On 14/07/18 11:33, Srivatsa S. Bhat wrote:
From: David Woodhouse dwmw@amazon.co.uk
commit def9331a12977770cc6132d79f8e6565871e8e38 upstream
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: Juergen Gross jgross@suse.com Reviewed-by: Boris Ostrovsky boris.ostrovsky@oracle.com Acked-by: Thomas Gleixner tglx@linutronix.de Signed-off-by: Juergen Gross jgross@suse.com Signed-off-by: David Woodhouse dwmw@amazon.co.uk Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org Signed-off-by: Srivatsa S. Bhat srivatsa@csail.mit.edu Reviewed-by: Matt Helsley (VMware) matt.helsley@gmail.com Reviewed-by: Alexey Makhalov amakhalov@vmware.com Reviewed-by: Bo Gan ganb@vmware.com
I believe you'll need upstream commit 60d3450167433f2d099ce2869dc52dd9e7dc9b29 ("x86/cpu: Re-apply forced caps every time CPU caps are re-read") for this patch to work as intended.
It was necessary for 4.9, at least.
Juergen