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

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Date:   Wed, 15 Jun 2022 14:55:19 +0530
From:   Viresh Kumar <viresh.kumar@...aro.org>
To:     Juergen Gross <jgross@...e.com>
Cc:     xen-devel@...ts.xenproject.org, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, Jonathan Corbet <corbet@....net>,
        Stefano Stabellini <sstabellini@...nel.org>,
        Oleksandr Tyshchenko <oleksandr_tyshchenko@...m.com>
Subject: Re: [PATCH] xen: don't require virtio with grants for non-PV guests

On 15-06-22, 10:48, 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.
> 
> The setting can be overridden by using the new "xen_virtio_grant"
> command line parameter.
> 
> 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")
> Signed-off-by: Juergen Gross <jgross@...e.com>
> ---
>  .../admin-guide/kernel-parameters.txt         |  6 +++++
>  drivers/xen/Kconfig                           |  9 ++++++++
>  drivers/xen/grant-dma-ops.c                   | 22 +++++++++++++++++++
>  include/xen/xen.h                             | 12 +++++-----
>  4 files changed, 42 insertions(+), 7 deletions(-)

Thanks for the quick fix.

With CONFIG_DEBUG_SECTION_MISMATCH=y, this generates a warning.

WARNING: modpost: vmlinux.o(.text+0x7a8270): Section mismatch in reference from the function xen_set_restricted_virtio_memory_access() to the variable .init.data:xen_virtio_grants
The function xen_set_restricted_virtio_memory_access() references
the variable __initdata xen_virtio_grants.
This is often because xen_set_restricted_virtio_memory_access lacks a __initdata
annotation or the annotation of xen_virtio_grants is wrong.

This can be fixed by:

diff --git a/drivers/xen/grant-dma-ops.c b/drivers/xen/grant-dma-ops.c
index d1fae789dfad..1099097b4515 100644
--- a/drivers/xen/grant-dma-ops.c
+++ b/drivers/xen/grant-dma-ops.c
@@ -42,7 +42,7 @@ static __init int parse_use_grants(char *arg)
 }
 early_param("xen_virtio_grant", parse_use_grants);

-void xen_set_restricted_virtio_memory_access(void)
+void __init xen_set_restricted_virtio_memory_access(void)
 {
        if (IS_ENABLED(CONFIG_XEN_VIRTIO_FORCE_GRANT) || xen_virtio_grants ||
            (!xen_virtio_grants_set && xen_pv_domain()))

With that:

Tested-by: Viresh Kumar <viresh.kumar@...aro.org>

-- 
viresh