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

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Date:   Thu, 16 Jun 2022 07:37:15 +0200
From:   Juergen Gross <jgross@...e.com>
To:     xen-devel@...ts.xenproject.org, linux-kernel@...r.kernel.org
Cc:     viresh.kumar@...aro.org, hch@...radead.org,
        Juergen Gross <jgross@...e.com>,
        Stefano Stabellini <sstabellini@...nel.org>,
        Oleksandr Tyshchenko <oleksandr_tyshchenko@...m.com>
Subject: [PATCH v2] xen: don't require virtio with grants for non-PV guests

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);
 }
 
-- 
2.35.3