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

[Paolo Bonzini <pbonzini@redhat.com>]

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
>