[Xen-changelog] [xen stable-4.5] xen/arm: Force broadcast of TLB and instruction cache maintenance instructions

[patchbot@xxxxxxx]

commit c0bb033951c7595a020025e2e765e0e5b4aa1312
Author:     Julien Grall <julien.grall@xxxxxxx>
AuthorDate: Wed Apr 27 12:22:53 2016 +0100
Commit:     Stefano Stabellini <sstabellini@xxxxxxxxxx>
CommitDate: Tue May 17 11:28:11 2016 +0100

    xen/arm: Force broadcast of TLB and instruction cache maintenance 
instructions
    
    UP guest may use TLB instructions to flush only on the local CPU.
    Therefore, TLB flush will not be broadcasted across all the CPUs within
    the same innershareable domain.
    
    When the vCPU is migrated between different CPUs, it may be rescheduled
    to a previous CPU where the TLB has not been flushed. The TLB may
    contain stale entries which will result to translate incorrectly a VA to
    IPA or even cause TLB conflicts.
    
    To avoid a such situation, it is possible to set HCR_EL2.FB, which will
    force the broadcast of TLB and instruction cache maintenance instructions.
    
    The performance impact of setting HCR_EL2.FB will depend on how often
    a guest makes use of local flush instructions.
    
    ARM64 Linux kernel is SMP-aware (no possibility to build only for UP).
    Most of the flush instructions are innershareable. The local flushes are
    limited to the boot (1 per CPU) and when a task is getting a new ASIC.
    Therefore the impact of setting HCR.FB for those guests is very limited.
    
    ARM32 Linux kernel offers the possibility to be built either for SMP or
    UP. The number of local flush is very limited in the former kernel
    whilst the latter will only issue local flushes. Therefore there will be
    an impact to set HCR.FB for guest kernel only built for UP.
    
    Note that the SMP kernel can run in a domain using 1 vCPU and it
    will still make use of innershareable flush instruction.
    
    Looking at other OSes, such as FreeBSD, they are very similar to ARM32
    Linux kernel (i.e offering two configuration: SMP and UP).
    
    However, nothing prevents an SMP-aware kernel to make more often use of
    local flush instrutions.
    
    In the case that HCR_EL2.FB is not set, Xen would need to:
        * Flush all the TLBs for the VMID associated to this domain
        * Invalidate all the entries from the branch predictor
        * Invalidate all the entries from the instruction cache
    Those actions would only be needed when the vCPU is migrating between 2
    physical CPUs.
    
    Whilst this solution would have a negative performance impact on kernels
    which do not heavily use local flush instructions, this may improve
    performance for kernels only built for UP system.
    
    For now implement the easiest solution (i.e setting HCR_EL2.FB). We can
    revisit it if the performance impact is too high for UP kernel.
    
    Signed-off-by: Julien Grall <julien.grall@xxxxxxx>
    Reviewed-by: Stefano Stabellini <sstabellini@xxxxxxxxxx>
---
 xen/arch/arm/traps.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/xen/arch/arm/traps.c b/xen/arch/arm/traps.c
index 2a8a442..2684d17 100644
--- a/xen/arch/arm/traps.c
+++ b/xen/arch/arm/traps.c
@@ -93,7 +93,8 @@ void __cpuinit init_traps(void)
 
     /* Setup hypervisor traps */
     WRITE_SYSREG(HCR_PTW|HCR_BSU_INNER|HCR_AMO|HCR_IMO|HCR_FMO|HCR_VM|
-                 HCR_TWE|HCR_TWI|HCR_TSC|HCR_TAC|HCR_SWIO|HCR_TIDCP, HCR_EL2);
+                 HCR_TWE|HCR_TWI|HCR_TSC|HCR_TAC|HCR_SWIO|HCR_TIDCP|HCR_FB,
+                 HCR_EL2);
     isb();
 }
 
--
generated by git-patchbot for /home/xen/git/xen.git#stable-4.5

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