Re: [PATCH] hostmem-file: add the 'hmem' option

[David Hildenbrand]

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?

> > 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?).

> > > 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.

--
Cheers,

David / dhildenb