Re: [Qemu-devel] [RFC PATCH qemu 1/4] memory: Postpone flatview and dispatch tree building till all devices are added

[Peter Maydell]

On 7 September 2017 at 10:20, Alexey Kardashevskiy <address@hidden> wrote:
> Most devices use at least one address space and every time a new address
> space is added, flat views and dispatch trees are rebuild for all address
> spaces. This is not a problem for a relatively small amount of devices but
> even 50 virtio-pci devices use more than 8GB of RAM.
>
> What happens that on every flatview/dispatch rebuild, new arrays are
> allocated and old ones release but the release is done via RCU so until
> an entire machine is build, they are not released.
>
> This wraps devices creation into memory_region_transaction_begin/commit
> to massively reduce amount of flat view/dispatch tree (re)allocations.
>
> Signed-off-by: Alexey Kardashevskiy <address@hidden>
> ---
>  vl.c | 4 ++++
>  1 file changed, 4 insertions(+)
>
> diff --git a/vl.c b/vl.c
> index 8e247cc2a2..3c39cc8b3a 100644
> --- a/vl.c
> +++ b/vl.c
> @@ -4655,12 +4655,16 @@ int main(int argc, char **argv, char **envp)
>      igd_gfx_passthru();
>
>      /* init generic devices */
> +    memory_region_transaction_begin();
> +
>      rom_set_order_override(FW_CFG_ORDER_OVERRIDE_DEVICE);
>      if (qemu_opts_foreach(qemu_find_opts("device"),
>                            device_init_func, NULL, NULL)) {
>          exit(1);
>      }
>
> +    memory_region_transaction_commit();

What happens if something in a device realize function tries
to do a read from an AddressSpace? I can't think of any examples
that need to do that and I suspect it's probably a bug if anybody
tries it, but I'm curious whether this change alters the failure
mode...

thanks
-- PMM