virtio via external shared ram

[Janne Karhunen]

Hi,

I have created an experimental setup for Linux where all the virtio
data structures and traffic can be allocated by the guest from a ram
blob outside of the guest default ram space. That ram blob can be
hotplugged to the guest or defined via the guests device tree.This is
done as some hypervisors, including tdx/sev/pkvm and others, would
probably benefit from a simple security policy that removes all
set_memory_{encrypted,decrypted} calls to open up the guest dma memory
in fragments that are not only likely to leak information due to the
widespread use of the DMA API but also slow things down for no obvious
reason. From the hypervisors point of view the fragmented shadow page
table space is also an unnecessary slowdown and a source of memory
waste.

I have seen forks of SWIOTLB that do similar things, but fundamentally
they are still SWIOTLB behind the curtains and as such unusable for
low latency / high bandwidth applications due to bouncing (copying)
data back and forth into those external buffers. The setup I have
created can act as virtio as it was designed to be, a zero copy data
transport path.

A trial integration into QEMU could probably look something like this
(in virt.c):

..
    emem_map = mmap(NULL, EMEM_SIZE, PROT_READ|PROT_WRITE, MAP_SHARED
| MAP_SYNC, fd, 0);
    memory_region_init_ram_ptr(sr, OBJECT(machine), "ext-mem",
EMEM_SIZE, emem_map);
..
    emem = g_new(MemoryRegion, 1);
    memory_region_add_subregion_overlap(sysmem, emem_physaddr, emem, 1000);
..

So the question I have is that did I understand the qemu RAM model
correctly and would something like that lead to known issues
somewhere?


--
Janne