Re: [PATCH 03/18] arm64: kvm: stop treating register x18 as caller save

Marc Zyngier
2019-10-22T17:23:01Z

On Fri, 18 Oct 2019 09:10:18 -0700
Sami Tolvanen <samitolvanen@google.com> wrote:

> From: Ard Biesheuvel <ard.biesheuvel@linaro.org>
>
> In preparation of using x18 as a task struct pointer register when
> running in the kernel, 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/
> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
> Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
> ---
> arch/arm64/kvm/hyp/entry.S | 12 +++++-------
> 1 file changed, 5 insertions(+), 7 deletions(-)
>
> diff --git a/arch/arm64/kvm/hyp/entry.S b/arch/arm64/kvm/hyp/entry.S
> index e5cc8d66bf53..20bd9a20ea27 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)]
> @@ -38,6 +39,7 @@
> ldp x25, x26, [\ctxt, #CPU_XREG_OFFSET(25)]
> ldp x27, x28, [\ctxt, #CPU_XREG_OFFSET(27)]
> ldp x29, lr, [\ctxt, #CPU_XREG_OFFSET(29)]
> + ldr x18, [\ctxt, #CPU_XREG_OFFSET(18)]

There is now an assumption that ctxt is x18 (otherwise why would it be
out of order?). Please add a comment to that effect.

> .endm
>
> /*
> @@ -87,12 +89,9 @@ alternative_else_nop_endif
> ldp x14, x15, [x18, #CPU_XREG_OFFSET(14)]
> ldp x16, x17, [x18, #CPU_XREG_OFFSET(16)]
>
> - // Restore guest regs x19-x29, lr
> + // Restore guest regs x18-x29, lr
> restore_callee_saved_regs x18

Or you could elect another register such as x29 as the base, and keep
the above in a reasonable order.

>
> - // Restore guest reg x18
> - ldr x18, [x18, #CPU_XREG_OFFSET(18)]
> -
> // Do not touch any register after this!
> eret
> sb
> @@ -114,7 +113,7 @@ 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 +122,8 @@ 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

Thanks,

M.
--
Jazz is not dead. It just smells funny...