Re: [PATCH v2] xen: don't require virtio with grants for non-PV guests

[Stefano Stabellini]

On Thu, 16 Jun 2022, Juergen Gross wrote:
> Commit fa1f57421e0b ("xen/virtio: Enable restricted memory access using
> Xen grant mappings") introduced a new requirement for using virtio
> devices: the backend now needs to support the VIRTIO_F_ACCESS_PLATFORM
> feature.
> 
> This is an undue requirement for non-PV guests, as those can be operated
> with existing backends without any problem, as long as those backends
> are running in dom0.
> 
> Per default allow virtio devices without grant support for non-PV
> guests.
> 
> Add a new config item to always force use of grants for virtio.
> 
> Fixes: fa1f57421e0b ("xen/virtio: Enable restricted memory access using Xen grant mappings")
> Reported-by: Viresh Kumar <viresh.kumar@xxxxxxxxxx>
> Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
> ---
> V2:
> - remove command line parameter (Christoph Hellwig)
> ---
>  drivers/xen/Kconfig | 9 +++++++++
>  include/xen/xen.h   | 2 +-
>  2 files changed, 10 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/xen/Kconfig b/drivers/xen/Kconfig
> index bfd5f4f706bc..a65bd92121a5 100644
> --- a/drivers/xen/Kconfig
> +++ b/drivers/xen/Kconfig
> @@ -355,4 +355,13 @@ config XEN_VIRTIO
>  
>  	  If in doubt, say n.
>  
> +config XEN_VIRTIO_FORCE_GRANT
> +	bool "Require Xen virtio support to use grants"
> +	depends on XEN_VIRTIO
> +	help
> +	  Require virtio for Xen guests to use grant mappings.
> +	  This will avoid the need to give the backend the right to map all
> +	  of the guest memory. This will need support on the backend side
> +	  (e.g. qemu or kernel, depending on the virtio device types used).
> +
>  endmenu
> diff --git a/include/xen/xen.h b/include/xen/xen.h
> index 0780a81e140d..4d4188f20337 100644
> --- a/include/xen/xen.h
> +++ b/include/xen/xen.h
> @@ -56,7 +56,7 @@ extern u64 xen_saved_max_mem_size;
>  
>  static inline void xen_set_restricted_virtio_memory_access(void)
>  {
> -	if (IS_ENABLED(CONFIG_XEN_VIRTIO) && xen_domain())
> +	if (IS_ENABLED(CONFIG_XEN_VIRTIO_FORCE_GRANT) || xen_pv_domain())
>  		platform_set(PLATFORM_VIRTIO_RESTRICTED_MEM_ACCESS);
>  }

Hi Juergen, you might have seen my email:
https://marc.info/?l=linux-kernel&m=165533636607801&w=2

Linux is always running as HVM on ARM, so if you want to introduce
XEN_VIRTIO_FORCE_GRANT, then XEN_VIRTIO_FORCE_GRANT should be
automatically selected on ARM. I don't think there should be a visible
menu option for XEN_VIRTIO_FORCE_GRANT on ARM.

I realize we have a conflict between HVM guests on ARM and x86:

- on ARM, PLATFORM_VIRTIO_RESTRICTED_MEM_ACCESS should be enabled when
  "xen,grant-dma" is present
- on x86, due to the lack of "xen,grant-dma", it should be off by
  default and based on a kconfig or command line option

To be honest, like Christoph suggested, I think even on x86 there should
be a firmware table to trigger setting
PLATFORM_VIRTIO_RESTRICTED_MEM_ACCESS. We have 2 Xen-specific ACPI
tables, and we could have 1 more to define this. Or an HVM param or
a feature flag?

I think that would be the cleanest way to do this, but it is a lot of
more work compared to adding a couple of lines of code to Linux, so this
is why I suggested:
https://marc.info/?l=linux-kernel&m=165533636607801&w=2

ARM uses "xen,grant-dma" to detect whether
PLATFORM_VIRTIO_RESTRICTED_MEM_ACCESS needs setting.

One day x86 could check an ACPI property or HVM param or feature flag.
None of them are available now, so for now use a command line option as
a workaround. It is totally fine to use an x86-only kconfig option
instead of a command line option.

Would you be OK with that?