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. Meanwhile I slightly prefer we don't touch
anything if SHARED|PRIVATE is set. 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. Should we fallback to
ram_block_add() below, just like a RAM_SHARED?
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
>
--
Peter Xu
