[Xen-devel] [PATCH v3] psr: fix bug which may cause crash

[Yi Sun <yi.y.sun@xxxxxxxxxxxxxxx>]

During test, we found a crash on Xen with below trace.
(XEN) Xen call trace:
(XEN)    [<ffff82d0802a065a>] R psr.c#l3_cdp_write_msr+0x1e/0x22
(XEN)    [<ffff82d0802a0858>] F psr.c#do_write_psr_msrs+0x6d/0x109
(XEN)    [<ffff82d08023e000>] F smp_call_function_interrupt+0x5a/0xac
(XEN)    [<ffff82d0802a2b89>] F call_function_interrupt+0x20/0x34
(XEN)    [<ffff82d080282c64>] F do_IRQ+0x175/0x6ae
(XEN)    [<ffff82d08038b8ba>] F common_interrupt+0x10a/0x120
(XEN)    [<ffff82d0802ec616>] F cpu_idle.c#acpi_idle_do_entry+0x9d/0xb1
(XEN)    [<ffff82d0802ecc01>] F cpu_idle.c#acpi_processor_idle+0x41d/0x626
(XEN)    [<ffff82d08027353b>] F domain.c#idle_loop+0xa5/0xa7
(XEN)
(XEN)
(XEN) ****************************************
(XEN) Panic on CPU 20:
(XEN) GENERAL PROTECTION FAULT
(XEN) [error_code=0000]
(XEN) ****************************************

The bug happens when CDP and MBA co-exist and MBA COS_MAX is bigger
than CDP COS_MAX. E.g. MBA has 8 COS registers but CDP only have 6.
When setting MBA throttling value for the 7th guest, the value array
would be:
    +------------------+------------------+--------------+
    | Data default val | Code default val | MBA throttle |
    +------------------+------------------+--------------+

Then, COS id 7 will be selected for writting the values. We should
avoid writting CDP data/code valules to COS id 7 MSR because it
exceeds the CDP COS_MAX.

Signed-off-by: Yi Sun <yi.y.sun@xxxxxxxxxxxxxxx>
---
 xen/arch/x86/psr.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/xen/arch/x86/psr.c b/xen/arch/x86/psr.c
index 5866a26..943a1e0 100644
--- a/xen/arch/x86/psr.c
+++ b/xen/arch/x86/psr.c
@@ -1271,7 +1271,13 @@ static void do_write_psr_msrs(void *data)
 
         for ( j = 0; j < cos_num; j++, index++ )
         {
-            if ( feat->cos_reg_val[cos * cos_num + j] != info->val[index] )
+            /*
+             * Multiple RDT features may co-exist and their COS_MAX may be
+             * different. So we should prevent one feature to write COS
+             * register which exceeds its COS_MAX. Otherwise, panic may happen.
+             */
+            if ( cos <= feat->cos_max &&
+                 feat->cos_reg_val[cos * cos_num + j] != info->val[index] )
             {
                 feat->cos_reg_val[cos * cos_num + j] = info->val[index];
                 props->write_msr(cos, info->val[index], props->type[j]);
-- 
1.9.1


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