48 lines
2.0 KiB
Zig
48 lines
2.0 KiB
Zig
//! User-space shared memory: `shm_create` / `shm_map`. A process creates a shareable,
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//! zeroed, cacheable RAM region and gets back a pointer plus a **capability handle**; it
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//! passes that handle to another process as an `ipc_call` send_cap, and the receiver
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//! `shm_map`s it to map the same physical pages. The kernel primitive under the display
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//! compositor↔native-driver and app↔compositor surface paths (docs/display-v2.md). The
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//! generalization of capability passing from endpoints to memory objects.
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const abi = @import("abi");
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const sc = @import("system-call.zig");
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const ipc = @import("ipc.zig");
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inline fn failed(r: usize) bool {
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return r > ~@as(usize, 0) - 4095; // a wrapped -errno lands in the top page
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}
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/// A shared region: the `ptr` the CPU touches, and the `handle` (a capability) to hand to
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/// another process as an `ipc_call` send_cap.
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pub const Region = struct {
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ptr: [*]u8,
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handle: ipc.Handle,
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len: usize,
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};
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/// Grant `len` bytes (rounded up to whole pages) of shareable, zeroed, cacheable RAM.
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/// Returns the region or null on failure. Two return values — vaddr in rax, handle in rdx —
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/// so this is a hand-written stub like `dma.alloc`.
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pub fn create(len: usize) ?Region {
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var rax: usize = undefined;
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var rdx: usize = undefined; // out: the capability handle
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asm volatile ("syscall"
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: [rax] "={rax}" (rax),
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[rdx] "={rdx}" (rdx),
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: [n] "{rax}" (@intFromEnum(abi.SystemCall.shm_create)),
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[a0] "{rdi}" (len),
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: .{ .rcx = true, .r11 = true, .memory = true });
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if (failed(rax)) return null;
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return .{ .ptr = @ptrFromInt(rax), .handle = rdx, .len = len };
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}
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/// Map the shared region named by a capability `handle` this process received (via an
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/// `ipc_call` send_cap) into its address space — the same physical pages the creator sees.
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/// Returns the pointer, or null on failure.
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pub fn map(handle: ipc.Handle) ?[*]u8 {
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const r = sc.systemCall1(.shm_map, handle);
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if (failed(r)) return null;
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return @ptrFromInt(r);
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}
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