On 10.12.24 22:51, Luo, Zhigang wrote:
[AMD Official Use Only - AMD Internal Distribution Only]
-----Original Message-----
From: David Hildenbrand <da...@redhat.com>
Sent: Tuesday, December 10, 2024 2:55 PM
To: Luo, Zhigang <zhigang....@amd.com>; qemu-devel@nongnu.org
Cc: kra...@redhat.com; Igor Mammedov <imamm...@redhat.com>
Subject: Re: [PATCH] hostmem-file: add the 'hmem' option
On 10.12.24 20:32, Luo, Zhigang wrote:
[AMD Official Use Only - AMD Internal Distribution Only]
Hi David,
Hi,
Thanks for your comments.
Let me give you some background for this patch.
I am currently engaged in a project that requires to pass the
EFI_MEMORY_SP
(Special Purpose Memory) type memory from host to a virtual machine
within QEMU. This memory needs to be EFI_MEMORY_SP type in the
virtual machine as well.
This particular memory type is essential for the functionality of my project.
Which exact guest memory will be backed by this memory? All guest-memory?
[Luo, Zhigang] not all guest-memory. Only the memory reserved for specific
device.
Can you show me an example QEMU cmdline, and how you would pass that
hostmem-file object to the device?
[Luo, Zhigang] the following is an example. m1 is the reserved memory for pci device
"0000:03:00.0". both the memory and pci device are set to same numa node.
-object memory-backend-ram,size=8G,id=m0 \
-object
memory-backend-file,size=16G,id=m1,mem-path=/dev/dax0.0,prealloc=on,align=1G,hmem=on
\
-numa node,nodeid=0,memdev=m0 -numa node,nodeid=1,memdev=m1 \
Okay, so you expose this memory as a second numa node, and want the
guest to identify the second numa node as SP to not use it during boot.
Let me CC Jonathan, I am pretty sure he has an idea what to do here.
-device pxb-pcie,id=pcie.1,numa_node=1,bus_nr=2,bus=pcie.0 \
-device ioh3420,id=pcie_port1,bus=pcie.1,chassis=1 \
-device vfio-pci,host=0000:03:00.0,id=hostdev0,bus=pcie_port1
And, what is the guest OS going to do with this memory?
[Luo, Zhigang] the device driver in guest will use this reserved memory.
Okay, so just like CXL memory.
Usually, this SP memory (dax, cxl, ...) is not used as boot memory.
Like on a bare metal system, one would expect that only CXL memory
will be marked as special and put aside to the cxl driver, such that
the OS can boot on ordinary DIMMs, such that cxl can online it etc.
So maybe you would want to expose this memory using CXL-mem device to
the VM? Or a DIMM?
I assume the alternative is to tell the VM on the Linux kernel
cmdline to set EFI_MEMORY_SP on this memory. I recall that there is a way to
achieve that.
[Luo, Zhigang] I know this option. but it requires the end user to know where
is the
memory location in guest side(start address, size).
Right.
In Linux, the SPM memory will be claimed by hmem-dax driver by
default. With
this patch I can use the following config to pass the SPM memory to guest VM.
-object
memory-backend-file,size=30G,id=m1,mem-path=/dev/dax0.0,prealloc=on,
al
ign=1G,hmem=on
I was thinking to change the option name from "hmem" to "spm" to
avoid
confusion.
Likely it should be specified elsewhere, that you want specific guest
RAM ranges to be EFI_MEMORY_SP. For a DIMM, it could be a property,
similarly maybe for CXL- mem devices (no expert on that).
For boot memory / machine memory it could be a machine property. But
I'll first have to learn which ranges you actually want to expose
that way, and what the VM will do with that information.
[Luo, Zhigang] we want to expose the SPM memory reserved for specific device.
And we will pass the SPM memory and the device to guest. Then the device driver
can use the SPM memory in guest side.
Then the device driver should likely have a way to configure that, not the
memory
backend.
After all, the device driver will map it somehow into guest physical address
space
(how?).
[Luo, Zhigang] from guest view, it's still system memory, but marked as SPM.
So, qemu will map the memory to guest physical address space.
The device driver just claims to use the SPM memory in guest side.
Do you have any suggestions to achieve this more reasonable?
The problem with qemu_ram_foreach_block() is that you would indicate
also DIMMs, virtio-mem, ... and even RAMBlocks that are not even used
for backing anything to the VM as EFI_MEMORY_SP, which is wrong.
[Luo, Zhigang] qemu_ram_foreach_block() will list all memory block, but in
pc_update_hmem_memory(), only the memory block with "hmem" flag will be
updated to SPM memory.
Yes, but imagine a user passing such a memory backend to a DIMM/virtio-mem/boot
memory etc. It will have very undesired side effects.
[Luo, Zhigang] the user should know what he/she is doing when he/she set the
flag for the memory region.
No, we must not allow to create insane configurations that don't make
any sense.
Sufficient to add:
-object memory-backend-file,size=16G,id=unused,mem-path=whatever,hmem=on
to the cmdline to cause a mess.
Maybe it should be a "numa" node configuration like
-numa node,nodeid=1,memdev=m1,sp=on
But I recall that we discussed something related with Jonathan, so I'm
hoping we can get his input.
--
Cheers,
David / dhildenb
