Re: Why does memblock only refer to E820 table and not EFI Memory Map?

[Dave Young <dyoung@redhat.com>]

Hi,
On 07/20/19 at 03:52pm, Sai Praneeth Prakhya wrote:
> Hi All,
> 
> Disclaimer:
> 1. Please note that this discussion is x86 specific
> 2. Below stated things are my understanding about kernel and I could have
> missed somethings, so please let me know if I understood something wrong.
> 3. I have focused only on memblock here because if I understand correctly,
> memblock is the base that feeds other memory management subsystems in kernel
> (like the buddy allocator).
> 
> On x86 platforms, there are two sources through which kernel learns about
> physical memory in the system namely E820 table and EFI Memory Map. Each table
> describes which regions of system memory is usable by kernel and which regions
> should be preserved (i.e. reserved regions that typically have BIOS code/data)
> so that no other component in the system could read/write to these regions. I
> think they are duplicating the information and hence I have couple of
> questions regarding these
> 
> 1. I see that only E820 table is being consumed by kernel [1] (i.e. memblock
> subsystem in kernel) to distinguish between "usable" vs "reserved" regions.
> Assume someone has called memblock_alloc(), the memblock subsystem would
> service the caller by allocating memory from "usable" regions and it knows
> this *only* from E820 table [2] (it does not check if EFI Memory Map also says
> that this region is usable as well). So, why isn't the kernel taking EFI
> Memory Map into consideration? (I see that it does happen only when
> "add_efi_memmap" kernel command line arg is passed i.e. passing this argument
> updates E820 table based on EFI Memory Map) [3]. The problem I see with
> memblock not taking EFI Memory Map into consideration is that, we are ignoring
> the main purpose for which EFI Memory Map exists.

https://blog.fpmurphy.com/2012/08/uefi-memory-v-e820-memory.html
Probably above blog can explain some background.

> 
> 2. Why doesn't the kernel have "add_efi_memmap" by default? From the commit
> "200001eb140e: x86 boot: only pick up additional EFI memmap if add_efi_memmap
> flag", I didn't understand why the decision was made so. Shouldn't we give
> more preference to EFI Memory map rather than E820 table as it's the latest
> and E820 is legacy?
> 
> 3. Why isn't kernel checking that both the tables E820 table and EFI Memory
> Map are in sync i.e. is there any *possibility* that a buggy BIOS could report
> a region as usable in E820 table and as reserved in EFI Memory Map?
> 
> [1] 
> https://elixir.bootlin.com/linux/latest/source/arch/x86/kernel/setup.c#L1106
> [2] 
> https://elixir.bootlin.com/linux/latest/source/arch/x86/kernel/e820.c#L1265
> [3] 
> https://elixir.bootlin.com/linux/latest/source/arch/x86/platform/efi/efi.c#L129
> 
> Regards,
> Sai
> 

Thanks
Dave