On 2025-01-24 at 08:14:41 -0800, Dave Hansen wrote:
On 11/27/24 09:35, Maciej Wieczor-Retman wrote:
-/* Check 5-level page table feature in CPUID.(EAX=07H, ECX=00H):ECX.[bit 16] */ static inline int cpu_has_la57(void) {
- unsigned int cpuinfo[4];
- __cpuid_count(0x7, 0, cpuinfo[0], cpuinfo[1], cpuinfo[2], cpuinfo[3]);
- return (cpuinfo[2] & (1 << 16));
- return !system("grep -wq la57 /proc/cpuinfo");
}
I would rather we find another way.
First, we've documented the behavior a bit in here:
https://docs.kernel.org/arch/x86/cpuinfo.html
The important part is:
"The absence of a flag in /proc/cpuinfo by itself means almost nothing to an end user."
Even worse, let's say there's a CPU bug and we say define a bug bit:
bugs : spectre_v1 spectre_v2 ... la57_is_broken
How is that grep going to work out? ;)
Could you poke around and see if there is any existing ABI that we can use to query LA57 support? Maybe one of the things KVM exports, or some TASK_SIZE_MAX comparisons?
Sure, I'll try to find some other way.
My previous tactic was to munmap() a high address and see if it works. Does that sound okay in case there isn't anything else would indicate la57 to userspace reliably?