Xsave

Summary

XSAVE stands for similar x86 instruction xsave which places extended processor state into a memory area. The saving can be initiated by any userspace application at any moment and size of the memory frame depends on processor features and may vary between different models. Thus if checkpoint and restore are done on different processors the next call to xsave may corrupt memory if sizes mismatch.

XSAVE frame size

Before fetching frame sizes one need to figure out if xsave is supported at all. There are several helpers we will refer on

static inline void native_cpuid(unsigned int *eax, unsigned int *ebx,
				unsigned int *ecx, unsigned int *edx)
{
	/* ecx is often an input as well as an output. */
	asm volatile("cpuid"
	    : "=a" (*eax),
	      "=b" (*ebx),
	      "=c" (*ecx),
	      "=d" (*edx)
	    : "0" (*eax), "2" (*ecx)
	    : "memory");
}

static inline void cpuid(unsigned int op,
			 unsigned int *eax, unsigned int *ebx,
			 unsigned int *ecx, unsigned int *edx)
{
	*eax = op;
	*ecx = 0;
	native_cpuid(eax, ebx, ecx, edx);
}

static inline void cpuid_count(unsigned int op, int count,
			       unsigned int *eax, unsigned int *ebx,
			       unsigned int *ecx, unsigned int *edx)
{
	*eax = op;
	*ecx = count;
	native_cpuid(eax, ebx, ecx, edx);
}

Run cpuid(0x1, &eax, &ebx, &ecx, &edx) and bits 26 and 27 are both set in ecx if xsave is supported (strictly speaking bit 27 is reserved for operating system which can clear it to indicate that instruction is disabled).

After that we can fetch maximal frame size which applications may use via cpuid_count(0xd, 0, &eax, &ebx, &ecx, &edx)