Re: [PATCH v2] KVM: VMX: Update instruction length on intercepted BP

Gleb Natapov
2010-02-16T08:24:56Z

On Tue, Feb 16, 2010 at 09:05:40AM +0100, Jan Kiszka wrote:
> Gleb Natapov wrote:
> > On Mon, Feb 15, 2010 at 03:53:04PM +0100, Jan Kiszka wrote:
> >> We intercept #BP while in guest debugging mode. As VM exits due to
> >> intercepted exceptions do not necessarily come with valid
> >> idt_vectoring, we have to update event_exit_inst_len explicitly in such
> >> cases. At least in the absence of migration, this ensures that
> >> re-injections of #BP will find and use the correct instruction length.
> >>
> > Thinking about it some more. Why do we exit to userspace at all if we
> > intercept wrong #DB? It seams to me not wise to have ability to inject
> > exceptions from userspace. Exceptions generation mechanism is a part of
> > CPU and we shouldn't outsource part of CPU functionality to userspace.
>
> The guest debugging API was design to avoid maintaining a "countless"
> number of breakpoints in kernel space and instead chose to loop over
> user space to decide about #DB & #BP. So this part is required even if
> we start thinking about an alternative interface in the future.
>
How much is "countless"? 10000? I am sure we can handle this.

> Jan
>
> >
> >> Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
> >> ---
> >>
> >> Changes in v2:
> >> - added comments
> >>
> >> arch/x86/kvm/vmx.c | 13 +++++++++++++
> >> 1 files changed, 13 insertions(+), 0 deletions(-)
> >>
> >> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> >> index f82b072..14873b9 100644
> >> --- a/arch/x86/kvm/vmx.c
> >> +++ b/arch/x86/kvm/vmx.c
> >> @@ -2775,6 +2775,12 @@ static int handle_rmode_exception(struct kvm_vcpu *vcpu,
> >> kvm_queue_exception(vcpu, vec);
> >> return 1;
> >> case BP_VECTOR:
> >> + /*
> >> + * Update instruction length as we may reinject the exception
> >> + * from user space while in guest debugging mode.
> >> + */
> >> + to_vmx(vcpu)->vcpu.arch.event_exit_inst_len =
> >> + vmcs_read32(VM_EXIT_INSTRUCTION_LEN);
> >> if (vcpu->guest_debug & KVM_GUESTDBG_USE_SW_BP)
> >> return 0;
> >> /* fall through */
> >> @@ -2897,6 +2903,13 @@ static int handle_exception(struct kvm_vcpu *vcpu)
> >> kvm_run->debug.arch.dr7 = vmcs_readl(GUEST_DR7);
> >> /* fall through */
> >> case BP_VECTOR:
> >> + /*
> >> + * Update instruction length as we may reinject #BP from
> >> + * user space while in guest debugging mode. Reading it for
> >> + * #DB as well causes no harm, it is not used in that case.
> >> + */
> >> + vmx->vcpu.arch.event_exit_inst_len =
> >> + vmcs_read32(VM_EXIT_INSTRUCTION_LEN);
> >> kvm_run->exit_reason = KVM_EXIT_DEBUG;
> >> kvm_run->debug.arch.pc = vmcs_readl(GUEST_CS_BASE) + rip;
> >> kvm_run->debug.arch.exception = ex_no;
> >
> > --
> > Gleb.
>

--
Gleb.