Date: Fri, 12 Jan 2018 02:11:51 +0300
From: Alexey Dobriyan <adobriyan@gmail.com>
To: w@1wt.eu
Cc: linux-kernel@vger.kernel.org
Subject: Re: [RFC PATCH v2 6/6] x86/entry/pti: don't switch PGD on when
 pti_disable is set
Message-ID: <20180111231151.GA12706@avx2>
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> I initially tried getpid() which I found to be cached by glibc, but I
> switched to write(-1, "a", 1) in the example in the cover letter and
> the test for 3 million runs roughly climbs from 200 ms to 900 ms under
> kvm (with PCID this time, I didn't retest without).

umask() is the fastest system call I think.
Also who cares about glibc caching?

static inline int sys_umask(int mask)
{
	int rv;
	asm volatile (
		"syscall"
		: "=a" (rv)
		: "0" (95), "D" (mask)
		: "rcx", "r11", "cc", "memory"
	);
	return rv;
}

int main(void)
{
	unsigned int i;

	for (i = 0; i < (1U << 24); i++) {
		sys_umask(0);
	}
	return 0;
}