On Tue, Jul 14, 2020 at 02:09:05PM +0200, Joerg Roedel wrote:> @@ -1028,6 +1036,16 @@ DEFINE_IDTENTRY_VC_SAFE_STACK(exc_vmm_communication) > struct ghcb *ghcb; > > lockdep_assert_irqs_disabled(); > + > + /* > + * #DB is special and needs to be handled outside of the intrumentation_begin()/end(). > + * Otherwise the #VC handler could be raised recursivly. > + */ > + if (error_code == SVM_EXIT_EXCP_BASE + X86_TRAP_DB) { > + vc_handle_trap_db(regs); > + return; > + } > + > instrumentation_begin();Wait what?! That makes no sense what so ever.
On Wed, Jul 15, 2020 at 10:47:52AM +0200, Peter Zijlstra wrote:> On Tue, Jul 14, 2020 at 02:09:05PM +0200, Joerg Roedel wrote: > > > @@ -1028,6 +1036,16 @@ DEFINE_IDTENTRY_VC_SAFE_STACK(exc_vmm_communication) > > struct ghcb *ghcb; > > > > lockdep_assert_irqs_disabled(); > > + > > + /* > > + * #DB is special and needs to be handled outside of the intrumentation_begin()/end(). > > + * Otherwise the #VC handler could be raised recursivly. > > + */ > > + if (error_code == SVM_EXIT_EXCP_BASE + X86_TRAP_DB) { > > + vc_handle_trap_db(regs); > > + return; > > + } > > + > > instrumentation_begin(); > > Wait what?! That makes no sense what so ever.Then my understanding of intrumentation_begin/end() is wrong, I thought that the kernel will forbid setting breakpoints before instrumentation_begin(), which is necessary here because a break-point in the #VC handler might cause recursive #VC-exceptions when #DB is intercepted. Maybe you can elaborate on why this makes no sense? Regards, Joerg
On Wed, Jul 15, 2020 at 11:13:37AM +0200, Joerg Roedel wrote:> On Wed, Jul 15, 2020 at 10:47:52AM +0200, Peter Zijlstra wrote: > > On Tue, Jul 14, 2020 at 02:09:05PM +0200, Joerg Roedel wrote: > > > > > @@ -1028,6 +1036,16 @@ DEFINE_IDTENTRY_VC_SAFE_STACK(exc_vmm_communication) > > > struct ghcb *ghcb; > > > > > > lockdep_assert_irqs_disabled(); > > > + > > > + /* > > > + * #DB is special and needs to be handled outside of the intrumentation_begin()/end(). > > > + * Otherwise the #VC handler could be raised recursivly. > > > + */ > > > + if (error_code == SVM_EXIT_EXCP_BASE + X86_TRAP_DB) { > > > + vc_handle_trap_db(regs); > > > + return; > > > + } > > > + > > > instrumentation_begin(); > > > > Wait what?! That makes no sense what so ever. > > Then my understanding of intrumentation_begin/end() is wrong, I thought > that the kernel will forbid setting breakpoints before > instrumentation_begin(), which is necessary here because a break-point > in the #VC handler might cause recursive #VC-exceptions when #DB is > intercepted. > Maybe you can elaborate on why this makes no sense?Kernel avoids breakpoints in any noinstr text, irrespective of instrumentation_begin(). instrumentation_begin() merely allows one to call !noinstr functions.