hvmloader - allow_memory_relocate overlaps

[Neowutran <xen@xxxxxxxxxxxxx>]

I am wondering if the variable "allow_memory_relocate" is still
relevant today and if its default value is still relevant. 
Should it be defined to 0 by default instead of 1 (it seems to be a
workaround for qemu-traditional, so maybe an outdated default value ? ) ? 

Code extract: 

tools/firmware/hvmloader/pci.c 
"
   /*
     * Do we allow hvmloader to relocate guest memory in order to
     * increase the size of the lowmem MMIO hole?  Defaulting to 1
     * here will
 mean that non-libxl toolstacks (including xend and
     * home-grown ones) means that those using qemu-xen will still
     * experience the memory relocation bug described below; but it
     * also means that those using q
emu-traditional will *not*
     * experience any change; and it also means that there is a
     * work-around for those using qemu-xen, namely switching to
     * qemu-traditional.
     *
     * If we defaulted to 0, and failing to resize the hole caused any
     * problems with qemu-traditional, then there is no work-around.
     *
     * Since xend can only use qemu-traditional, I think this is the
     * option that will have the least impact.
     */
    bool allow_memory_relocate = 1;
"

"
        /*
         * At the moment qemu-xen can't deal with relocated memory regions.
         * It's too close to the release to make a proper fix; for now,
         * only allow t
he MMIO hole to grow large enough to move guest memory
         * if we're running qemu-traditional.  Items that don't fit will be
         * relocated into the 64-bit address space.
         *
         * This loop now does the following:
         * - If allow_memory_relocate, increase the MMIO hole until it's
         *   big enough, or 
until it's 2GiB
         * - If !allow_memory_relocate, increase the MMIO hole until it's
         *   big enough, or until it's 2GiB, or until it overlaps guest
         *   memory
         */
        while ( (mmio_total > (pci_mem_end - pci_mem_start))
                && ((pci_mem_start << 1) != 0)
                && (allow_memory_relocate
                    || (((pci_mem_start << 1) >> PAGE_SHIFT)
                        >= hvm_info->low_mem_pgend)) )
            pci_mem_start <<= 1;
"

The issue it cause is documented in the source code: guest memory can
be trashed by the hvmloader. 

Due to this issue, it is impossible to passthrough a large PCI device to a HVM 
with a lot of ram.
(https://github.com/QubesOS/qubes-issues/issues/4321). 

(Forcing "allow_memory_relocate" to be 0 seems to solve the issue
linked)



Thanks, 
Neowutran