[Xen-devel] [PATCH] x86/ats: Disable Address Translation Services by default

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

Xen cannot safely use any ATS functionality until it gains asynchronous queued
invalidation support, because of the current synchronous wait for completion.

Do not turn ATS on by default.

While editing the default in the command line documentation, correct the
statement regarding PCI Passthrough.  ATS is purely a performance
optimisation, and is certainly not required for PCI Passthrough to function.

Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
CC: Jan Beulich <JBeulich@xxxxxxxx>
---
 docs/misc/xen-command-line.markdown |    9 ++++++---
 xen/drivers/passthrough/x86/ats.c   |    2 +-
 2 files changed, 7 insertions(+), 4 deletions(-)

diff --git a/docs/misc/xen-command-line.markdown 
b/docs/misc/xen-command-line.markdown
index a8cab59..5f4680f 100644
--- a/docs/misc/xen-command-line.markdown
+++ b/docs/misc/xen-command-line.markdown
@@ -176,10 +176,13 @@ developers wishing Xen to fall back to older timing 
methods on newer hardware.
 ### ats
 > `= <boolean>`
 
-> Default: `true`
+> Default: `false`
+
+Permits Xen to set up and use PCI Address Translation Services.  This is a
+performance optimisation for PCI Passthrough.
 
-Permits Xen to set up and use PCI Address Translation Services, which
-is required for PCI Passthrough.
+**WARNING: Xen cannot currently safely use ATS because of its synchronous wait
+loops for Queued Invalidation completions.**
 
 ### availmem
 > `= <size>`
diff --git a/xen/drivers/passthrough/x86/ats.c 
b/xen/drivers/passthrough/x86/ats.c
index 1e3e03a..436eada 100644
--- a/xen/drivers/passthrough/x86/ats.c
+++ b/xen/drivers/passthrough/x86/ats.c
@@ -20,7 +20,7 @@
 
 LIST_HEAD(ats_devices);
 
-bool_t __read_mostly ats_enabled = 1;
+bool_t __read_mostly ats_enabled = 0;
 boolean_param("ats", ats_enabled);
 
 int enable_ats_device(int seg, int bus, int devfn, const void *iommu)
-- 
1.7.10.4


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