linux-kernel - Re: SNP guest crash in memblock with unaccepted memory

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Message-ID: <b037ffeb-bfeb-41a6-b200-d8c57076370f@redhat.com>
Date: Mon, 28 Apr 2025 16:04:50 +0200
From: David Hildenbrand <david@...hat.com>
To: Tom Lendacky <thomas.lendacky@....com>,
 "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, Mike Rapoport
 <rppt@...nel.org>, Vlastimil Babka <vbabka@...e.cz>,
 Mel Gorman <mgorman@...hsingularity.net>,
 "Kalra, Ashish" <ashish.kalra@....com>,
 Rick Edgecombe <rick.p.edgecombe@...el.com>, linux-mm@...ck.org,
 linux-coco@...ts.linux.dev, linux-kernel@...r.kernel.org,
 Michael Roth <michael.roth@....com>
Subject: Re: SNP guest crash in memblock with unaccepted memory

On 27.04.25 17:01, Tom Lendacky wrote:
> Hi Kirill,
> 
> Every now and then I experience an SNP guest boot failure for accessing
> memory that hasn't been accepted. I managed to get a back trace:
> 
>    RIP: 0010:memcpy_orig+0x68/0x130
>    Code: ...
>    RSP: 0000:ffffffff9cc03ce8 EFLAGS: 00010006
>    RAX: ff11001ff83e5000 RBX: 0000000000000000 RCX: fffffffffffff000
>    RDX: 0000000000000bc0 RSI: ffffffff9dba8860 RDI: ff11001ff83e5c00
>    RBP: 0000000000002000 R08: 0000000000000000 R09: 0000000000002000
>    R10: 000000207fffe000 R11: 0000040000000000 R12: ffffffff9d06ef78
>    R13: ff11001ff83e5000 R14: ffffffff9dba7c60 R15: 0000000000000c00
>    memblock_double_array+0xff/0x310
>    memblock_add_range+0x1fb/0x2f0
>    memblock_reserve+0x4f/0xa0
>    memblock_alloc_range_nid+0xac/0x130
>    memblock_alloc_internal+0x53/0xc0
>    memblock_alloc_try_nid+0x3d/0xa0
>    swiotlb_init_remap+0x149/0x2f0
>    mem_init+0xb/0xb0
>    mm_core_init+0x8f/0x350
>    start_kernel+0x17e/0x5d0
>    x86_64_start_reservations+0x14/0x30
>    x86_64_start_kernel+0x92/0xa0
>    secondary_startup_64_no_verify+0x194/0x19b
> 
> I don't know a lot about memblock, but it appears that it needs to
> allocate more memory for it's regions array and returns a range of memory
> that hasn't been accepted. When the memcpy() runs, the SNP guest gets a
> #VC 0x404 because of this.
> 
> Do you think it is as simple as calling accept_memory() on the memory
> range returned from memblock_find_in_range() in memblock_double_array()?

(not Kirill, but replying :) )

Yeah, we seem to be effectively allocating memory from memblock ("from 
ourselves") without considering that memory must be accepted first.

accept_memory() on the new memory (in case of !slab) should be the right 
thing to do.

-- 
Cheers,

David / dhildenb