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Commit def9331a authored by Juergen Gross's avatar Juergen Gross
Browse files

x86/amd: don't set X86_BUG_SYSRET_SS_ATTRS when running under Xen 



When running as Xen pv guest X86_BUG_SYSRET_SS_ATTRS must not be set
on AMD cpus.

This bug/feature bit is kind of special as it will be used very early
when switching threads. Setting the bit and clearing it a little bit
later leaves a critical window where things can go wrong. This time
window has enlarged a little bit by using setup_clear_cpu_cap() instead
of the hypervisor's set_cpu_features callback. It seems this larger
window now makes it rather easy to hit the problem.

The proper solution is to never set the bit in case of Xen.

Signed-off-by: default avatarJuergen Gross <jgross@suse.com>
Reviewed-by: default avatarBoris Ostrovsky <boris.ostrovsky@oracle.com>
Acked-by: default avatarThomas Gleixner <tglx@linutronix.de>
Signed-off-by: default avatarJuergen Gross <jgross@suse.com>
parent d162809f

arch/x86/kernel/cpu/amd.c

+3 −2
Original line number Diff line number Diff line
@@ -799,7 +799,8 @@ static void init_amd(struct cpuinfo_x86 *c) 
		if (cpu_has(c, X86_FEATURE_3DNOW) || cpu_has(c, X86_FEATURE_LM))

			set_cpu_cap(c, X86_FEATURE_3DNOWPREFETCH);



	/* AMD CPUs don't reset SS attributes on SYSRET */

	/* AMD CPUs don't reset SS attributes on SYSRET, Xen does. */

	if (!cpu_has(c, X86_FEATURE_XENPV))

		set_cpu_bug(c, X86_BUG_SYSRET_SS_ATTRS);

}

arch/x86/xen/enlighten_pv.c

+0 −1
Original line number Diff line number Diff line
@@ -290,7 +290,6 @@ static bool __init xen_check_xsave(void) 


static void __init xen_init_capabilities(void)

{

	setup_clear_cpu_cap(X86_BUG_SYSRET_SS_ATTRS);

	setup_force_cpu_cap(X86_FEATURE_XENPV);

	setup_clear_cpu_cap(X86_FEATURE_DCA);

	setup_clear_cpu_cap(X86_FEATURE_APERFMPERF);