Re: Generating host code in TCG mode

[沈梦姣]

Thanks Peter

在 2023年3月18日，上午12:18，Peter Maydell <peter.maydell@linaro.org> 写道：

On Fri, 17 Mar 2023 at 15:17, 沈梦姣 <shen.mengjiao3@icloud.com> wrote:

Hi,

When generating host code, sometimes it has to jmp to qemu common

code(e.g. memory management code to get host virtual address), but

it seems before the jmp the caller saved registers haven’t been

pushed to stack, qemu common code is function so it can use caller

saved registers.

TCG knows what registers may be clobbered by a call into
host C code (this is set up in the tcg_target_call_clobber_regs
regset by code in tcg_target_init()), so the register allocator
will either avoid using those registers

Look at tcg_target_reg_alloc_order in i386 host, it has these clobber registers though it first allocates callee saved registers.

for values that must
be preserved across the call, or else it will spill them to
memory.

I didn’t see these clobber registers being saved to memory before call/jmp Host C code in i386(x86-64) host, and copy these memory value back to these registers

-- PMM