[xen staging] xen/arm: drop unreachable EL0 check when emulating ACTLR

[patchbot@xxxxxxx]

commit ddd5011e6266e768afd14c0fe66a122a000eee54
Author:     Michal Orzel <michal.orzel@xxxxxxx>
AuthorDate: Tue Aug 11 15:08:17 2026 +0200
Commit:     Michal Orzel <michal.orzel@xxxxxxx>
CommitDate: Mon Aug 17 08:29:57 2026 +0200

    xen/arm: drop unreachable EL0 check when emulating ACTLR
    
    do_sysreg() and do_cp15_32() inject an undefined exception when the
    trapped access to ACTLR_EL1 (resp. ACTLR) originates from EL0. That
    branch cannot be taken. ACTLR_EL1 can be accessed by EL1 and above, and no
    enable makes them accessible at EL0. The only trap covering them,
    HCR_EL2.TACR (HCR.TAC on AArch32), applies to accesses from EL1 only.
    
    Refer Arm ARM (DDI 0487M.b) D1.4.5.6 "Prioritization of Synchronous
    exceptions": "attempting to execute an instruction that is defined to
    never be accessible at the current Exception level and Security state,
    regardless of any enables or traps" is priority 18, whereas an exception
    taken to EL2 as the result of a configuration control in HCR_EL2 is
    priority 24. The former wins, so an access from EL0 is UNDEFINED and the
    TACR trap is not taken.
    
    An EL0 access therefore never reaches EL2: it is taken to EL1, as Xen
    never sets HCR_EL2.TGE for guests, and it is reported with EC=0x00
    (Unknown reason) rather than EC=0x18/0x03.
    
    Signed-off-by: Michal Orzel <michal.orzel@xxxxxxx>
    Reviewed-by: Stefano Stabellini <sstabellini@xxxxxxxxxx>
---
 xen/arch/arm/arm64/vsysreg.c | 2 --
 xen/arch/arm/vcpreg.c        | 2 --
 2 files changed, 4 deletions(-)

diff --git a/xen/arch/arm/arm64/vsysreg.c b/xen/arch/arm/arm64/vsysreg.c
index d14258290f..d55a538ec4 100644
--- a/xen/arch/arm/arm64/vsysreg.c
+++ b/xen/arch/arm/arm64/vsysreg.c
@@ -94,8 +94,6 @@ void do_sysreg(struct cpu_user_regs *regs,
      * ARMv8 (DDI 0487A.d): D7.2.1
      */
     case HSR_SYSREG_ACTLR_EL1:
-        if ( regs_mode_is_user(regs) )
-            return inject_undef_exception(regs);
         if ( hsr.sysreg.read )
             set_user_reg(regs, regidx, v->arch.actlr);
         break;
diff --git a/xen/arch/arm/vcpreg.c b/xen/arch/arm/vcpreg.c
index e7c484f2c1..59a2b89a0e 100644
--- a/xen/arch/arm/vcpreg.c
+++ b/xen/arch/arm/vcpreg.c
@@ -216,8 +216,6 @@ void do_cp15_32(struct cpu_user_regs *regs, const union hsr 
hsr)
      * ARMv8 (DDI 0487A.d): G6.2.1
      */
     case HSR_CPREG32(ACTLR):
-        if ( regs_mode_is_user(regs) )
-            return inject_undef_exception(regs);
         if ( cp32.read )
             set_user_reg(regs, regidx, v->arch.actlr);
         break;
--
generated by git-patchbot for /home/xen/git/xen.git#staging