Messages in this thread | | | From | Andrew Lutomirski <> | Date | Wed, 27 Jul 2011 13:05:18 -0400 | Subject | Re: [Xen-devel] Re: [PATCH 0/5] Collected vdso/vsyscall fixes for 3.1 |
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On Wed, Jul 27, 2011 at 12:58 PM, Konrad Rzeszutek Wilk <konrad.wilk@oracle.com> wrote: >> >> $ test_vsyscall test >> >> Testing gettimeofday... >> >> vDSO offset = 0.000001s >> >> vsyscall offset = 0.000001s >> >> >> >> Testing time... >> >> vDSO offset = 0 >> >> vsyscall offset = 0 >> >> Testing getcpu... >> >> ok! cpu=6 node=0 > >> I bet if you pull a new copy or remove -mavx from Makefile it will >> work. I got a grossly hacked-up Xen domU booted and everything seems >> to work. > > It did. Both Dom0 and DomU work on AMD and Intel. > > In regards to the last pv-ops patch - is there no better way? The reason I am asking > is the pv-ops hook is just a bandaid for the problem. Is the Xen syscall suppose to > be doingsomething extra with the stack perhaps? >
The Xen code in question is:
restore_all_guest: ASSERT_INTERRUPTS_DISABLED RESTORE_ALL testw $TRAP_syscall,4(%rsp) jz iret_exit_to_guest
addq $8,%rsp popq %rcx # RIP popq %r11 # CS cmpw $FLAT_USER_CS32,%r11 popq %r11 # RFLAGS popq %rsp # RSP je 1f sysretq 1: sysretl
So with VCGF_in_syscall set, the ireq hypercall will return via sysretq if the saved CS is __USER_CS or FLAT_USER_CS64. This is faster than iretq.
The hypervisor doesn't allow the guest OS to override the values in MSR_STAR, so FLAT_USER_CS64 gets returned to userspace. And sysretq is probably much faster than iretq, so unsetting VCGF_in_syscall is probably a bad idea.
The ideal solution would be to allow the kernel to change MSR_STAR, but this would require changing the hypervisor and the kernel.
--Andy -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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