Astral/kernel-src/arch/x86-64/context.c

#include <arch/context.h>
#include <stdbool.h>
#include <arch/cpuid.h>
#include <kernel/slab.h>
#include <logging.h>
#include <arch/gdt.h>

#define XCR0_X87 (1ull << 0)
#define XCR0_SSE (1ull << 1)
#define XCR0_AVX (1ull << 2)
#define XCR0_AVX512 (7ull << 5)

#define XCR0_MASK (XCR0_AVX512 | XCR0_AVX | XCR0_SSE | XCR0_X87)

static enum {
	FXSAVE,
	XSAVE,
	XSAVEOPT,
} xsave_method;
size_t arch_xsave_size;
static uint64_t xcr0_value;
static scache_t *xsave_cache;

typedef struct {
	uint16_t fcw;
	uint16_t fsw;
	uint8_t ftw;
	uint8_t reserved1;
	uint16_t fop;
	uint64_t fip;
	uint64_t fdp;
	uint32_t mxcsr;
	uint32_t mxcsr_mask;
	struct {
		uint64_t data[2];
	} mm[8];
	struct {
		uint64_t data[2];
	} xmm[16];
	uint64_t reserved2[6];
	uint64_t unused[6];
} fxsave_area_t;

void arch_extracontext_detect(void) {
	if (!arch_xsave_size) {
		cpuid_results_t results;
		cpuid(1, &results);
		if (cpuid_base_max_leaf() >= 0x0d && (results.ecx & (1 << 26))) {
			xsave_method = XSAVE;

			cpuid_with_ecx(0x0d, 0, &results);
			xcr0_value = ((uint64_t)results.edx << 32) | results.eax;
			xcr0_value &= XCR0_MASK;
		} else {
			xsave_method = FXSAVE;
			arch_xsave_size = 512;
		}
	}

	if (xsave_method == FXSAVE) return;

	size_t cr4;
	asm volatile("mov %%cr4, %0" : "=r" (cr4));
	cr4 |= (1u << 18);
	asm volatile("mov %0, %%cr4" :: "r" (cr4));

	uint32_t low = xcr0_value;
	uint32_t high = xcr0_value >> 32;
	asm volatile("xsetbv" :: "a" (low), "d" (high), "c" (0u));

	if (!arch_xsave_size) {
		// after xsetbv ebx is updated to the size of the context for the enabled features
		cpuid_results_t results;
		cpuid_with_ecx(0x0d, 0, &results);
		arch_xsave_size = results.ebx;

		cpuid_with_ecx(0x0d, 1, &results);
		if (results.eax & 1) xsave_method = XSAVEOPT;
	}
}

int arch_extracontext_init(extracontext_t *context) {
	if (!xsave_cache) {
		xsave_cache = slab_newcache(arch_xsave_size, xsave_method == FXSAVE ? 16 : 64, NULL, NULL);
		__assert(xsave_cache);
	}

	context->xsave = slab_allocate(xsave_cache);
	if (!context->xsave) return 1;

	// all sse exceptions masked
	// initialise the x87 FPU state as it would be after the FNINIT instruction
	memset(context->xsave, 0, arch_xsave_size);
	fxsave_area_t *area = context->xsave;
	area->mxcsr = 0x1f80;
	area->fcw = 0x37f;

	return 0;
}

void arch_extracontext_free(extracontext_t *context) {
	slab_free(xsave_cache, context->xsave);
}

void arch_extracontext_save(extracontext_t *context) {
	context->gsbase = rdmsr(MSR_KERNELGSBASE);
	context->fsbase = rdmsr(MSR_FSBASE);

	asm volatile("mov %%gs, %0" : "=r"(context->gs));
	asm volatile("mov %%fs, %0" : "=r"(context->fs));

	switch (xsave_method) {
		case FXSAVE: asm("fxsaveq %0" ::"m"(*(char(*)[arch_xsave_size])context->xsave) : "memory"); break;
		case XSAVE: asm("xsaveq %0" ::"m"(*(char(*)[arch_xsave_size])context->xsave), "d"(-1), "a"(-1) : "memory"); break;
		case XSAVEOPT: asm("xsaveoptq %0" ::"m"(*(char(*)[arch_xsave_size])context->xsave), "d"(-1), "a"(-1) : "memory"); break;
	}
}

void arch_extracontext_load(extracontext_t *context) {
	if (current_thread()->proc && current_thread()->proc->ldt)
		arch_gdt_set_ldt(current_thread()->proc->ldt, 0xffff);
	else
		arch_gdt_set_ldt(NULL, 0);

	void *old_base = current_cpu();
	if (!load_seg_gs(context->gs)) load_seg_gs(0);
	wrmsr(MSR_GSBASE, (uint64_t)old_base);
	wrmsr(MSR_KERNELGSBASE, context->gsbase);

	if (!load_seg_fs(context->fs)) load_seg_fs(0);	
	wrmsr(MSR_FSBASE, context->fsbase);

	switch (xsave_method) {
		case FXSAVE: asm("fxrstorq %0" ::"m"(*(char(*)[arch_xsave_size])context->xsave) : "memory"); break;
		case XSAVE:
		case XSAVEOPT: asm("xrstorq %0" ::"m"(*(char(*)[arch_xsave_size])context->xsave), "d"(-1), "a"(-1) : "memory"); break;
	}
}

void arch_extracontext_copy(extracontext_t *dst, const extracontext_t *src) {
	// XXX is this behavior correct? ucontext doesnt seem to save it on linux and so it will not be saved
	dst->gsbase = src->gsbase;
	dst->fsbase = src->fsbase;
	dst->fs = dst->fs;
	dst->gs = dst->gs;
	memcpy(dst->xsave, src->xsave, arch_xsave_size);
}