arch, arm: Preserve TLB bootUncacheability when switching CPUs

The ARM TLBs have a bootUncacheability flag used to make some loads
and stores become uncacheable when booting in FS mode. Later the
flag is cleared to let those loads and stores operate as normal.  When
doing a takeOverFrom(), this flag's state is not preserved and is
momentarily reset until the CPSR is touched. On single core runs this
is a non-issue. On multi-core runs this can lead to crashes on the O3
CPU model from the following series of events:
 1) takeOverFrom executed to switch from Atomic -> O3
 2) All bootUncacheability flags are reset to true
 3) Core2 tries to execute a load covered by bootUncacheability, it
    is flagged as uncacheable
 4) Core2's load needs to replay due to a pipeline flush
 3) Core1 core does an action on CPSR
 4) The handling code for CPSR then checks all other cores
    to determine if bootUncacheability can be set to false
 5) Asynchronously set bootUncacheability on all cores to false
 6) Core2 replays load previously set as uncacheable and notices
    it is now flagged as cacheable, leads to a panic.
This patch implements takeOverFrom() functionality for the ARM TLBs
to preserve flag values when switching from atomic -> detailed.

1 files changed, 2 insertions, 0 deletions

diff --git a/src/arch/x86/tlb.hh b/src/arch/x86/tlb.hh
index ea2d50ec2..6b65b6f37 100644
--- a/src/arch/x86/tlb.hh
+++ b/src/arch/x86/tlb.hh
@@ -75,6 +75,8 @@ namespace X86ISA
         typedef X86TLBParams Params;
         TLB(const Params *p);
 
+        void takeOverFrom(BaseTLB *otlb) {}
+
         TlbEntry *lookup(Addr va, bool update_lru = true);
 
         void setConfigAddress(uint32_t addr);