Re: SNP guest crash in memblock with unaccepted memory

[Mike Rapoport <rppt@kernel.org>]

On Mon, Apr 28, 2025 at 11:00:36PM +0200, David Hildenbrand wrote:
> On 28.04.25 20:10, Tom Lendacky wrote:
> > On 4/28/25 09:04, David Hildenbrand wrote:
> > > 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.
> > 
> > Thanks, David. Let me add a call in for accept_memory in the !slab case
> > and see if that resolves it. May take a bit to repro, but should find
> > out eventually.
> > 
> > I'll submit a patch once I verify.
> 
> BTW, I was wondering if we could use memblock_alloc_range_nid() in
> memblock_double_array(); maybe not that easy, just a thought.

Not easy at all for memblock.reserved, memblock_double_array() makes sure
to avoid memory that's being reserved in this call chain:

memblock_alloc_range_nid()
	memblock_reserve()
		memblock_add_range()
			memblock_double_array()

 
> -- 
> Cheers,
> 
> David / dhildenb
> 

-- 
Sincerely yours,
Mike.