[Xen-changelog] [xen master] xen/arm: ctxt_switch: Document the erratum #852523 related to Cortex A57

[patchbot@xxxxxxx]

commit b50c03d281fd1cfaf1f99654a2d6987a702b41a3
Author:     Julien Grall <julien.grall@xxxxxxxxxx>
AuthorDate: Thu Oct 8 20:22:37 2015 +0100
Commit:     Ian Campbell <ian.campbell@xxxxxxxxxx>
CommitDate: Wed Oct 14 11:06:54 2015 +0100

    xen/arm: ctxt_switch: Document the erratum #852523 related to Cortex A57
    
    When restoring the system register state for an AArch32 guest at EL2,
    writes to DACR32_EL2 may not be correctly synchronised by Cortex-A57,
    which can lead to the guest effectively running into unexpected domain
    faults.
    
    Thankfully, we don't hit this erratum in Xen. Nonetheless, document the
    code to prevent any introduction of the erratum if the context switch
    code is re-ordered.
    
    Link: http://lists.xen.org/archives/html/xen-devel/2015-09/msg01746.html
    Signed-off-by: Julien Grall <julien.grall@xxxxxxxxxx>
    Acked-by: Ian Campbell <ian.campbell@xxxxxxxxxx>
    [ ijc -- comment nits ]
---
 xen/arch/arm/domain.c |   10 ++++++++++
 1 files changed, 10 insertions(+), 0 deletions(-)

diff --git a/xen/arch/arm/domain.c b/xen/arch/arm/domain.c
index 575745c..4c08073 100644
--- a/xen/arch/arm/domain.c
+++ b/xen/arch/arm/domain.c
@@ -181,6 +181,12 @@ static void ctxt_switch_to(struct vcpu *n)
     WRITE_SYSREG(n->arch.ttbcr, TCR_EL1);
     WRITE_SYSREG64(n->arch.ttbr0, TTBR0_EL1);
     WRITE_SYSREG64(n->arch.ttbr1, TTBR1_EL1);
+
+    /*
+     * Erratum #852523: DACR32_EL2 must be restored before one of the
+     * following sysregs: SCTLR_EL1, TCR_EL1, TTBR0_EL1, TTBR1_EL1 or
+     * CONTEXTIDR_EL1.
+     */
     if ( is_32bit_domain(n->domain) )
         WRITE_SYSREG(n->arch.dacr, DACR32_EL2);
     WRITE_SYSREG64(n->arch.par, PAR_EL1);
@@ -198,6 +204,10 @@ static void ctxt_switch_to(struct vcpu *n)
     /* Control Registers */
     WRITE_SYSREG(n->arch.cpacr, CPACR_EL1);
 
+    /*
+     * This write to sysreg CONTEXTIDR_EL1 ensures we don't hit erratum
+     * #852523. I.e DACR32_EL2 is not correctly synchronized.
+     */
     WRITE_SYSREG(n->arch.contextidr, CONTEXTIDR_EL1);
     WRITE_SYSREG(n->arch.tpidr_el0, TPIDR_EL0);
     WRITE_SYSREG(n->arch.tpidrro_el0, TPIDRRO_EL0);
--
generated by git-patchbot for /home/xen/git/xen.git#master

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