PMD_MASK) == (end & PMD_MASK)) {
> > + npages = (end - start) / PAGE_SIZE;
> > + __accept_memory(start, start + npages * PAGE_SIZE);
> > + return;
> > + }
>
> Hmm, is it possible to have this case hit, but neither of the two below
> cases? This seems to be looking for a case where the range is somehow
> entirely contained in one PMD_SIZE area, but where it doesn't consume a
> whole area.
>
> Wouldn't that mean that 'start' or 'end' must be unaligned?

The problem is that if both of them unaligned round_up() and round_down()
in the cases below would step outside the requested range.

> > + if (start & ~PMD_MASK) {
> > + npages = (round_up(start, PMD_SIZE) - start) / PAGE_SIZE;
> > + __accept_memory(start, start + npages * PAGE_SIZE);
> > + start = round_up(start, PMD_SIZE);
> > + }
> > +
> > + if (end & ~PMD_MASK) {
> > + npages = (end - round_down(end, PMD_SIZE)) / PAGE_SIZE;
> > + end = round_down(end, PMD_SIZE);
> > + __accept_memory(end, end + npages * PAGE_SIZE);
> > + }
> > + npages = (end - start) / PMD_SIZE;
> > + bitmap_set((unsigned long *)params->unaccepted_memory,
> > + start / PMD_SIZE, npages);
> > +}
>
> Even though it's changed right there, it's a bit cruel to change the
> units of 'npages' right in the middle of a function. It's just asking
> for bugs.
>
> It would only take a single extra variable declaration to make this
> unambiguous:
>
> u64 nr_unaccepted_bits;
>
> or something, then you can do:
>
> nr_unaccepted_bits = (end - start) / PMD_SIZE;
> bitmap_set((unsigned long *)params->unaccepted_memory,
> start / PMD_SIZE, nr_unaccepted_bits);

Okay.

>
> ...
> > static efi_status_t allocate_e820(struct boot_params *params,
> > + struct efi_boot_memmap *map,
> > struct setup_data **e820ext,
> > u32 *e820ext_size)
> > {
> > - unsigned long map_size, desc_size, map_key;
> > efi_status_t status;
> > - __u32 nr_desc, desc_version;
> > -
> > - /* Only need the size of the mem map and size of each mem descriptor */
> > - map_size = 0;
> > - status = efi_bs_call(get_memory_map, &map_size, NULL, &map_key,
> > - &desc_size, &desc_version);
> > - if (status != EFI_BUFFER_TOO_SMALL)
> > - return (status != EFI_SUCCESS) ? status : EFI_UNSUPPORTED;
>
> I noticed that there's no reference to EFI_BUFFER_TOO_SMALL in the hunks
> you added back. That makes me a bit nervous that this is going to
> unintentionally change behavior.
>
> It might be worth having a preparatory reorganization patch for
> allocate_e820() before this new feature is added to make this more clear.

Okay. Will do.
>
> > + __u32 nr_desc;
> > + bool unaccepted_memory_present = false;
> > + u64 max_addr = 0;
> > + int i;
> >
> > - nr_desc = map_size / desc_size + EFI_MMAP_NR_SLACK_SLOTS;
> > + status = efi_get_memory_map(map);
> > + if (status != EFI_SUCCESS)
> > + return status;
> >
> > - if (nr_desc > ARRAY_SIZE(params->e820_table)) {
> > - u32 nr_e820ext = nr_desc - ARRAY_SIZE(params->e820_table);
> > + nr_desc = *map->map_size / *map->desc_size;
> > + if (nr_desc > ARRAY_SIZE(params->e820_table) - EFI_MMAP_NR_SLACK_SLOTS) {
> > + u32 nr_e820ext = nr_desc - ARRAY_SIZE(params->e820_table) -
> > + EFI_MMAP_NR_SLACK_SLOTS;
> >
> > status = alloc_e820ext(nr_e820ext, e820ext, e820ext_size);
> > if (status != EFI_SUCCESS)
> > return status;
> > }
> >
> > + if (!IS_ENABLED(CONFIG_UNACCEPTED_MEMORY))
> > + return EFI_SUCCESS;
> > +
> > + /* Check if there's any unaccepted memory and find the max address */
> > + for (i = 0; i < nr_desc; i++) {
> > + efi_memory_desc_t *d;
> > +
> > + d = efi_early_memdesc_ptr(*map->map, *map->desc_size, i);
> > + if (d->type == EFI_UNACCEPTED_MEMORY)
> > + unaccepted_memory_present = true;
> > + if (d->phys_addr + d->num_pages * PAGE_SIZE > max_addr)
> > + max_addr = d->phys_addr + d->num_pages * PAGE_SIZE;
> > + }
>
> This 'max_addr' variable looks a bit funky.
>
> It *seems* like it's related only to EFI_UNACCEPTED_MEMORY, but it's not
> underneath the EFI_UNACCEPTED_MEMORY check. Is this somehow assuming
> that once unaccepted memory as been found that *all* memory found in
> later descriptors at higher addresses is also going to be unaccepted?

You got it right below :P

> > + /*
> > + * If unaccepted memory present allocate a bitmap to track what memory
> > + * has to be accepted before access.
> > + *
> > + * One bit in the bitmap represents 2MiB in the address space: one 4k
> > + * page is enough to track 64GiB or physical address space.
> > + *
> > + * In the worst case scenario -- a huge hole in the middle of the
> > + * address space -- we would need 256MiB to handle 4PiB of the address
> > + * space.
> > + *
> > + * TODO: handle situation if params->unaccepted_memory has already set.
> > + * It's required to deal with kexec.
> > + */
> > + if (unaccepted_memory_present) {
> > + unsigned long *unaccepted_memory = NULL;
> > + u64 size = DIV_ROUND_UP(max_addr, PMD_SIZE * BITS_PER_BYTE);
>
> Oh, so the bitmap has to be present for *all* memory, not just
> unaccepted memory. So, we really do need to know the 'max_addr' so that
> we can allocate the bitmap for so that can be marked in the bitmap has
> having been accepted.

Right we need a bit for every 2M. Accepted or not.

> > + status = efi_allocate_pages(size,
> > + (unsigned long *)&unaccepted_memory,
> > + ULONG_MAX);
> > + if (status != EFI_SUCCESS)
> > + return status;
> > + memset(unaccepted_memory, 0, size);
> > + params->unaccepted_memory = (u64)unaccepted_memory;
> > + }
>
> It might be nice to refer to setup_e820() here to mention that it is the
> thing that actually fills out the bitmap.

Okay.

--
Kirill A. Shutemov