On 01/12/16 17:08, Andy Lutomirski wrote: > On Thu, Dec 1, 2016 at 1:22 AM, Borislav Petkov <[email protected]> wrote: >> On Wed, Nov 30, 2016 at 12:34:55PM -0800, Andy Lutomirski wrote: >>> Aside from being excessively slow, CPUID is problematic: Linux runs >>> on a handful of CPUs that don't have CPUID. MOV to CR2 is always >>> available, so use it instead. >>> >>> Signed-off-by: Andy Lutomirski <[email protected]> >>> --- >>> arch/x86/include/asm/processor.h | 31 ++++++++----------------------- >>> 1 file changed, 8 insertions(+), 23 deletions(-) >> Looks nice. >> >> I'm wondering if we should leave this one in tip for an additional cycle >> to have it tested on more hw. I know, it is architectural and so on but >> who knows what every implementation actually does... > I want the Xen opinion as well. > > Xen folks, can Linux use write_cr2 to serialize the CPU core on Xen PV > or do we need something a bit heavier weight like native_write_cr2?
To sum up our conversation on IRC. xen_write_cr2() is not serialising; it is just a write into a shared page. native_write_cr2() would trap and be emulated. This will incur #GP[0] due to cpl, although not necessarily an iret on the way back out of Xen. Something like an iret-to-self would be far quicker, and avoid trapping into the hypervisor. ~Andrew
