On 1/12/26 09:45, Shameer Kolothum wrote:
-----Original Message-----
From: Duan, Zhenzhong <[email protected]>
Sent: 12 January 2026 02:33
To: Shameer Kolothum <[email protected]>; qemu-
[email protected]; [email protected]
Cc: [email protected]; [email protected]; [email protected];
[email protected]; [email protected]; Nicolin Chen <[email protected]>;
Nathan Chen <[email protected]>; Matt Ochs <[email protected]>;
Jason Gunthorpe <[email protected]>; Krishnakant Jaju <[email protected]>
Subject: RE: [PATCH 3/3] hw/vfio/region: Create dmabuf for PCI BAR per
region
External email: Use caution opening links or attachments
@@ -305,6 +345,21 @@ int vfio_region_mmap(VFIORegion *region)
region->mmaps[i].size - 1);
}
+ ret = vfio_region_create_dma_buf(region);
+ if (ret < 0) {
+ if (ret == -ENOTTY) {
+ warn_report_once("VFIO dmabuf not supported in
kernel");
+ } else {
+ error_report("%s: failed to create dmabuf: %s",
+ memory_region_name(region->mem),
strerror(errno));
+ }
+ } else {
+ MemoryRegion *mr = ®ion->mmaps[0].mem;
Do we need to support region->mmaps[1]?
My understanding is all region->mmaps[] entries for a VFIO region share
the same RAMBlock. And the kernel returns a single dmabuf fd per
region, not per subrange.
Not get, can RAMBlock have holes?
Yes, a RAMBlock can effectively have holes, but in this context
that is not what is happening.
IIUC, for a VFIO PCI BAR region, all region->mmaps[] entries
correspond to subranges of the same BAR and are backed by the
same MemoryRegion and therefore the same RAMBlock. The sparse
mmap layout (nr_mmaps > 1) exists to describe which parts of the
BAR are mappable, not to represent distinct backing memory objects.
So while sparse regions may look like "holes" at the mmap level, there
are no holes in the RAMBlock abstraction itself. All region->mmaps[]
entries share the same RAMBlock, which is why attaching the returned
dmabuf fd to region->mmaps[0].mem.ram_block is sufficient, I think.
However, possible I may be missing the case you are concerned about here.
Please let me know.
I see memory_region_init_ram_device_ptr() is called for each region-
mmaps[x].mem,
and RAMBlock is allocated in each call.
Ah.. I see. It does allocate RAMBlock per mmaps[i].
IIUC, we should set fd and fd_offset in each RAMBlock.
Ok. Will update in v2.
I'd like to send a vfio PR soon and this v2 looks a like good
candidate for it.
Thanks,
C.
