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

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Date:   Thu, 16 Jun 2022 10:31:42 +0300
From:   Oleksandr <olekstysh@...il.com>
To:     Juergen Gross <jgross@...e.com>
Cc:     xen-devel@...ts.xenproject.org, linux-kernel@...r.kernel.org,
        viresh.kumar@...aro.org, hch@...radead.org,
        Stefano Stabellini <sstabellini@...nel.org>,
        Oleksandr Tyshchenko <oleksandr_tyshchenko@...m.com>
Subject: Re: [PATCH v2] xen: don't require virtio with grants for non-PV
 guests


On 16.06.22 08:37, Juergen Gross wrote:


Hello Juergen

> 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@...aro.org>
> Signed-off-by: Juergen Gross <jgross@...e.com>
> ---
> 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);


Looks like, the flag will be *always* set for paravirtualized guests 
even if CONFIG_XEN_VIRTIO disabled.

Maybe we should clarify the check?


if (IS_ENABLED(CONFIG_XEN_VIRTIO_FORCE_GRANT) || 
IS_ENABLED(CONFIG_XEN_VIRTIO) && xen_pv_domain())

     platform_set(PLATFORM_VIRTIO_RESTRICTED_MEM_ACCESS);


>   }
>   

-- 
Regards,

Oleksandr Tyshchenko