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



On 16.06.22 09:31, Oleksandr wrote:

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@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);


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);


Yes, we should. I had the function in grant-dma-ops.c in V1, and could drop the
CONFIG_XEN_VIRTIO dependency for that reason.

I'll wait for more comments before sending V3, though.


Juergen

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