Re: [PATCH V3 01/16] machine: anon-alloc option

[Steven Sistare <steven.sistare@oracle.com>]

On 11/6/2024 3:41 PM, Peter Xu wrote:
> On Wed, Nov 06, 2024 at 03:12:20PM -0500, Steven Sistare wrote:
>> On 11/4/2024 4:36 PM, David Hildenbrand wrote:
>>> On 04.11.24 21:56, Steven Sistare wrote:
>>>> On 11/4/2024 3:15 PM, David Hildenbrand wrote:
>>>>> On 04.11.24 20:51, David Hildenbrand wrote:
>>>>>> On 04.11.24 18:38, Steven Sistare wrote:
>>>>>>> On 11/4/2024 5:39 AM, David Hildenbrand wrote:
>>>>>>>> On 01.11.24 14:47, Steve Sistare wrote:
>>>>>>>>> Allocate anonymous memory using mmap MAP_ANON or memfd_create depending
>>>>>>>>> on the value of the anon-alloc machine property.  This option applies to
>>>>>>>>> memory allocated as a side effect of creating various devices. It does
>>>>>>>>> not apply to memory-backend-objects, whether explicitly specified on
>>>>>>>>> the command line, or implicitly created by the -m command line option.
>>>>>>>>>
>>>>>>>>> The memfd option is intended to support new migration modes, in which the
>>>>>>>>> memory region can be transferred in place to a new QEMU process, by sending
>>>>>>>>> the memfd file descriptor to the process.  Memory contents are preserved,
>>>>>>>>> and if the mode also transfers device descriptors, then pages that are
>>>>>>>>> locked in memory for DMA remain locked.  This behavior is a pre-requisite
>>>>>>>>> for supporting vfio, vdpa, and iommufd devices with the new modes.
>>>>>>>>
>>>>>>>> A more portable, non-Linux specific variant of this will be using shm,
>>>>>>>> similar to backends/hostmem-shm.c.
>>>>>>>>
>>>>>>>> Likely we should be using that instead of memfd, or try hiding the
>>>>>>>> details. See below.
>>>>>>>
>>>>>>> For this series I would prefer to use memfd and hide the details.  It's a
>>>>>>> concise (and well tested) solution albeit linux only.  The code you supply
>>>>>>> for posix shm would be a good follow on patch to support other unices.
>>>>>>
>>>>>> Unless there is reason to use memfd we should start with the more
>>>>>> generic POSIX variant that is available even on systems without memfd.
>>>>>> Factoring stuff out as I drafted does look quite compelling.
>>>>>>
>>>>>> I can help with the rework, and send it out separately, so you can focus
>>>>>> on the "machine toggle" as part of this series.
>>>>>>
>>>>>> Of course, if we find out we need the memfd internally instead under
>>>>>> Linux for whatever reason later, we can use that instead.
>>>>>>
>>>>>> But IIUC, the main selling point for memfd are additional features
>>>>>> (hugetlb, memory sealing) that you aren't even using.
>>>>>
>>>>> FWIW, I'm looking into some details, and one difference is that shmem_open() under Linux (glibc) seems to go to /dev/shmem and memfd/SYSV go to the internal tmpfs mount. There is not a big difference, but there can be some difference (e.g., sizing of the /dev/shm mount).
>>>>
>>>> Sizing is a non-trivial difference.  One can by default allocate all memory using memfd_create.
>>>> To do so using shm_open requires configuration on the mount.  One step harder to use.
>>>
>>> Yes.
>>>
>>>>
>>>> This is a real issue for memory-backend-ram, and becomes an issue for the internal RAM
>>>> if memory-backend-ram has hogged all the memory.
>>>>
>>>>> Regarding memory-backend-ram,share=on, I assume we can use memfd if available, but then fallback to shm_open().
>>>>
>>>> Yes, and if that is a good idea, then the same should be done for internal RAM
>>>> -- memfd if available and fallback to shm_open.
>>>
>>> Yes.
>>>
>>>>
>>>>> I'm hoping we can find a way where it just all is rather intuitive, like
>>>>>
>>>>> "default-ram-share=on": behave for internal RAM just like "memory-backend-ram,share=on"
>>>>>
>>>>> "memory-backend-ram,share=on": use whatever mechanism we have to give us "anonymous" memory that can be shared using an fd with another process.
>>>>>
>>>>> Thoughts?
>>>>
>>>> Agreed, though I thought I had already landed at the intuitive specification in my patch.
>>>> The user must explicitly configure memory-backend-* to be usable with CPR, and anon-alloc
>>>> controls everything else.  Now we're just riffing on the details: memfd vs shm_open, spelling
>>>> of options and words to describe them.
>>>
>>> Well, yes, and making it all a bit more consistent and the "machine option" behave just like "memory-backend-ram,share=on".
>>
>> Hi David and Peter,
>>
>> I have implemented and tested the following, for both qemu_memfd_create
>> and qemu_shm_alloc.  This is pseudo-code, with error conditions omitted
>> for simplicity.
> 
> I'm ok with either shm or memfd, as this feature only applies to Linux
> anyway.  I'll leave that part to you and David to decide.
> 
>>
>> Any comments before I submit a complete patch?
>>
>> ----
>> qemu-options.hx:
>>      ``aux-ram-share=on|off``
>>          Allocate auxiliary guest RAM as an anonymous file that is
>>          shareable with an external process.  This option applies to
>>          memory allocated as a side effect of creating various devices.
>>          It does not apply to memory-backend-objects, whether explicitly
>>          specified on the command line, or implicitly created by the -m
>>          command line option.
>>
>>          Some migration modes require aux-ram-share=on.
>>
>> qapi/migration.json:
>>      @cpr-transfer:
>>           ...
>>           Memory-backend objects must have the share=on attribute, but
>>           memory-backend-epc is not supported.  The VM must be started
>>           with the '-machine aux-ram-share=on' option.
>>
>> Define RAM_PRIVATE
>>
>> Define qemu_shm_alloc(), from David's tmp patch
>>
>> ram_backend_memory_alloc()
>>      ram_flags = backend->share ? RAM_SHARED : RAM_PRIVATE;
>>      memory_region_init_ram_flags_nomigrate(ram_flags)
> 
> Looks all good until here.
> 
>>
>> qemu_ram_alloc_internal()
>>      ...
>>      if (!host && !(ram_flags & RAM_PRIVATE) && current_machine->aux_ram_share)
> 
> Nitpick: could rely on flags-only, rather than testing "!host", AFAICT
> that's equal to RAM_PREALLOC.  

IMO testing host is clearer and more future proof, regardless of how flags
are currently used.  If the caller passes host, then we should not allocate
memory here, full stop.

> Meanwhile I slightly prefer we don't touch
> anything if SHARED|PRIVATE is set.  

OK, if SHARED is already set I will not set it again.

> All combined, it could be:
> 
>      if (!(ram_flags & (RAM_PREALLOC | RAM_PRIVATE | RAM_SHARED))) {
>          // ramblock to be allocated, with no share/private request, aka,
>          // aux memory chunk...
>      }
> 
>>          new_block->flags |= RAM_SHARED;
>>
>>      if (!host && (new_block->flags & RAM_SHARED)) {
>>          qemu_ram_alloc_shared(new_block);
> 
> I'm not sure whether this needs its own helper.  

Reserve judgement until you see the full patch.  The helper is a
non-trivial subroutine and IMO it improves readability.  Also the
cpr find/save hooks are confined to the subroutine.

> Should we fallback to
> ram_block_add() below, just like a RAM_SHARED?

I thought we all discussed and agreed that the allocation should be performed
above ram_block_add.  David's suggested patch does it here also.

- Steve

> IIUC, we could start to create RAM_SHARED in qemu_anon_ram_alloc() and
> always cache the fd (even if we don't do that before)?
> 
>>      } else
>>          new_block->fd = -1;
>>          new_block->host = host;
>>      }
>>      ram_block_add(new_block);
>>
>> qemu_ram_alloc_shared()
>>      if qemu_memfd_check()
>>          new_block->fd = qemu_memfd_create()
>>      else
>>          new_block->fd = qemu_shm_alloc()
>>      new_block->host = file_ram_alloc(new_block->fd)
>> ----
>>
>> - Steve
>>
>