Re: [PATCH v2 6/9] KVM: x86: implement KVM_GUESTDBG_BLOCKEVENTS

Paolo Bonzini
2021-04-02T17:07:43Z

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
>