[Xen-changelog] [xen stable-4.10] iommu: leave IOMMU enabled by default during kexec crash transition

[patchbot@xxxxxxx]

commit d06f5613e5df529f3e6aea94d648a3c62e65a403
Author:     Igor Druzhinin <igor.druzhinin@xxxxxxxxxx>
AuthorDate: Fri May 3 11:03:52 2019 +0200
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Fri May 3 11:03:52 2019 +0200

    iommu: leave IOMMU enabled by default during kexec crash transition
    
    It's unsafe to disable IOMMU on a live system which is the case
    if we're crashing since remapping hardware doesn't usually know what
    to do with ongoing bus transactions and frequently raises NMI/MCE/SMI,
    etc. (depends on the firmware configuration) to signal these abnormalities.
    This, in turn, doesn't play well with kexec transition process as there is
    no handling available at the moment for this kind of events resulting
    in failures to enter the kernel.
    
    Modern Linux kernels taught to copy all the necessary DMAR/IR tables
    following kexec from the previous kernel (Xen in our case) - so it's
    currently normal to keep IOMMU enabled. It might require minor changes to
    kdump command line that enables IOMMU drivers (e.g. intel_iommu=on /
    intremap=on) but recent kernels don't require any additional changes for
    the transition to be transparent.
    
    A fallback option is still left for compatibility with ancient crash
    kernels which didn't like to have IOMMU active under their feet on boot.
    
    Signed-off-by: Igor Druzhinin <igor.druzhinin@xxxxxxxxxx>
    Acked-by: Jan Beulich <jbeulich@xxxxxxxx>
    master commit: 12c36f577d454996c882ecdc5da8113ca2613646
    master date: 2019-03-12 14:38:12 +0100
---
 docs/misc/xen-command-line.markdown | 12 +++++++++++-
 xen/arch/x86/crash.c                |  7 +++++--
 xen/drivers/passthrough/iommu.c     |  8 ++++++++
 3 files changed, 24 insertions(+), 3 deletions(-)

diff --git a/docs/misc/xen-command-line.markdown 
b/docs/misc/xen-command-line.markdown
index 470da80174..ffa55a8345 100644
--- a/docs/misc/xen-command-line.markdown
+++ b/docs/misc/xen-command-line.markdown
@@ -1078,7 +1078,7 @@ debug hypervisor only).
 > Default: `new` unless directed-EOI is supported
 
 ### iommu
-> `= List of [ <boolean> | force | required | intremap | intpost | qinval | 
snoop | sharept | dom0-passthrough | dom0-strict | amd-iommu-perdev-intremap | 
workaround_bios_bug | igfx | verbose | debug ]`
+> `= List of [ <boolean> | force | required | intremap | intpost | qinval | 
snoop | sharept | dom0-passthrough | dom0-strict | amd-iommu-perdev-intremap | 
workaround_bios_bug | igfx | crash-disable | verbose | debug ]`
 
 > Sub-options:
 
@@ -1168,6 +1168,16 @@ debug hypervisor only).
 >> to workaround graphics issues. If adding `no-igfx` fixes anything, you
 >> should file a bug reporting the problem.
 
+> `crash-disable`
+
+> Default: `false`
+
+>> This option controls disabling IOMMU functionality (DMAR/IR/QI) before
+>> switching to a crash kernel. This option is inactive by default and
+>> is for compatibility with older kdump kernels only. Modern kernels copy
+>> all the necessary tables from the previous one following kexec which makes
+>>  the transition transparent for them with IOMMU functions still on.
+
 > `verbose`
 
 > Default: `false`
diff --git a/xen/arch/x86/crash.c b/xen/arch/x86/crash.c
index 82535c4df0..b3337afcff 100644
--- a/xen/arch/x86/crash.c
+++ b/xen/arch/x86/crash.c
@@ -168,8 +168,11 @@ static void nmi_shootdown_cpus(void)
         printk("Failed to shoot down CPUs {%s}\n", keyhandler_scratch);
     }
 
-    /* Crash shutdown any IOMMU functionality as the crashdump kernel is not
-     * happy when booting if interrupt/dma remapping is still enabled */
+    /*
+     * Try to crash shutdown IOMMU functionality as some old crashdump
+     * kernels are not happy when booting if interrupt/dma remapping
+     * is still enabled.
+     */
     iommu_crash_shutdown();
 
     __stop_this_cpu();
diff --git a/xen/drivers/passthrough/iommu.c b/xen/drivers/passthrough/iommu.c
index 98fdb14b40..b5f8044439 100644
--- a/xen/drivers/passthrough/iommu.c
+++ b/xen/drivers/passthrough/iommu.c
@@ -60,6 +60,7 @@ bool_t __read_mostly iommu_passthrough;
 bool_t __read_mostly iommu_snoop = 1;
 bool_t __read_mostly iommu_qinval = 1;
 bool_t __read_mostly iommu_intremap = 1;
+bool_t __read_mostly iommu_crash_disable;
 
 /*
  * In the current implementation of VT-d posted interrupts, in some extreme
@@ -112,6 +113,10 @@ static int __init parse_iommu_param(const char *s)
             iommu_intremap = val;
         else if ( !cmdline_strcmp(s, "intpost") )
             iommu_intpost = val;
+#ifdef CONFIG_KEXEC
+        else if ( !cmdline_strcmp(s, "crash-disable") )
+            iommu_crash_disable = val;
+#endif
         else if ( !cmdline_strcmp(s, "debug") )
         {
             iommu_debug = val;
@@ -452,6 +457,9 @@ void iommu_share_p2m_table(struct domain* d)
 
 void iommu_crash_shutdown(void)
 {
+    if ( !iommu_crash_disable )
+        return;
+
     if ( iommu_enabled )
         iommu_get_ops()->crash_shutdown();
     iommu_enabled = iommu_intremap = iommu_intpost = 0;
--
generated by git-patchbot for /home/xen/git/xen.git#stable-4.10

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