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    Subject[PATCH v3 03/17] arm64: kvm: stop treating register x18 as caller save
    From
    In preparation of reserving x18, stop treating it as caller save in
    the KVM guest entry/exit code. Currently, the code assumes there is
    no need to preserve it for the host, given that it would have been
    assumed clobbered anyway by the function call to __guest_enter().
    Instead, preserve its value and restore it upon return.

    Link: https://patchwork.kernel.org/patch/9836891/
    Co-developed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
    [ updated commit message, switched from x18 to x29 for the guest context ]
    Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
    ---
    arch/arm64/kvm/hyp/entry.S | 41 +++++++++++++++++++-------------------
    1 file changed, 20 insertions(+), 21 deletions(-)

    diff --git a/arch/arm64/kvm/hyp/entry.S b/arch/arm64/kvm/hyp/entry.S
    index e5cc8d66bf53..c3c2d842c609 100644
    --- a/arch/arm64/kvm/hyp/entry.S
    +++ b/arch/arm64/kvm/hyp/entry.S
    @@ -23,6 +23,7 @@
    .pushsection .hyp.text, "ax"

    .macro save_callee_saved_regs ctxt
    + str x18, [\ctxt, #CPU_XREG_OFFSET(18)]
    stp x19, x20, [\ctxt, #CPU_XREG_OFFSET(19)]
    stp x21, x22, [\ctxt, #CPU_XREG_OFFSET(21)]
    stp x23, x24, [\ctxt, #CPU_XREG_OFFSET(23)]
    @@ -32,6 +33,8 @@
    .endm

    .macro restore_callee_saved_regs ctxt
    + // We assume \ctxt is not x18-x28
    + ldr x18, [\ctxt, #CPU_XREG_OFFSET(18)]
    ldp x19, x20, [\ctxt, #CPU_XREG_OFFSET(19)]
    ldp x21, x22, [\ctxt, #CPU_XREG_OFFSET(21)]
    ldp x23, x24, [\ctxt, #CPU_XREG_OFFSET(23)]
    @@ -48,7 +51,7 @@ ENTRY(__guest_enter)
    // x0: vcpu
    // x1: host context
    // x2-x17: clobbered by macros
    - // x18: guest context
    + // x29: guest context

    // Store the host regs
    save_callee_saved_regs x1
    @@ -67,31 +70,28 @@ alternative_else_nop_endif
    ret

    1:
    - add x18, x0, #VCPU_CONTEXT
    + add x29, x0, #VCPU_CONTEXT

    // Macro ptrauth_switch_to_guest format:
    // ptrauth_switch_to_guest(guest cxt, tmp1, tmp2, tmp3)
    // The below macro to restore guest keys is not implemented in C code
    // as it may cause Pointer Authentication key signing mismatch errors
    // when this feature is enabled for kernel code.
    - ptrauth_switch_to_guest x18, x0, x1, x2
    + ptrauth_switch_to_guest x29, x0, x1, x2

    // Restore guest regs x0-x17
    - ldp x0, x1, [x18, #CPU_XREG_OFFSET(0)]
    - ldp x2, x3, [x18, #CPU_XREG_OFFSET(2)]
    - ldp x4, x5, [x18, #CPU_XREG_OFFSET(4)]
    - ldp x6, x7, [x18, #CPU_XREG_OFFSET(6)]
    - ldp x8, x9, [x18, #CPU_XREG_OFFSET(8)]
    - ldp x10, x11, [x18, #CPU_XREG_OFFSET(10)]
    - ldp x12, x13, [x18, #CPU_XREG_OFFSET(12)]
    - ldp x14, x15, [x18, #CPU_XREG_OFFSET(14)]
    - ldp x16, x17, [x18, #CPU_XREG_OFFSET(16)]
    -
    - // Restore guest regs x19-x29, lr
    - restore_callee_saved_regs x18
    -
    - // Restore guest reg x18
    - ldr x18, [x18, #CPU_XREG_OFFSET(18)]
    + ldp x0, x1, [x29, #CPU_XREG_OFFSET(0)]
    + ldp x2, x3, [x29, #CPU_XREG_OFFSET(2)]
    + ldp x4, x5, [x29, #CPU_XREG_OFFSET(4)]
    + ldp x6, x7, [x29, #CPU_XREG_OFFSET(6)]
    + ldp x8, x9, [x29, #CPU_XREG_OFFSET(8)]
    + ldp x10, x11, [x29, #CPU_XREG_OFFSET(10)]
    + ldp x12, x13, [x29, #CPU_XREG_OFFSET(12)]
    + ldp x14, x15, [x29, #CPU_XREG_OFFSET(14)]
    + ldp x16, x17, [x29, #CPU_XREG_OFFSET(16)]
    +
    + // Restore guest regs x18-x29, lr
    + restore_callee_saved_regs x29

    // Do not touch any register after this!
    eret
    @@ -114,7 +114,7 @@ ENTRY(__guest_exit)
    // Retrieve the guest regs x0-x1 from the stack
    ldp x2, x3, [sp], #16 // x0, x1

    - // Store the guest regs x0-x1 and x4-x18
    + // Store the guest regs x0-x1 and x4-x17
    stp x2, x3, [x1, #CPU_XREG_OFFSET(0)]
    stp x4, x5, [x1, #CPU_XREG_OFFSET(4)]
    stp x6, x7, [x1, #CPU_XREG_OFFSET(6)]
    @@ -123,9 +123,8 @@ ENTRY(__guest_exit)
    stp x12, x13, [x1, #CPU_XREG_OFFSET(12)]
    stp x14, x15, [x1, #CPU_XREG_OFFSET(14)]
    stp x16, x17, [x1, #CPU_XREG_OFFSET(16)]
    - str x18, [x1, #CPU_XREG_OFFSET(18)]

    - // Store the guest regs x19-x29, lr
    + // Store the guest regs x18-x29, lr
    save_callee_saved_regs x1

    get_host_ctxt x2, x3
    --
    2.24.0.rc0.303.g954a862665-goog
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