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Re: [PATCH v4 09/14] vfio-user: handle DMA mappings
From: |
Stefan Hajnoczi |
Subject: |
Re: [PATCH v4 09/14] vfio-user: handle DMA mappings |
Date: |
Thu, 16 Dec 2021 13:24:06 +0000 |
On Wed, Dec 15, 2021 at 10:35:33AM -0500, Jagannathan Raman wrote:
> Define and register callbacks to manage the RAM regions used for
> device DMA
>
> Signed-off-by: Elena Ufimtseva <elena.ufimtseva@oracle.com>
> Signed-off-by: John G Johnson <john.g.johnson@oracle.com>
> Signed-off-by: Jagannathan Raman <jag.raman@oracle.com>
> Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
> ---
> hw/remote/vfio-user-obj.c | 48 +++++++++++++++++++++++++++++++++++++++
> hw/remote/trace-events | 2 ++
> 2 files changed, 50 insertions(+)
>
> diff --git a/hw/remote/vfio-user-obj.c b/hw/remote/vfio-user-obj.c
> index c6d0c675b7..46f2251a68 100644
> --- a/hw/remote/vfio-user-obj.c
> +++ b/hw/remote/vfio-user-obj.c
> @@ -208,6 +208,47 @@ static ssize_t vfu_object_cfg_access(vfu_ctx_t *vfu_ctx,
> char * const buf,
> return count;
> }
>
> +static void dma_register(vfu_ctx_t *vfu_ctx, vfu_dma_info_t *info)
> +{
> + MemoryRegion *subregion = NULL;
> + g_autofree char *name = NULL;
> + static unsigned int suffix;
> + struct iovec *iov = &info->iova;
> +
> + if (!info->vaddr) {
> + return;
> + }
> +
> + name = g_strdup_printf("remote-mem-%u", suffix++);
> +
> + subregion = g_new0(MemoryRegion, 1);
> +
> + memory_region_init_ram_ptr(subregion, NULL, name,
> + iov->iov_len, info->vaddr);
> +
> + memory_region_add_subregion(get_system_memory(), (hwaddr)iov->iov_base,
> + subregion);
> +
> + trace_vfu_dma_register((uint64_t)iov->iov_base, iov->iov_len);
> +}
> +
> +static void dma_unregister(vfu_ctx_t *vfu_ctx, vfu_dma_info_t *info)
> +{
> + MemoryRegion *mr = NULL;
> + ram_addr_t offset;
> +
> + mr = memory_region_from_host(info->vaddr, &offset);
> + if (!mr) {
> + return;
> + }
> +
> + memory_region_del_subregion(get_system_memory(), mr);
> +
> + object_unparent((OBJECT(mr)));
> +
> + trace_vfu_dma_unregister((uint64_t)info->iova.iov_base);
> +}
This does not support hot unplug (memory regions pointing to memory
mapped by libvfio-user are left registered). The code should keep a list
(e.g. https://docs.gtk.org/glib/struct.SList.html) of memory regions and
automatically remove them before destroying the vfu context.
It also doesn't support multiple vfio-user server instances running in
the same QEMU process. get_system_memory() is global but the memory
regions provided by vfio-user are per-client (i.e. VM). If multiple VMs
are connected to one vfio-user server process then they conflict.
I don't know the best way to support multiple vfio-user server
instances, it would be straightforward if QEMU supported multiple
MachineStates and didn't use global get_system_memory()/get_io_memory()
APIs. It would be nice to solve that in the future.
Maybe it's too hard to change that, I haven't looked. An alternative is
to make the x-remote machine empty (it doesn't create any devices) and
instead create a new PCI bus, interrupt controller, memory MemoryRegion,
and io MemoryRegion in VfuObject. Stop using get_system_memory() and
instead use the per-VfuObject memory MemoryRegion.
In either of those approaches it's probably necessary to specify the PCI
bus ID in --device and device_add so it's clear which vfio-user server
the PCI device is associated with.
The multiple vfio-user server instance limitation doesn't need to be
solved now, but I wanted to share some ideas around it. Maybe someone
has better ideas or is aware of limitations preventing what I described.
Stefan
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- [PATCH v4 07/14] vfio-user: run vfio-user context, (continued)
[PATCH v4 08/14] vfio-user: handle PCI config space accesses, Jagannathan Raman, 2021/12/15
[PATCH v4 09/14] vfio-user: handle DMA mappings, Jagannathan Raman, 2021/12/15
- Re: [PATCH v4 09/14] vfio-user: handle DMA mappings,
Stefan Hajnoczi <=
[PATCH v4 10/14] vfio-user: handle PCI BAR accesses, Jagannathan Raman, 2021/12/15
[PATCH v4 11/14] vfio-user: IOMMU support for remote device, Jagannathan Raman, 2021/12/15
[PATCH v4 12/14] vfio-user: handle device interrupts, Jagannathan Raman, 2021/12/15