Messages in this thread | | | Subject | Re: [PATCH v2 6/9] KVM: x86: implement KVM_GUESTDBG_BLOCKEVENTS | From | Paolo Bonzini <> | Date | Fri, 2 Apr 2021 19:07:32 +0200 |
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On 01/04/21 15:54, Maxim Levitsky wrote: > KVM_GUESTDBG_BLOCKEVENTS is a guest debug feature that > will allow KVM to block all interrupts while running. > It is mostly intended to be used together with single stepping, > to make it more robust, and has the following benefits: > > * Resuming from a breakpoint is much more reliable: > When resuming execution from a breakpoint, with interrupts enabled, > more often than not, KVM would inject an interrupt and make the CPU > jump immediately to the interrupt handler and eventually return to > the breakpoint, only to trigger it again. > > From the gdb's user point of view it looks like the CPU has never > executed a single instruction and in some cases that can even > prevent forward progress, for example, when the breakpoint > is placed by an automated script (e.g lx-symbols), which does > something in response to the breakpoint and then continues > the guest automatically. > If the script execution takes enough time for another interrupt to > arrive, the guest will be stuck on the same breakpoint forever. > > * Normal single stepping is much more predictable, since it won't > land the debugger into an interrupt handler. > > * Chances of RFLAGS.TF being leaked to the guest are reduced: > > KVM sets that flag behind the guest's back to single step it, > but if the single step lands the vCPU into an > interrupt/exception handler the RFLAGS.TF will be leaked to the > guest in the form of being pushed to the stack. > This doesn't completely eliminate this problem as exceptions > can still happen, but at least this eliminates the common > case. > > Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
The patch uses BLOCKIRQ instead of BLOCKEVENTS.
Paolo
> --- > Documentation/virt/kvm/api.rst | 1 + > arch/x86/include/asm/kvm_host.h | 3 ++- > arch/x86/include/uapi/asm/kvm.h | 1 + > arch/x86/kvm/x86.c | 4 ++++ > 4 files changed, 8 insertions(+), 1 deletion(-) > > diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst > index 9778b2434c03..a4f2dc84741f 100644 > --- a/Documentation/virt/kvm/api.rst > +++ b/Documentation/virt/kvm/api.rst > @@ -3338,6 +3338,7 @@ flags which can include the following: > - KVM_GUESTDBG_INJECT_DB: inject DB type exception [x86] > - KVM_GUESTDBG_INJECT_BP: inject BP type exception [x86] > - KVM_GUESTDBG_EXIT_PENDING: trigger an immediate guest exit [s390] > + - KVM_GUESTDBG_BLOCKIRQ: avoid injecting interrupts/NMI/SMI [x86] > > For example KVM_GUESTDBG_USE_SW_BP indicates that software breakpoints > are enabled in memory so we need to ensure breakpoint exceptions are > diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h > index cc7c82a449d5..8c529ae9dbbe 100644 > --- a/arch/x86/include/asm/kvm_host.h > +++ b/arch/x86/include/asm/kvm_host.h > @@ -227,7 +227,8 @@ enum x86_intercept_stage; > KVM_GUESTDBG_USE_HW_BP | \ > KVM_GUESTDBG_USE_SW_BP | \ > KVM_GUESTDBG_INJECT_BP | \ > - KVM_GUESTDBG_INJECT_DB) > + KVM_GUESTDBG_INJECT_DB | \ > + KVM_GUESTDBG_BLOCKIRQ) > > > #define PFERR_PRESENT_BIT 0 > diff --git a/arch/x86/include/uapi/asm/kvm.h b/arch/x86/include/uapi/asm/kvm.h > index 5a3022c8af82..b0f9945067f7 100644 > --- a/arch/x86/include/uapi/asm/kvm.h > +++ b/arch/x86/include/uapi/asm/kvm.h > @@ -282,6 +282,7 @@ struct kvm_debug_exit_arch { > #define KVM_GUESTDBG_USE_HW_BP 0x00020000 > #define KVM_GUESTDBG_INJECT_DB 0x00040000 > #define KVM_GUESTDBG_INJECT_BP 0x00080000 > +#define KVM_GUESTDBG_BLOCKIRQ 0x00100000 > > /* for KVM_SET_GUEST_DEBUG */ > struct kvm_guest_debug_arch { > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c > index 956e8e0bd6af..3627ce8fe5bb 100644 > --- a/arch/x86/kvm/x86.c > +++ b/arch/x86/kvm/x86.c > @@ -8460,6 +8460,10 @@ static void inject_pending_event(struct kvm_vcpu *vcpu, bool *req_immediate_exit > can_inject = false; > } > > + /* Don't inject interrupts if the user asked to avoid doing so */ > + if (vcpu->guest_debug & KVM_GUESTDBG_BLOCKIRQ) > + return; > + > /* > * Finally, inject interrupt events. If an event cannot be injected > * due to architectural conditions (e.g. IF=0) a window-open exit >
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