[PATCH v1 2/2] memory-device: reintroduce memory region size check

[David Hildenbrand <david@redhat.com>]

We used to check that the memory region size is multiples of the overall
requested address alignment for the device memory address.

We removed that check, because there are cases (i.e., hv-balloon) where
devices unconditionally request an address alignment that has a very large
alignment (i.e., 32 GiB), but the actual memory device size might not be
multiples of that alignment.

However, this change:

(a) allows for some practically impossible DIMM sizes, like "1GB+1 byte".
(b) allows for DIMMs that partially cover hugetlb pages, previously
    reported in [1].

Both scenarios don't make any sense: we might even waste memory.

So let's reintroduce that check, but only check that the
memory region size is multiples of the memory region alignment (i.e.,
page size, huge page size), but not any additional memory device
requirements communicated using md->get_min_alignment().

The following examples now fail again as expected:

(a) 1M with 2M THP
 qemu-system-x86_64 -m 4g,maxmem=16g,slots=1 -S -nodefaults -nographic \
                     -object memory-backend-ram,id=mem1,size=1M \
                     -device pc-dimm,id=dimm1,memdev=mem1
 -> backend memory size must be multiple of 0x200000

(b) 1G+1byte

 qemu-system-x86_64 -m 4g,maxmem=16g,slots=1 -S -nodefaults -nographic \
                   -object memory-backend-ram,id=mem1,size=1073741825B \
                   -device pc-dimm,id=dimm1,memdev=mem1
 -> backend memory size must be multiple of 0x200000

(c) Unliagned hugetlb size (2M)

 qemu-system-x86_64 -m 4g,maxmem=16g,slots=1 -S -nodefaults -nographic \
                   -object memory-backend-file,id=mem1,mem-path=/dev/hugepages/tmp,size=511M \
                   -device pc-dimm,id=dimm1,memdev=mem1
 backend memory size must be multiple of 0x200000

(d) Unliagned hugetlb size (1G)

 qemu-system-x86_64 -m 4g,maxmem=16g,slots=1 -S -nodefaults -nographic \
                    -object memory-backend-file,id=mem1,mem-path=/dev/hugepages1G/tmp,size=2047M \
                    -device pc-dimm,id=dimm1,memdev=mem1
 -> backend memory size must be multiple of 0x40000000

Note that this fix depends on a hv-balloon change to communicate its
additional alignment requirements using get_min_alignment() instead of
through the memory region.

[1] https://lkml.kernel.org/r/f77d641d500324525ac036fe1827b3070de75fc1.1701088320.git.mprivozn@redhat.com

Reported-by: Zhenyu Zhang <zhenyzha@redhat.com>
Reported-by: Michal Privoznik <mprivozn@redhat.com>
Fixes: eb1b7c4bd413 ("memory-device: Drop size alignment check")
Signed-off-by: David Hildenbrand <david@redhat.com>
---
 hw/mem/memory-device.c | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/hw/mem/memory-device.c b/hw/mem/memory-device.c
index a1b1af26bc..e098585cda 100644
--- a/hw/mem/memory-device.c
+++ b/hw/mem/memory-device.c
@@ -374,6 +374,20 @@ void memory_device_pre_plug(MemoryDeviceState *md, MachineState *ms,
         goto out;
     }
 
+    /*
+     * We always want the memory region size to be multiples of the memory
+     * region alignment: for example, DIMMs with 1G+1byte size don't make
+     * any sense. Note that we don't check that the size is multiples
+     * of any additional alignment requirements the memory device might
+     * have when it comes to the address in physical address space.
+     */
+    if (!QEMU_IS_ALIGNED(memory_region_size(mr),
+                         memory_region_get_alignment(mr))) {
+        error_setg(errp, "backend memory size must be multiple of 0x%"
+                   PRIx64, memory_region_get_alignment(mr));
+        return;
+    }
+
     if (legacy_align) {
         align = *legacy_align;
     } else {
-- 
2.43.0