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Re: [Qemu-riscv] [Qemu-devel] [PATCH v1 10/28] target/riscv: Convert mie

From: Richard Henderson
Subject: Re: [Qemu-riscv] [Qemu-devel] [PATCH v1 10/28] target/riscv: Convert mie and mstatus to pointers
Date: Thu, 19 Sep 2019 07:50:12 -0700
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.8.0

On 9/18/19 4:47 PM, Alistair Francis wrote:
> I'm not a fan of the pointer method that I'm using, but to me it seems
> the least worst in terms of handling future code, keeping everythign
> consistnent and avoiding complex access rules.

FWIW, I prefer the "banked" register method used by ARM.

enum {
    M_REG_NS = 0,    /* non-secure mode */
    M_REG_S = 1,     /* secure mode */
    M_REG_NUM_BANKS = 2,


        uint32_t vecbase[M_REG_NUM_BANKS];
        uint32_t basepri[M_REG_NUM_BANKS];
        uint32_t control[M_REG_NUM_BANKS];

The major difference that I see is that a pointer can only represent a single
state at a single time.  With an index, different parts of the code can ask
different questions that may have different states.  E.g. "are we currently in
secure mode" vs "will the exception return to secure mode".


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