On Tue, May 20, 2025 at 03:53:10PM +0200, Paolo Bonzini wrote:
> On 5/20/25 13:30, Magnus Kulke wrote:
> > Qemu maps regions of userland multiple times into the guest. The MSHV
> > kernel driver detects those overlapping regions and rejects those
> > mappings.
>
> Can you explain what you see? QEMU doesn't do that, just look at KVM code:
Hey Paolo, I appreciate that you took a look so swiftly, we'll try to
accomodate and post a fixed series up soon.
I think what I am referring to is a "memory region alias", e.g. in this
mtree output (machine q35 + seabios):
00000000000e0000-00000000000fffff (prio 1, rom): alias isa-bios @pc.bios
0000000000020000-000000000003ffff
...
00000000fffc0000-00000000ffffffff (prio 0, rom): pc.bios
parts of the bios are mapped into different regions on the guest. a
code path for such a mapping that is refused by the MSHV kernel driver
would start in hw/i386/pc.c:894
memory_region_init_alias(ram_below_4g, NULL, "ram-below-4g", machine->ram,
0, x86ms->below_4g_mem_size);
memory_region_add_subregion(system_memory, 0, ram_below_4g);
eventually that ends up in a hv call that registers a region, but the
userspace_addr of pc.bios is already registered, so the mapping of an alias
slice is rejected by the kernel driver.
best,
magnus
>
> static bool kvm_check_memslot_overlap(struct kvm_memslots *slots, int id,
> gfn_t start, gfn_t end)
> {
> struct kvm_memslot_iter iter;
>
> kvm_for_each_memslot_in_gfn_range(&iter, slots, start, end) {
> if (iter.slot->id != id)
> return true;
> }
>
> return false;
> }
>
> ...
>
> if ((change == KVM_MR_CREATE || change == KVM_MR_MOVE) &&
> kvm_check_memslot_overlap(slots, id, base_gfn, base_gfn + npages))
> return -EEXIST;
>
>
> Paolo
>
