Re: [RFC PATCH v3 3/5] xen: add option to disable legacy backends

["Woodhouse, David" <dwmw@xxxxxxxxxxxx>]

On Fri, 2023-11-24 at 23:24 +0000, Volodymyr Babchuk wrote:
> This patch makes legacy backends optional. As was discussed at [1]
> this is a solution to a problem when we can't run QEMU as a device
> model in a non-privileged domain. This is because legacy backends
> assume that they are always running in domain with ID = 0. Actually,
> this may prevent running QEMU in a privileged domain with ID not equal
> to zero.
> 
> To be able to disable legacy backends we need to alter couple of
> source files that unintentionally depend on them. For example
> xen-all.c used xen_pv_printf to report errors, while not providing any
> additional like xendev pointer. Also, we need to move xenstore
> structure from xen-legacy-backend.c, because it is apparently used in
> xen-all.c.
> 
> With this patch it is possible to provide
> "--disable-xen-legacy-backends" configure option to get QEMU binary
> that can run in a driver domain. With price of not be able to use
> legacy backends of course.
> 
> [1]
> https://lists.gnu.org/archive/html/qemu-devel/2023-11/msg05022.html
> 
> Signed-off-by: Volodymyr Babchuk <volodymyr_babchuk@xxxxxxxx>
> 
> ---
> 
> I am not sure if I made correct changes to the build system, thus this
> patch is tagged as RFC.

Hm, I was imagining a new CONFIG_LEGACY_XEN_BACKENDS option which would
look a lot like CONFIG_XEN_BUS (which would now be only for the new
XenBus code).

This looks weird to me:

> --- a/hw/display/meson.build
> +++ b/hw/display/meson.build
> @@ -14,7 +14,9 @@ system_ss.add(when: 'CONFIG_PL110', if_true:
> files('pl110.c'))
>  system_ss.add(when: 'CONFIG_SII9022', if_true: files('sii9022.c'))
>  system_ss.add(when: 'CONFIG_SSD0303', if_true: files('ssd0303.c'))
>  system_ss.add(when: 'CONFIG_SSD0323', if_true: files('ssd0323.c'))
> -system_ss.add(when: 'CONFIG_XEN_BUS', if_true: files('xenfb.c'))
> +if have_xen_legacy_backends
> +  system_ss.add(when: 'CONFIG_XEN_BUS', if_true: files('xenfb.c'))
> +endif
> 
>  system_ss.add(when: 'CONFIG_VGA_PCI', if_true: files('vga-pci.c'))
>  system_ss.add(when: 'CONFIG_VGA_ISA', if_true: files('vga-isa.c'))

I'd prefer to see just:

-system_ss.add(when: 'CONFIG_XEN_BUS', if_true: files('xenfb.c'))
+system_ss.add(when: 'CONFIG_XEN_LEGACY_BACKENDS', if_true: files('xenfb.c'))


Probably also better to split out the bits in accel/xen/xen-all.c and
hw/xen/xen-legacy-backend.c to a separate preparatory commit.

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