On Fri, Mar 6, 2020 at 6:26 PM Andy Lutomirski luto@kernel.org wrote:
The ABI is broken and we cannot support it properly. Turn it off.
If this causes a meaningful performance regression for someone, KVM can introduce an improved ABI that is supportable.
Cc: stable@vger.kernel.org Signed-off-by: Andy Lutomirski luto@kernel.org
arch/x86/kernel/kvm.c | 21 ++++++++++++++++++--- 1 file changed, 18 insertions(+), 3 deletions(-)
diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c index 93ab0cbd304e..e6f2aefa298b 100644 --- a/arch/x86/kernel/kvm.c +++ b/arch/x86/kernel/kvm.c @@ -318,11 +318,26 @@ static void kvm_guest_cpu_init(void)
pa = slow_virt_to_phys(this_cpu_ptr(&apf_reason));-#ifdef CONFIG_PREEMPTION
pa |= KVM_ASYNC_PF_SEND_ALWAYS;-#endif pa |= KVM_ASYNC_PF_ENABLED;
/** We do not set KVM_ASYNC_PF_SEND_ALWAYS. With the current* KVM paravirt ABI, the following scenario is possible:** #PF: async page fault (KVM_PV_REASON_PAGE_NOT_PRESENT)* NMI before CR2 or KVM_PF_REASON_PAGE_NOT_PRESENT* NMI accesses user memory, e.g. due to perf* #PF: normal page fault* #PF reads CR2 and apf_reason -- apf_reason should be 0** outer #PF reads CR2 and apf_reason -- apf_reason should be* KVM_PV_REASON_PAGE_NOT_PRESENT** There is no possible way that both reads of CR2 and* apf_reason get the correct values. Fixing this would* require paravirt ABI changes.*/
Upon re-reading my own comment, I think the problem is real, but I don't think my patch fixes it. The outer #PF could just as easily have come from user mode. We may actually need the NMI code (and perhaps MCE and maybe #DB too) to save, clear, and restore apf_reason. If we do this, then maybe CPL0 async PFs are actually okay, but the semantics are so poorly defined that I'm not very confident about that.