Re: [PATCH] x86/unaccepted_memory: Do not accept ranges that are no longer E820 RAM

["Gupta, Pankaj" <pankaj.gupta@amd.com>]

Hi Ard,

> Hello Pankaj,
>
> On Mon, 2 Nov 2026, at 12:31, Pankaj Gupta wrote:
>> The EFI stub builds the unaccepted memory bitmap before the kernel parses
>> its command line.  Option memmap=<size>$<addr> removes RAM from the E820
>> map while its bits are still set in the bitmap.
>>
>> The removed range, such as a memmap=...$... hole, is then still treated as
>> unaccepted RAM and accept_memory() tries to accept it.  The hole is not
>> mapped, so on SEV-SNP the PVALIDATE faults and the guest crashes early in
>> the boot:
>>
>>    BUG: unable to handle page fault for address: ffff8a14bfe00000
>>    RIP: 0010:pvalidate_pages+0xc0/0x250
>>     set_pages_state+0x41a/0x440
>>     snp_accept_memory+0x3e/0x50
>>     accept_memory+0x186/0x260
>>     __free_pages_core+0xd2/0x240
>>     memblock_free_pages+0x1b/0x30
>>     memblock_free_all+0x275/0x2d0
>>
>> This was seen with memmap=2M$0x1ffe00000, a 2 MiB hole in RAM, on a 19 GiB
>> SNP guest with lazy acceptance.
>>
>> Fixes: 2053bc57f367 ("efi: Add unaccepted memory support")
>> Signed-off-by: Pankaj Gupta <pankaj.gupta@amd.com>
>> ---
>>   arch/x86/include/asm/unaccepted_memory.h | 23 ++++++++++++++++++++++-
>>   1 file changed, 22 insertions(+), 1 deletion(-)
>>
> Can we just disallow memmap= when running in a confidential VM?

My experiments showed: memmap itself don't have a problem but memap with 
a hole is causing crash.

There might be some usecases which are already using this, e.g:

https://lore.kernel.org/all/20261029-cma-multi-v1-0-2cbced8b5cb7@kernel.org/

Thanks,

Pankaj


>