Date:   Tue, 10 Aug 2021 10:50:33 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     "Kirill A. Shutemov" <kirill@...temov.name>,
        Borislav Petkov <bp@...en8.de>,
        Andy Lutomirski <luto@...nel.org>,
        Sean Christopherson <seanjc@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Joerg Roedel <jroedel@...e.de>
Cc:     Andi Kleen <ak@...ux.intel.com>,
        Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        David Rientjes <rientjes@...gle.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Tom Lendacky <thomas.lendacky@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Ingo Molnar <mingo@...hat.com>,
        Varad Gautam <varad.gautam@...e.com>,
        Dario Faggioli <dfaggioli@...e.com>, x86@...nel.org,
        linux-mm@...ck.org, linux-coco@...ts.linux.dev,
        linux-kernel@...r.kernel.org,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: [PATCH 2/5] efi/x86: Implement support for unaccepted memory

...
> +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?

> +	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: */

> +	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?


> +	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);

...
>  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.

> +	__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?

> +	/*
> +	 * 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.

> +		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.

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