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

[Kirill A. Shutemov]

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