Re: [PATCH 12/32] x86/boot/e820: Clean up confusing and self-contradictory verbiage around E820 related resource allocations

[Nikolay Borisov <nik.borisov@suse.com>]

On 5/15/25 15:05, Ingo Molnar wrote:
> So the E820 code has a rather confusing area of code at around
> e820__reserve_resources(), which is, by its plain reading,
> rather self-contradictory. For example, the comment explaining
> e820__reserve_resources() claims:
> 
>   - '* Mark E820 reserved areas as busy for the resource manager'
> 
> By 'E820 reserved areas' one can naively conclude that it's
> talking about E820_TYPE_RESERVED areas - while those areas
> are treated in exactly the opposite fashion by do_mark_busy():
> 
>          switch (type) {
>          case E820_TYPE_RESERVED:
>          case E820_TYPE_SOFT_RESERVED:
>          case E820_TYPE_PRAM:
>          case E820_TYPE_PMEM:
>                  return false;
> 
> Ie. E820_TYPE_RESERVED areas are *not* marked busy for the
> resource manager, because E820_TYPE_RESERVED areas are
> device regions that might eventually be claimed by a device driver.
> 
> This type of confusion permeates this whole area of code,
> making it exceedingly difficult to read (for me at least).
> 
> So untangle it bit by bit:
> 
>   - Instead of talking about ambiguous 'reserved areas',
>     talk about 'E820 device address regions' instead,
>     and 'register'/'lock' them.
> 
>   - The do_mark_busy() function is a misnomer as well, because despite
>     its name it 'does' nothing - it only determines what type
>     of resource handling an E820 type should receive from the
>     kernel. Rename it to e820_device_region() and negate its
>     meaning, to avoid the 'busy/reserved' confusion. Because
>     that's what this code is really about: filtering out
>     device regions such as E820_TYPE_RESERVED, E820_TYPE_PRAM,
>     E820_TYPE_PMEM, etc., and allowing them to be claimed
>     by device drivers later on.
> 
>   - All other E820 regions (system regions) are registered and
>     locked early on, before the PCI resource manager does its
>     search for device BAR addresses, etc.
> 
> Also fix this somewhat misleading comment:
> 
> 	/*
> 	 * Try to bump up RAM regions to reasonable boundaries, to
> 	 * avoid stolen RAM:
> 	 */
> 
> and explain that here we register artificial 'gap' resources
> at the end of suspiciously sized RAM regions, as heuristics
> to try to avoid buggy firmware with undeclared 'stolen RAM' regions:
> 
> 	/*
> 	 * Create additional 'gaps' at the end of RAM regions,
> 	 * rounding them up to 64k/1MB/64MB boundaries, should
> 	 * they be weirdly sized, and register extra, locked
> 	 * resource regions for them, to make sure drivers
> 	 * won't claim those addresses.
> 	 *
> 	 * These are basically blind guesses and heuristics to
> 	 * avoid resource conflicts with broken firmware that
> 	 * doesn't properly list 'stolen RAM' as a system region
> 	 * in the E820 map.
> 	 */
> 
> Also improve the printout of this extra resource a bit: make the
> message more unambiguous, and upgrade it from pr_debug() (where
> very few people will see it), to pr_info() (where it will make
> it into the syslog on default distro configs).
> 
> Also fix spelling and improve comment placement.
> 
> No change in functionality intended.
> 
> Signed-off-by: Ingo Molnar <mingo@kernel.org>
> Cc: Andy Shevchenko <andy@kernel.org>
> Cc: Arnd Bergmann <arnd@kernel.org>
> Cc: David Woodhouse <dwmw@amazon.co.uk>
> Cc: H. Peter Anvin <hpa@zytor.com>
> Cc: Kees Cook <keescook@chromium.org>
> Cc: Linus Torvalds <torvalds@linux-foundation.org>
> Cc: Mike Rapoport (Microsoft) <rppt@kernel.org>
> ---
>   arch/x86/kernel/e820.c | 55 +++++++++++++++++++++++++++++++++-----------------
>   1 file changed, 37 insertions(+), 18 deletions(-)
> 
> diff --git a/arch/x86/kernel/e820.c b/arch/x86/kernel/e820.c
> index 5eb0849b492f..c9bb808c4888 100644
> --- a/arch/x86/kernel/e820.c
> +++ b/arch/x86/kernel/e820.c
> @@ -1106,37 +1106,44 @@ static unsigned long __init e820_type_to_iores_desc(struct e820_entry *entry)
>   	}
>   }
>   
> -static bool __init do_mark_busy(enum e820_type type, struct resource *res)
> +/*
> + * We assign one resource entry for each E820 map entry:
> + */
> +static struct resource __initdata *e820_res;
> +
> +/*
> + * Is this a device address region that should not be marked busy?
> + * (Versus system address regions that we register & lock early.)
> + */
> +static bool __init e820_device_region(enum e820_type type, struct resource *res)
>   {
> -	/* this is the legacy bios/dos rom-shadow + mmio region */
> +	/* This is the legacy BIOS/DOS ROM-shadow + MMIO region: */
>   	if (res->start < (1ULL<<20))

nit: While at it, change this to also use SZ_1M define rather than this 
shift.

<snip>