fix(risc-v, virt64, plic): use volatile rw for claim and complete

[Tang-Haojin]

拉取/合并请求描述：(PR description)

[

为什么提交这份PR (why to submit this PR)

观察到开启编译优化时，中断只能触发一次。定位到是由于对 plic 的 claim/complete 寄存器的操作没有被标记为 volatile，导致编译器优化掉了对 mmio 寄存器的写。

• 问题代码序列

void plic_handle_irq(void)
{
    int plic_irq = plic_claim();
    plic_complete(plic_irq);
    irq_desc[plic_irq].handler(plic_irq, irq_desc[plic_irq].param);
}

• 反汇编

00000000802264e6 <plic_handle_irq>:
    802264e6:	0001e797          	auipc	a5,0x1e
    802264ea:	b627a783          	lw	a5,-1182(a5) # 80244048 <__text_end>
    802264ee:	8181b683          	ld	a3,-2024(gp) # 80245720 <plic_base>
    802264f2:	00201737          	lui	a4,0x201
    802264f6:	00d7979b          	slliw	a5,a5,0xd
    802264fa:	97b6                	add	a5,a5,a3
    802264fc:	97ba                	add	a5,a5,a4
    802264fe:	43c8                	lw	a0,4(a5)
    80226500:	00037797          	auipc	a5,0x37
    80226504:	69078793          	addi	a5,a5,1680 # 8025db90 <irq_desc>
    80226508:	00451713          	slli	a4,a0,0x4
    8022650c:	97ba                	add	a5,a5,a4
    8022650e:	6398                	ld	a4,0(a5)
    80226510:	678c                	ld	a1,8(a5)
    80226512:	8702                	jr	a4

可以看到，对寄存器的写（sw）操作被优化掉了。

你的解决方案是什么 (what is your solution)

使用 readl() 和 writel() 对 mmio 寄存器进行操作。

• 修改后的反汇编

00000000802264e6 <plic_handle_irq>:
    802264e6:	0001e697          	auipc	a3,0x1e
    802264ea:	b7a68693          	addi	a3,a3,-1158 # 80244060 <__text_end>
    802264ee:	81818613          	addi	a2,gp,-2024 # 80245738 <plic_base>
    802264f2:	4288                	lw	a0,0(a3)
    802264f4:	621c                	ld	a5,0(a2)
    802264f6:	00201737          	lui	a4,0x201
    802264fa:	0711                	addi	a4,a4,4 # 201004 <__text_size+0x1bcfa4>
    802264fc:	97ba                	add	a5,a5,a4
    802264fe:	00d5151b          	slliw	a0,a0,0xd
    80226502:	953e                	add	a0,a0,a5
    80226504:	4108                	lw	a0,0(a0)
    80226506:	2501                	sext.w	a0,a0
    80226508:	0820000f          	fence	i,r
    8022650c:	4294                	lw	a3,0(a3)
    8022650e:	0140000f          	fence	w,o
    80226512:	621c                	ld	a5,0(a2)
    80226514:	00d6969b          	slliw	a3,a3,0xd
    80226518:	97ba                	add	a5,a5,a4
    8022651a:	97b6                	add	a5,a5,a3
    8022651c:	c388                	sw	a0,0(a5)
    8022651e:	00037797          	auipc	a5,0x37
    80226522:	6b278793          	addi	a5,a5,1714 # 8025dbd0 <irq_desc>
    80226526:	00451713          	slli	a4,a0,0x4
    8022652a:	97ba                	add	a5,a5,a4
    8022652c:	6398                	ld	a4,0(a5)
    8022652e:	678c                	ld	a1,8(a5)
    80226530:	8702                	jr	a4

请提供验证的bsp和config (provide the config and bsp)

• BSP: bsp/qemu-virt64-riscv

• .config: 无变化

• action:

]

当前拉取/合并请求的状态 Intent for your PR

必须选择一项 Choose one (Mandatory):

• 本拉取/合并请求是一个草稿版本 This PR is for a code-review and is intended to get feedback
• 本拉取/合并请求是一个成熟版本 This PR is mature, and ready to be integrated into the repo

代码质量 Code Quality：

我在这个拉取/合并请求中已经考虑了 As part of this pull request, I've considered the following:

• 已经仔细查看过代码改动的对比 Already check the difference between PR and old code
• 代码风格正确，包括缩进空格，命名及其他风格 Style guide is adhered to, including spacing, naming and other styles
• 没有垃圾代码，代码尽量精简，不包含#if 0代码，不包含已经被注释了的代码 All redundant code is removed and cleaned up
• 所有变更均有原因及合理的，并且不会影响到其他软件组件代码或BSP All modifications are justified and not affect other components or BSP
• 对难懂代码均提供对应的注释 I've commented appropriately where code is tricky
• 代码是高质量的 Code in this PR is of high quality
• 已经使用formatting 等源码格式化工具确保格式符合RT-Thread代码规范 This PR complies with RT-Thread code specification
• 如果是新增bsp, 已经添加ci检查到.github/ALL_BSP_COMPILE.json 详细请参考链接BSP自查

[github-actions]

📌 Code Review Assignment

🏷️ Tag: libcpu_riscv

Reviewers: @Yaochenger

Changed Files (Click to expand)

• libcpu/risc-v/virt64/plic.c

---

📊 Current Review Status (Last Updated: 2025-09-02 22:16 CST)

• ⌛ @Yaochenger Pending Review

---

📝 Review Instructions

1. 维护者可以通过单击此处来刷新审查状态: 🔄 刷新状态
   Maintainers can refresh the review status by clicking here: 🔄 Refresh Status

2. 确认审核通过后评论 LGTM/lgtm
   Comment LGTM/lgtm after confirming approval

3. PR合并前需至少一位维护者确认
   PR must be confirmed by at least one maintainer before merging

ℹ️ 刷新CI状态操作需要具备仓库写入权限。
ℹ️ Refresh CI status operation requires repository Write permission.