Hi Stanislav,
On 30.08.23 00:07, Stanislav Kinsburskii wrote:
On Mon, Aug 28, 2023 at 10:50:19PM +0200, Alexander Graf wrote:
+kexec, iommu, kvm
On 23.08.23 04:45, Stanislav Kinsburskii wrote:
+akpm, +linux-mm
On Fri, Aug 25, 2023 at 01:32:40PM +0000, Gowans, James wrote:
On Fri, 2023-08-25 at 10:05 +0200, Greg Kroah-Hartman wrote:
Thanks for adding me to this thread Greg!
On Tue, Aug 22, 2023 at 11:34:34AM -0700, Stanislav Kinsburskii wrote:
This patch addresses the need for a memory allocator dedicated to
persistent memory within the kernel. This allocator will preserve
kernel-specific states like DMA passthrough device states, IOMMU state, and
more across kexec.
The proposed solution offers a foundational implementation for potential
custom solutions that might follow. Though the implementation is
intentionally kept concise and straightforward to foster discussion and
feedback, it's fully functional in its current state.
Hi Stanislav, it looks like we're working on similar things. I'm looking
to develop a mechanism to support hypervisor live update for when KVM is
running VMs with PCI device passthrough. VMs with device passthrough
also necessitates passing and re-hydrating IOMMU state so that DMA can
continue during live update.
Planning on having an LPC session on this topic:
https://lpc.events/event/17/abstracts/1629/ (currently it's only a
submitted abstract so not sure if visible, hopefully it will be soon).
We are looking at implementing persistence across kexec via an in-memory
filesystem on top of reserved memory. This would have files for anything
that needs to be persisted. That includes files for IOMMU pgtables, for
guest memory or userspace-accessible memory.
It may be nice to solve all kexec persistence requirements with one
solution, but we can consider IOMMU separately. There are at least three
ways that this can be done:
a) carving out reserved memory for pgtables. This is done by your
proposal here, as well as my suggestion of a filesystem.
b) pre/post kexec hooks for drivers to serialise state and pass it
across in a structured format from old to new kernel.
c) Reconstructing IOMMU state in the new kernel by starting at the
hardware registers and walking the page tables. No state passing needed.
Have you considered option (b) and (c) here? One of the implications of
(b) and (c) are that they would need to hook into the buddy allocator
really early to be able to carve out the reconstructed page tables
before the allocator is used. Similar to how pkram [0] hooks in early to
carve out pages used for its filesystem.
Hi James,
We are indeed working on similar things, so thanks for chiming in.
I've seen pkram proposal as well as your comments there.
I think (b) will need some persistent-over-kexec memory to pass the
state across kexec as well as some key-value store persisted as well.
And the proposed persistent memory pool is aimed exactly for this
purpose.
Or do you imply some other way to pass driver's data accross kexec?
If I had to build this, I'd probably do it just like device tree passing on
ARM. It's a single, physically contiguous blob of data whose entry point you
pass to the target kernel. IIRC ACPI passing works similarly. This would
just be one more opaque data structure that then needs very strict
versioning and forward/backward compat guarantees.
Device tree or ACPI are options indeed. However AFAIU in case of DT user
space has to involved into the picture to modify and complie it, while
ACPI isn't flexible or easily extendable.
Also, AFAIU both these standards were designed with passing
hardware-specific data in mind from bootstrap software to an OS kernel
and thus were never really intended to be used for creating a persistent
state accross kexec.
To me, an attempt to use either of them to pass kernel-specific data looks
like an abuse (or misuse) excused by the simplicity of implementation.
What I was describing above is that the Linux boot protocol already has
natural ways to pass a DT (arm) or set of ACPI tables (x86) to the
target kernel. Whatever we do here should either piggy back on top of
those natural mechanisms (e.g. /chosen node in DT) or be on the same
level (e.g. pass DT in one register, pass metadata structure in another
register).
When it comes to the actual content of the metadata, I'm personally also
leaning towards DT. We already have libfdt inside the kernel. It gives
is a very simple, well understood structured file format that you can
extend, version, etc etc. And the kernel has mechanisms to modify fdt
contents.
I dind't consider (c) yet, thanks for for the pointer.
I have a question in this scope: how is PCI devices registers state is persisted
across kexec with the files system you are working on? I.e. how does
driver know, that the device shouldn't not be reinitialized?
The easiest way to do it initially would be kernel command line options that
hack up the drivers. But I suppose depending on the option we go with, you
can also use the respective "natural" path:
(a) A special metadata file that explains the state to the driver
(b) An entry in the structured file format that explains the state to the
target driver
(c) Compatible target drivers try to enumerate state from the target
device's register file
Command line option is the simplest way to go indeed, but from my POV
it's good only for pointing to a particualr object, which is persisted
somehow else. But it we have a persistence mechanism, then I think we
can make another step forward and don't use command line at all (which
is a bit cumbersome and errorprone due to it's human-readable and
serialized nature).
I'm leaning towards some kind of "natural" path you mentioned... I guess
I'm a bit confused with the word "file" here, as it sounds line it
implies a file system driver, and I'm not sure that's what we want for
driver specific data.
Oh, I was just referring to the set of registers a device exposes :).
Potential applications include:
1. Allowing various in-kernel entities to allocate persistent pages from
a singular memory pool, eliminating the need for multiple region
reservations.
2. For in-kernel components that require the allocation address to be
available on kernel kexec, this address can be exposed to user space and
then passed via the command line.
Do you have specific examples of other state that needs to be passed
across? Trying to see whether tailoring specifically to the IOMMU case
is okay. Conceptually IOMMU state can be reconstructed starting with
hardware registers, not needing reserved memory. Other use-cases may not
have this option.
Well, basically it's IOMMU state and PCI devices to skip/avoid
initializing.
I bet there can be other misc (and unrelated things) like persistent
filesystems, block devices, etc. But I don't have a solid set of use
cases to present.
Would be great if you could think through the problem space until LPC so we
can have a solid conversation there :)
Yeah, I have a few ideas I'll try to implement and share before LPC.
Unfortunatelly I'm not planning to attend it this year, so this
conversation will be without me.
But I'll do my best to provide as much content to discuss as I can.
Thanks, looking forward to it! :)
Alex
Amazon Development Center Germany GmbH
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